View source for Astronomy:Near-Earth object

{{short description|Small Solar System body whose orbit brings it close to Earth}}
{{Multiple image
| perrow = 2/1
| total_width = 300
| image1 = Asteroid2006DP14.jpg
| caption1 = Radar-imaging of {{mp|(388188) 2006 DP|14}} by a [[Astronomy:NASA Deep Space Network|DSN]] antenna
| image2 = The VLT images the very faint Near-Earth Object 2009 FD.jpg
| caption2 = Very Large Telescope image of the very faint near-Earth asteroid {{mpl|2009 FD}} as seen by the [[Astronomy:Very Large Telescope|VLT]] telescope
| image3 = Comet Hartley 2.jpg
| caption3 = Near-Earth comet 103P/Hartley as seen by NASA's ''[[Engineering:Deep Impact (spacecraft)|Deep Impact]]'' probe 
}}
{{Pie chart
| caption = 34,000+ known NEOs, divided into several orbital subgroups<ref name="neo-jpl-stats" />
| label1 = [[Astronomy:Apollo asteroid|Apollos]]: 19,323
| value1 = 56.39
| color1 = #009a39
| label2 = [[Astronomy:Amor asteroid|Amors]]: 12,094
| value2 = 35.29
| color2 = #66CCFF
| label3 = [[Astronomy:Aten asteroid|Atens]]: 2,692	
| value3 = 7.86
| color3 = #f7c35a
| label4 = [[Astronomy:Comet|Comet]]s: 123
| value4 = 0.36
| color4 = #de2821
| label5 = [[Astronomy:Atira asteroid|Atiras]]: 34
| value5 = 0.01
| color5 = black
}}

A '''near-Earth object''' ('''NEO''') is any [[Astronomy:Small Solar System body|small Solar System body]] whose orbit brings it into proximity with [[Astronomy:Earth|Earth]]. By convention, a Solar System body is a NEO if its closest approach to the Sun ([[Astronomy:Apsis|perihelion]]) is less than 1.3&nbsp;[[Astronomy:Astronomical unit|astronomical unit]]s (AU).<ref name="CNEOS-NEO-groups"/> If a NEO's orbit crosses the Earth's orbit, and the object is larger than {{convert|140|meter|ft}} across, it is considered a [[Astronomy:Potentially hazardous object|potentially hazardous object]] (PHO).<ref name="CHAPMAN04"/> Most known PHOs and NEOs are [[Astronomy:Asteroid|asteroid]]s, but a small fraction are [[Astronomy:Comet|comet]]s.<ref name="neo-jpl-stats" />

There are over 34,000 known near-Earth asteroids (NEAs) and over 120 known short-period near-Earth comets (NECs).<ref name="neo-jpl-stats" /> A number of solar-orbiting [[Astronomy:Meteoroid|meteoroid]]s were large enough to be tracked in space before striking Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth.<ref name="BROWN02">{{cite news |url=https://www.sciencenews.org/archive/call-catastrophes |title=The Call of Catastrophes |author=Richard Monastersky |work=Science News Online |date=March 1, 1997 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20040313165341/http://www.sciencenews.org/pages/sn_arc97/75th/rm_essay.htm |archive-date=2004-03-13}}</ref> Asteroids as small as {{convert|20|m|ft}} in diameter can cause significant damage to the local environment and human populations.<ref>{{Cite journal |last1=Rumpf |first1=Clemens M.|last2=Lewis |first2=Hugh G.|last3=Atkinson |first3=Peter M. |date=March 23, 2017 |title=Asteroid impact effects and their immediate hazards for human populations |journal=[[Earth:Geophysical Research Letters|Geophysical Research Letters]] |language=en |volume=44 |issue=8 |pages=3433–3440 |doi=10.1002/2017gl073191 |issn=0094-8276 |arxiv=1703.07592 |bibcode=2017GeoRL..44.3433R |s2cid=34867206}}</ref> Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or [[Tsunami|tsunami]]s if they impact the sea. Interest in NEOs has increased since the 1980s because of greater awareness of this potential danger. [[Astronomy:Asteroid impact avoidance|Asteroid impact avoidance]] by deflection is possible in principle, and methods of mitigation are being researched.<ref name="tsr20120514"/>

Two scales, the simple [[Astronomy:Torino scale|Torino scale]] and the more complex [[Palermo Technical Impact Hazard Scale|Palermo scale]], rate the risk presented by an identified NEO based on the probability of it impacting the Earth and on how severe the consequences of such an impact would be. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery.

Since 1998, the United States, the European Union, and other nations are scanning the sky for NEOs in an effort called [[Astronomy:Spaceguard|Spaceguard]].<ref name="spaceguard-2004" /> The initial US Congress mandate to [[Organization:NASA|NASA]] to catalog at least 90% of NEOs that are at least {{convert|1|km|ft}} in diameter, sufficient to cause a global catastrophe, was met by 2011.<ref name="pia14734" /> In later years, the survey effort was expanded<ref name="law-109-155"/> to include smaller objects<ref name="ETech"/> which have the potential for large-scale, though not global, damage.

NEOs have low surface gravity, and many have Earth-like orbits that make them easy targets for spacecraft.<ref name="USAToday-NEA"/><ref name="wired20130323"/> {{As of|2024|01}}, five near-Earth comets<ref name="TaskForceReport"/><ref name="DeepImpactHartley"/><ref name="newsci20160930"/> and six near-Earth asteroids<ref name="Eros-NEAR"/><ref name="Toutatis-Change"/><ref name="SFN-Hayabusa2"/><ref name="space-osiris-overview"/><ref name="NASA220927"/>, one of them with a moon,<ref name="NASA220927"/> have been visited by spacecraft. Samples of two have been returned to Earth,<ref name="SFN-Hayabusa2"/> and one successful deflection test was conducted.<ref name="NASA221215"/> Similar missions are in progress. Preliminary plans for commercial [[Astronomy:Asteroid mining|asteroid mining]] have been drafted by private startup companies, but few of these plans were pursued.<ref name="Forbes210831"/>

== Definitions ==
[[File:Potentially Hazardous Asteroids 2013.png|thumb|Plot of orbits of known potentially hazardous asteroids (size over {{convert|460|ft|m|order=flip|abbr=on}} and passing within {{convert|4.7|e6mi|e6km|order=flip|abbr=on}} of Earth's orbit) as of early 2013 (alternate image)]]

Near-Earth objects (NEOs) are by convention technically defined by the [[Organization:International Astronomical Union|International Astronomical Union]] (IAU) as all small Solar System bodies with orbits around the Sun that are partially closer than 1.3 [[Astronomy:Astronomical unit|astronomical unit]]s (AU; Sun–Earth distance) away from the Sun.<ref name="IAU-NEOs"/> The organisations cataloging NEOs further limit their definition to objects with an orbital period under 200 years, a restriction that applies to comets in particular,<ref name="CNEOS-NEO-groups">{{cite web |title=NEO Basics. NEO Groups |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/about/neo_groups.html |access-date=2024-01-27}}</ref><ref name="NEOCC-DA">{{cite web |title=Definitions & Assumptions |publisher=ESA NEOCC |url=https://neo.ssa.esa.int/definitions-assumptions |access-date=2024-01-27}}</ref> but this approach is not universal.<ref name="IAU-NEOs"/> Some authors further restrict the definition to orbits that are at least partly further than 0.983 AU away from the Sun.<ref name="MorbidelliAstIII"/><ref>{{Cite journal |last1=Waszczak |first1=Adam |last2=Prince |first2=Thomas A. |last3=Laher |first3=Russ |last4=Masci |first4=Frank |last5=Bue |first5=Brian |last6=Rebbapragada |first6=Umaa |last7=Barlow |first7=Tom |last8=Jason Surace |last9=Helou |first9=George |date=2017 |title=Small Near-Earth Asteroids in the Palomar Transient Factory Survey: A Real-Time Streak-detection System |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=129 |issue=973 |at=part 034402 |doi=10.1088/1538-3873/129/973/034402 |issn=1538-3873 |arxiv=1609.08018|bibcode=2017PASP..129c4402W |s2cid=43606524}}</ref> NEOs are thus not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. Many NEOs have complex orbits due to constant perturbation by the Earth's gravity, and some of them can temporarily change from an orbit around the Sun to one around the Earth, but the term is applied flexibly for these objects, too.<ref name="ST111230"/>

When a NEO is detected, like all other small Solar System bodies, its positions and brightness are submitted to the (IAU's) [[Organization:Minor Planet Center|Minor Planet Center]] (MPC) for cataloging. The MPC maintains separate lists of confirmed NEOs and potential NEOs.<ref name="MPC-NEO-confirm">{{cite web |title=The NEO Confirmation Page |publisher=IAU/MPC |url=http://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html |access-date=2024-01-26}}</ref><ref>{{cite journal |last1=Marsden |first1=B. G. |last2=Williams |first2=G. V. |title=The NEO Confirmation Page |journal=Planetary and Space Science |volume=46 |issue=2 |pages=299 |bibcode=1998P&SS...46..299M |year=1998 |doi=10.1016/S0032-0633(96)00153-5 }}</ref> The orbits of some NEOs intersect that of the Earth, so they pose a collision danger.<ref name="CHAPMAN04">{{cite journal |title=The hazard of near-Earth asteroid impacts on earth |author=Clark R. Chapman |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=222 |issue=1 |pages=1–15 |date=May 2004 |doi=10.1016/j.epsl.2004.03.004 |bibcode=2004E&PSL.222....1C}}</ref> These are considered [[Astronomy:Potentially hazardous object|potentially hazardous object]]s (PHOs) if their estimated diameter is above 140 meters. The MPC maintains a separate list for the asteroids among PHOs, the potentially hazardous asteroids (PHAs).<ref name="MPC-PHA-list">{{cite web |title=List Of Potentially Hazardous Minor Planets (by designation) |publisher=IAU/MPC |url=https://www.minorplanetcenter.net/iau/lists/t_phas.html |access-date=2024-01-26}}</ref> NEOs are also catalogued by two separate units of the [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] (JPL) of the National Aeronautics and Space Administration ([[Organization:NASA|NASA]]): the Center for Near Earth Object Studies (CNEOS)<ref name="neo-jpl-intro" /> and the Solar System Dynamics Group.<ref name="JPL-SSD-NEA"/> NEOs are also catalogued by a unit of the [[Organization:European Space Agency|European Space Agency]] (ESA), the Near-Earth Objects Coordination Centre (NEOCC).<ref name="NEOCC-about">{{cite web |title=About NEOCC |publisher=ESA NEOCC |url=https://neo.ssa.esa.int/about-neocc |access-date=2024-01-27}}</ref>

PHAs are defined based on two parameters relating to respectively their potential to approach the Earth dangerously closely and the estimated consequences that an impact would have if it occurs.<ref name="CNEOS-NEO-groups"/> Objects with both an Earth [[Astronomy:Minimum orbit intersection distance|minimum orbit intersection distance]] (MOID) of 0.05&nbsp;AU or less and an [[Astronomy:Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of 22.0 or brighter (a rough indicator of large size) are considered PHAs. Objects that either cannot approach closer to the Earth than {{convert |0.05 |AU |km mi |abbr=on |lk=in}}, or which are fainter than H = 22.0 (about {{convert|140|m|ft|abbr=on|lk=off}} in diameter with assumed [[Earth:Albedo|albedo]] of 14%), are not considered PHAs.<ref name="CNEOS-NEO-groups"/>

== History of human awareness of NEOs ==
[[File:PSM V76 D020 Path of halley comet.png|thumb|1910 drawing of the path of Halley's Comet]]
[[File:Eros - PIA02923 (color).jpg|thumb|The near Earth asteroid 433 Eros was visited by a probe in the 1990s]]
The first near-Earth objects to be observed by humans were comets. Their extraterrestrial nature was recognised and confirmed only after [[Biography:Tycho Brahe|Tycho Brahe]] tried to measure the distance of a comet through its [[Astronomy:Parallax|parallax]] in 1577 and the lower limit he obtained was well above the Earth diameter; the periodicity of some comets was first recognised in 1705, when [[Biography:Edmond Halley|Edmond Halley]] published his orbit calculations for the returning object now known as [[Astronomy:Halley's Comet|Halley's Comet]].<ref name="Halley-synopsis">{{cite book |last=Halley |first=Edmund |date=1705 |title=A synopsis of the astronomy of comets |url=https://library.si.edu/digital-library/book/synopsisofastron00hall |access-date=2024-01-26 |location=London |publisher=John Senex |url-status=live |archive-url=https://web.archive.org/web/20171201055040/https://library.si.edu/digital-library/book/synopsisofastron00hall |archive-date=2017-12-01 }}</ref> The 1758–1759 return of Halley's Comet was the first comet appearance predicted.<ref>{{cite book |last=Stoyan |first=Ronald |title=Atlas of Great Comets |date=2015 |location=Cambridge |publisher=Cambridge University Press |url=https://books.google.com/books?id=WAZEBgAAQBAJ&pg=PA101 |access-date=2024-01-26 |pages=101–103 |isbn=978-1-107-09349-2 |url-status=live |archive-url=https://web.archive.org/web/20180301114413/https://books.google.com/books?id=WAZEBgAAQBAJ&pg=PA101&lpg=PA101 |archive-date=2018-03-01 }}</ref> It has been said that [[Astronomy:Lexell's Comet|Lexell's comet]] of 1770 was the first discovered Near-Earth object.<ref>{{Cite journal |last1=Ye |first1=Quan-Zhi |last2=Wiegert |first2=Paul A. |last3=Hui |first3=Man-To |date=March 22, 2018 |title=Finding Long Lost Lexell's Comet: The Fate of the First Discovered Near-Earth Object |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=155 |issue=4 |pages=163 |doi=10.3847/1538-3881/aab1f6 |issn=1538-3881 |arxiv=1802.08904 |bibcode=2018AJ....155..163Y |s2cid=118895688 |doi-access=free }}</ref>

The first near-Earth asteroid to be discovered was [[Astronomy:433 Eros|433 Eros]] in 1898.<ref name=Scholl>{{Cite journal |last1=Scholl |first1=Hans |last2=Schmadel |first2=Lutz D. |title=Discovery Circumstances of the First Near-Earth Asteroid (433) Eros |journal=Acta Historica Astronomiae |bibcode=2002AcHA...15..210S |volume=15 |pages=210–220 |date=2002}}</ref> The asteroid was subject to several extensive observation campaigns, primarily because measurements of its orbit enabled a precise determination of the then imperfectly known distance of the Earth from the Sun.<ref>{{cite web |url=http://near.jhuapl.edu/eros/history/eros_useful.html |title=Eros comes on stage, finally a useful asteroid |publisher=Johns Hopkins University Applied Physics Laboratory |access-date=2024-01-26}}</ref>

In 1937, asteroid [[Astronomy:69230 Hermes|69230 Hermes]] was discovered when it passed the Earth at twice the [[Astronomy:Lunar distance|distance of the Moon]].<ref name="RadarHermes">{{cite web |title=Radar observations of long-lost asteroid 1937 UB (Hermes) |publisher=[[Organization:University of California, Los Angeles|UCLA]] |url=http://www2.ess.ucla.edu/~jlm/research/NEAs/Hermes/ |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20230123110938/http://www2.ess.ucla.edu/~jlm/research/NEAs/Hermes/ |archive-date=2023-01-23 }}</ref> Hermes was considered a threat because it was lost after its discovery; thus its orbit and potential for collision with Earth were not known precisely.<ref name="Marsden1998"/> Hermes was only re-discovered in 2003, and it is now known to be no threat for at least the next century.<ref name="RadarHermes"/>

On June 14, 1968, the 1.4&nbsp;km diameter asteroid [[Astronomy:1566 Icarus|1566 Icarus]] passed Earth at a distance of {{convert |0.042 |AU |km |abbr=on |lk=off}}, or 16 times the distance of the Moon.<ref name=jpl-close>{{cite web |url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1566&view=OPC |title=Small-Body Database Lookup. 1566 Icarus (1949 MA) |publisher=NASA/JPL |date=January 20, 2024 |access-date=2024-01-26 }}</ref> During this approach, Icarus became the first minor planet to be observed using [[Engineering:Radar|radar]], with measurements obtained at the [[Astronomy:Haystack Observatory|Haystack Observatory]]<ref name="Pettengill-1969">{{Cite journal |display-authors=6 |first1=G. H. |last1=Pettengill |first2=I. I. |last2=Shapiro |first3=M. E. |last3=Ash |first4=R. P. |last4=Ingalls |first5=L. P. |last5=Rainville |first6=W. B. |last6=Smith |first7=M. L. |last7=Stone |date=May 1969 |title=Radar observations of Icarus |journal=Icarus |volume=10 |issue=3 |pages=432–435 |bibcode=1969Icar...10..432P |doi= 10.1016/0019-1035(69)90101-8 |issn=0019-1035}}</ref> and the Goldstone Tracking Station.<ref name="Goldstein-1968">{{Cite journal |last=Goldstein |first= R. M. |date=November 1968 |title=Radar Observations of Icarus |journal=Science |volume=162 |issue=3856 |pages=903–904 |bibcode=1968Sci...162..903G |doi=10.1126/science.162.3856.903 |pmid=17769079|s2cid=129644095 }}</ref> This was the first close approach predicted years in advance (Icarus had been discovered in 1949), and also earned significant public attention, due to alarmist news reports.<ref name="Marsden1998"/> A year before the approach, MIT students launched Project Icarus, devising a plan to deflect the asteroid with rockets in case it was found to be on a collision course with Earth.<ref>{{cite news |url=http://www.thespacereview.com/article/175/1 |title=Giant bombs on giant rockets: Project Icarus |author=Dwayne A. Day |work=The Space Review |date=July 5, 2004 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20160415041026/http://www.thespacereview.com/article/175/1 |archive-date=2016-04-15 }}</ref> Project Icarus received wide media coverage, and inspired the 1979 disaster movie ''Meteor'', in which the US and the USSR join forces to blow up an Earth-bound fragment of an asteroid hit by a comet.<ref>{{cite news |url=http://tech.mit.edu/V99/PDF/V99-N43.pdf |title=MIT Course precept for movie |work=The Tech |publisher=MIT |date=October 30, 1979 |access-date=2017-11-15 |url-status=dead |archive-url=https://web.archive.org/web/20140811005933/http://tech.mit.edu/V99/PDF/V99-N43.pdf |archive-date=2014-08-11 }}</ref>

On March 23, 1989, the {{convert|300|m|ft|abbr=on}} diameter Apollo asteroid [[Astronomy:4581 Asclepius|4581 Asclepius]] (1989 FC) missed the Earth by {{convert|700,000|km|mi|abbr=on}}. If the asteroid had impacted it would have created the largest explosion in recorded history, equivalent to 20,000 [[Physics:TNT equivalent|megatons of TNT]]. It attracted widespread attention because it was discovered only after the closest approach.<ref>{{cite news |url=https://www.nytimes.com/1989/04/20/us/big-asteroid-passes-near-earth-unseen-in-a-rare-close-call.html |title=Big Asteroid Passes Near Earth Unseen In a Rare Close Call |author=Warren E. Leary |work={{wipe|The New York Times}} |date=April 20, 1989 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20171109191344/http://www.nytimes.com/1989/04/20/us/big-asteroid-passes-near-earth-unseen-in-a-rare-close-call.html |archive-date=2017-11-09 }}</ref>

In March 1998, early orbit calculations for recently discovered asteroid {{mpl|(35396) 1997 XF|11}} showed a potential 2028 close approach {{convert |0.00031 |AU |km |abbr=on |lk=off}} from the Earth, well within the orbit of the Moon, but with a large error margin allowing for a direct hit. Further data allowed a revision of the 2028 approach distance to {{convert |0.0064 |AU |km |abbr=on |lk=off}}, with no chance of collision. By that time, inaccurate reports of a potential impact had caused a media storm.<ref name="Marsden1998">{{cite news |date=March 29, 1998 |title=How the Asteroid Story Hit: An Astronomer Reveals How a Discovery Spun Out of Control |work=The Boston Globe |author=Brian G. Marsden |url=http://www.minorplanetcenter.net/iau/pressinfo/1997XF11Globe.html |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20120617210302/http://www.minorplanetcenter.net/iau/pressinfo/1997XF11Globe.html |archive-date=2012-06-17 }}</ref>

=== Risk ===
[[File:Toutatis.jpg|thumb|Asteroid 4179 Toutatis is a potentially hazardous object that passed within 4 lunar distances in September 2004 and currently has a minimum possible distance of 2.5 lunar distances.]]

From the late 1990s, a typical frame of reference in searches for NEOs has been the scientific concept of [[Risk|risk]]. The risk that any near-Earth object poses is viewed having regard to both the [[Social:Culture|culture]] and the [[Engineering:Technology|technology]] of human society. Through history, humans have associated NEOs with changing risks, based on religious, philosophical or scientific views, as well as humanity's technological or economical capability to deal with such risks.<ref name="tsr20120514">
{{cite journal |last=Fernández Carril |first=Luis |title=The evolution of near Earth objects risk perception |journal=The Space Review |date=May 14, 2012 |url=http://www.thespacereview.com/article/2080/1 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20170629205156/http://www.thespacereview.com/article/2080/1 |archive-date=2017-06-29 }}</ref> Thus, NEOs have been seen as [[Unsolved:Omen|omen]]s of natural disasters or wars; harmless spectacles in an unchanging universe; the source of era-changing cataclysms<ref name="tsr20120514"/> or potentially poisonous fumes (during Earth's passage through the tail of Halley's Comet in 1910);<ref>{{cite news |author=Stuart Clark |url=https://www.theguardian.com/science/across-the-universe/2012/dec/20/apocalypse-postponed-halley-comet |title=Apocalypse postponed: how Earth survived Halley's comet in 1910 |date=December 20, 2012 |work={{wipe|The Guardian}} |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20171222021944/https://www.theguardian.com/science/across-the-universe/2012/dec/20/apocalypse-postponed-halley-comet |archive-date=2017-12-22 }}</ref> and finally as a possible cause of a crater-forming impact that could even cause [[Biology:Extinction|extinction]] of humans and other life on Earth.<ref name="tsr20120514"/>

The potential of catastrophic impacts by near-Earth comets was recognised as soon as the first orbit calculations provided an understanding of their orbits: in 1694, Edmond Halley presented a theory that [[Religion:Genesis flood narrative|Noah's flood]] in the [[Religion:Bible|Bible]] was caused by a comet impact.<ref>{{cite web |url=http://www.jasoncolavito.com/halley-on-noahs-comet.html |title=Noah's Comet. Edmond Halley 1694 |first=Jason |last=Colavito |work=Jasoncolavito.com |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20171001192755/http://www.jasoncolavito.com/halley-on-noahs-comet.html |archive-date=2017-10-01 }}</ref> Human [[Perception|perception]] of near-Earth asteroids as benign objects of fascination or killer objects with high risk to human society has ebbed and flowed during the short time that NEAs have been scientifically observed.<ref name="wired20130323">{{cite news |last=Portree |first=David S. |title=Earth-Approaching Asteroids as Targets for Exploration (1978) |url=https://www.wired.com/wiredscience/2013/03/earth-approaching-asteroids-as-targets-for-exploration-1978/ |access-date=2023-01-26 |magazine=Wired |date=March 23, 2013 |quote=People in the early 21st century have been encouraged to see asteroids as the interplanetary equivalent of sea monsters. We often hear talk of “killer asteroids,” when in fact there exists no conclusive evidence that any asteroid has killed anyone in all of human history. … In the 1970s, asteroids had yet to gain their present fearsome reputation … most astronomers and planetary scientists who made a career of studying asteroids rightfully saw them as sources of fascination, not of worry. |url-status=live |archive-url=https://web.archive.org/web/20140112133317/http://www.wired.com/wiredscience/2013/03/earth-approaching-asteroids-as-targets-for-exploration-1978/ |archive-date=2014-01-12 }}</ref> Scientists have recognised the threat of impacts that create craters much bigger than the impacting bodies and have indirect effects on an even wider area since the 1980s, after the confirmation of a theory that the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] (in which the non-avian dinosaurs died out) 65 million years ago was caused by a [[Earth:Chicxulub crater|large asteroid impact]].<ref name="tsr20120514"/><ref name="Chapman1998">{{cite web |author=Clark R. Chapman |title=History of The Asteroid/Comet Impact Hazard |publisher=Southwest Research Institute |date=October 7, 1998 |url=http://www.boulder.swri.edu/clark/ncarhist.html |access-date=2024-01-26}}</ref>

The awareness of the wider public of the impact risk rose after the observation of the impact of the fragments of [[Astronomy:Comet Shoemaker–Levy 9|Comet Shoemaker–Levy 9]] into Jupiter in July 1994.<ref name="tsr20120514"/><ref name="Chapman1998"/> In 1998, the movies ''Deep Impact'' and ''Armageddon'' popularised the notion that near-Earth objects could cause catastrophic impacts.<ref name="Chapman1998"/> Also at that time, a [[Unsolved:Conspiracy theory|conspiracy theory]] arose about the supposed 2003 impact of the fictitious [[Unsolved:Nibiru cataclysm|planet Nibiru]], which persisted on the internet as the predicted impact date was moved to 2012 and then 2017.<ref name="Molloy2017">{{cite news |last=Molloy |first=Mark |title=Nibiru: How the nonsense Planet X Armageddon and Nasa fake news theories spread globally |url=https://www.telegraph.co.uk/news/2017/09/21/nibiru-nonsense-planet-x-armageddon-nasa-fake-news-theories/ |archive-url=https://ghostarchive.org/archive/20220111/https://www.telegraph.co.uk/news/2017/09/21/nibiru-nonsense-planet-x-armageddon-nasa-fake-news-theories/ |archive-date=2022-01-11 |url-access=subscription |url-status=live |work=The Daily Telegraph |date=September 24, 2017 |access-date=2024-01-26}}{{cbignore}}</ref>

==== Risk scales ====

There are two schemes for the scientific classification of impact hazards from NEOs:
* the simple [[Astronomy:Torino scale|Torino scale]], which rates the risks of impacts in the next 100 years according to impact energy and impact probability, using integer numbers between 0 and 10;<ref name="torino">{{cite web |url=http://cneos.jpl.nasa.gov/torino_scale.html |title=Torino Impact Hazard Scale |publisher=NASA/JPL CNEOS |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20240103160751/https://cneos.jpl.nasa.gov/sentry/torino_scale.html |archive-date=2024-01-03}}</ref><ref>{{cite journal|title=Torino Impact Hazard Scale |journal=Planetary and Space Science |volume=48 |issue=4 |pages=297–303 |bibcode=2000P&SS...48..297B |last1=Binzel |first1=Richard P. |year=2000 |doi=10.1016/S0032-0633(00)00006-4 }}</ref> and
* the more complex [[Palermo Technical Impact Hazard Scale]], which ascribes ratings that can be any positive or negative real number; these ratings depend on the background impact frequency, impact probability and time until possible impact.<ref name="palermo">{{cite web |url=http://cneos.jpl.nasa.gov/palermo_scale.html |title=Palermo Technical Impact Hazard Scale |publisher=NASA/JPL CNEOS |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20231001191140/https://cneos.jpl.nasa.gov/sentry/palermo_scale.html |archive-date=2023-10-01}}</ref>

On both scales, risks of any concern are indicated by values above zero.<ref name="torino"/><ref name="palermo"/>

==== Magnitude of risk ====
The annual background frequency used in the Palermo scale for impacts of energy greater than ''E'' megatonnes is estimated as:<ref name="palermo"/>

:<math>f_B = 0.03E^{-0.8} \;</math>

For instance, this formula implies that the [[Expected value|expected value]] of the time from now until the next impact greater than 1 megatonne is 33 years, and that when it occurs, there is a 50% chance that it will be above 2.4 megatonnes. This formula is only valid over a certain range of ''E''.

However, another paper<ref>{{cite journal |title=The flux of small near-Earth objects colliding with the Earth |author=P. Brown |display-authors=0 |journal=Nature |volume=420 |issue=6913 |pages=294–296 |date=November 2002 |bibcode=2002Natur.420..294B |doi=10.1038/nature01238 |pmid=12447433 |s2cid=4380864}}</ref> published in 2002 – the same year as the paper on that the Palermo scale is based – found a power law with different constants:

:<math>f_B = 0.00737 E^{-0.9} \;</math>

This formula gives considerably lower rates for a given ''E''. For instance, it gives the rate for [[Astronomy:Bolide|bolide]]s of 10 megatonnes or more (like the Tunguska explosion) as 1 per thousand years, rather than 1 per 210 years as in the Palermo formula. However, the authors give a rather large uncertainty (once in 400 to 1800 years for 10 megatonnes), due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination.

==== Highly rated risks ====

[[Organization:NASA|NASA]] maintains an automated system to evaluate the threat from known NEOs over the next 100 years, which generates the continuously updated Sentry Risk Table.<ref name="Current_Impact_Risks" /> All or nearly all of the objects are highly likely to drop off the list eventually as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions.<ref name="Current_Impact_Risks"/><ref name="2006HZ51">{{cite news |url=https://www.newscientist.com/article/dn9095-big-new-asteroid-has-slim-chance-of-hitting-earth.html |title=Big new asteroid has slim chance of hitting Earth |author=David Chandler |work=New Scientist |date=May 2, 2006 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20150531181008/http://www.newscientist.com/article/dn9095-big-new-asteroid-has-slim-chance-of-hitting-earth.html |archive-date=2015-05-31 }}</ref>

In March 2002, {{mpl|(163132) 2002 CU|11}} became the first asteroid with a temporarily positive rating on the Torino Scale, with about a 1 in 9,300 chance of an impact in 2049.<ref>{{cite news |author1=Andrea Milani |author2=Giovanni Valsecchi |author3=Maria Eugenia Sansaturio |title=The problem with 2002 CU11 |work=Tumbling Stone |volume=12 |publisher=[[Astronomy:NEODyS|NEODyS]] |date=March 12, 2002 |url=http://spaceguard.rm.iasf.cnr.it/tumblingstone/issues/num12/eng/2002cu11.htm |access-date=2018-01-29 |url-status=live |archive-url=https://web.archive.org/web/20160304023838/http://spaceguard.rm.iasf.cnr.it/tumblingstone/issues/num12/eng/2002cu11.htm |archive-date=2016-03-04 }}</ref> Additional observations reduced the estimated risk to zero, and the asteroid was removed from the Sentry Risk Table in April 2002.<ref name="removed">{{cite web |title=Date/Time Removed |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/sentry/removed.html |date=January 24, 2024 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20240126214410/https://cneos.jpl.nasa.gov/sentry/removed.html |archive-date=2024-01-26 }}</ref> It is now known that within the next two centuries, {{mp|2002 CU|11}} will pass the Earth at a safe closest distance (perigee) of {{convert|0.00425|AU|km mi|abbr=on|lk=off}} on August 31, 2080.<ref>{{cite web |url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2002CU11&view=OPC |title=Small-Body Database Lookup. 163132 (2002 CU11) |publisher=NASA/JPL |date=September 13, 2023 |access-date=2024-01-26}}</ref>

[[File:1950 DA (color).png|thumb|Radar image of asteroid {{mpl-|29075|1950 DA}}]]
Asteroid {{mpl-|29075|1950 DA}} was lost after its 1950 discovery, since its observations over just 17 days were insufficient to precisely determine its orbit; it was rediscovered on December 31, 2000. It has a diameter of about a kilometer (0.6 miles), and an impact would therefore be globally catastrophic. It was observed by radar during its close 2001 approach, allowing much more precise orbit calculations. Although this asteroid will not strike for at least 800 years and thus has no Torino scale rating, it was added to the Sentry list in April 2002 as the first object with a Palermo scale value greater than zero.<ref name="IAU-NEOs"/><ref name="NEO-1950DA">{{cite web |title=29075 (1950 DA) Analyses, 2001-2007 |publisher=NASA/JPL CNEOS |url=http://cneos.jpl.nasa.gov/doc/1950da/ |access-date=2024-01-26}}</ref> The then-calculated 1 in 300 maximum chance of impact and +0.17 Palermo scale value was roughly 50% greater than the background risk of impact by all similarly large objects until 2880.<ref name="NEO-1950DA"/><ref>{{cite journal |last1=Giorgini |first1=J. D. |last2=Ostro |first2=S. J. |last3=Benner |first3=L. A. M. |last4=Chodas |first4=P. W. |last5=Chesley |first5=S. R. |last6=Hudson |first6=R. S. |last7=Nolan |first7=M. C. |last8=Klemola |first8=A. R. |last9=Standish |first9=E. M. |last10=Jurgens |first10=R. F. |last11=Rose |first11=R |last12=Chamberlin |first12=A. B. |last13=Yeomans |first13=D. K. |last14=Margot |first14=J. L. |date=April 5, 2002 |title=Asteroid 1950 DA's Encounter with Earth in 2880: Physical Limits of Collision Probability Prediction |journal=Science |volume=296 |pages=132–136 |url=https://cneos.jpl.nasa.gov/doc/1950da/1950da_published.pdf |access-date=2024-01-26 |doi=10.1126/science.1068191 |pmid=11935024 |issue=5565|bibcode = 2002Sci...296..132G |s2cid=8689246 |display-authors=8 }}</ref> Uncertainties in the orbit calculations were further reduced using additional radar observations in 2012, and this decreased the odds of an impact.<ref name=Farnocchia2013>{{Cite journal |last1=Farnocchia |first1=Davide |last2=Chesley |first2=Steven R. |title=Assessment of the 2880 impact threat from asteroid (29075) 1950 DA |date=2013 |arxiv=1310.0861 |doi=10.1016/j.icarus.2013.09.022 |bibcode=2014Icar..229..321F |volume=229 |journal=Icarus |pages=321–327|s2cid=56453734 }}</ref> Taking all radar and optical observations through 2021 into account, the probability of impact in March 2880 is, {{As of|2024|01|lc=y}}, assessed at 1 in 34,000.<ref name="Current_Impact_Risks"/> The corresponding Palermo scale value of −2.05 is still the second highest for all objects on the Sentry List Table.<ref name="Current_Impact_Risks"/>

On December 24, 2004, {{convert|370|m|ft|abbr=on}} asteroid [[Astronomy:99942 Apophis|99942 Apophis]] (at the time known only by its provisional designation {{mp|2004 MN|4}}) was assigned a 4 on the Torino scale, the highest rating given to date, as the information available at the time translated to a 1.6% chance of Earth impact on Friday, April 13, 2029.<ref>{{cite news |first1=D. |last1=Yeomans |first2=S. |last2=Chesley |first3=P. |last3=Chodas |title=Near-Earth Asteroid 2004 MN4 Reaches Highest Score To Date On Hazard Scale |publisher=NASA/JPL CNEOS |date=December 23, 2004 |url=https://cneos.jpl.nasa.gov/news/news146.html |access-date=2024-01-31 }}</ref> The calculated chance of impact increased to as high as 2.7%,<ref name="cneosnews164">{{cite news |first1=Dwayne |last1=Brown |first2=DC |last2=Agle |title=NASA Refines Asteroid Apophis' Path Toward Earth |publisher=NASA/JPL CNEOS |date=October 7, 2009 |url=https://cneos.jpl.nasa.gov/news/news164.html |access-date=2024-01-31 }}</ref> but by December 27, 2004, additional observations had significantly reduced the uncertainty zone for the 2029 approach and it no longer included the Earth.<ref>{{cite news |first1=D. |last1=Yeomans |first2=S. |last2=Chesley |first3=P. |last3=Chodas |title=Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN4 |publisher=NASA/JPL CNEOS |date=December 27, 2004 |url=https://cneos.jpl.nasa.gov/news/news148.html |access-date=2024-01-31 }}</ref> The 2029 risk of impact consequently dropped to zero, but later potential impact dates were still rated 1 on the Torino scale. Further observations lowered the 2036 risk to a Torino rating of 0 in August 2006.<ref>{{cite web |title=99942 Apophis (2004 MN4) Earth Impact Risk Summary |publisher=NASA/JPL |date=August 4, 2006 |url=http://neo.jpl.nasa.gov/risk/a99942.html |access-date=January 13, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20060805063943/http://neo.jpl.nasa.gov/risk/a99942.html |archive-date=2006-08-05}}</ref> A small risk of an impact in 2068 remained, but this was eliminated with orbital calculations enhanced further by using observations taken during the March 2021 flyby of the asteroid near Earth.<ref>{{cite news |title=NASA Analysis: Earth Is Safe From Asteroid Apophis for 100-Plus Years |date=March 25, 2021 |work=News |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/nasa-analysis-earth-is-safe-from-asteroid-apophis-for-100-plus-years |access-date=2024-01-31 }}</ref> Consequently, Apophis was removed from the Sentry Risk Table.<ref name="removed"/>

In February 2006, {{mpl|(144898) 2004 VD|17}} was assigned a Torino Scale rating of 2 due to a close encounter predicted for May 4, 2102.<ref name="Morrison-VD17">{{cite news |date=March 1, 2006 |title=Asteroid 2004 VD17 classed as Torino Scale 2 |work=Asteroid and Comet Impact Hazards |publisher=NASA |author=David Morrison |url=http://impact.arc.nasa.gov/news_detail.cfm?ID=167 |access-date=2017-11-10 |url-status=dead |archive-url=https://web.archive.org/web/20111014235559/https://impact.arc.nasa.gov/news_detail.cfm?ID=167 |archive-date=2011-10-14}}</ref> After additional observations allowed increasingly precise predictions, the Torino rating was lowered first to 1 in May 2006, then to 0 in October 2006, and the asteroid was removed from the Sentry Risk Table entirely in February 2008.<ref name=removed/>

In 2021, {{mpl|2010 RF|12}} was listed with the highest chance of impacting Earth, at 1 in 22 on September 5, 2095. At only {{convert|7|m|ft|abbr=on}} across, the asteroid however is much too small to be considered a [[Astronomy:Potentially hazardous asteroid|potentially hazardous asteroid]] and it poses no serious threat: the possible 2095 impact therefore rated only −3.32 on the Palermo Scale.<ref name="Current_Impact_Risks"/> Observations during the August 2022 close approach were expected to ascertain whether the asteroid will impact or miss Earth in 2095.<ref name="2022recovery">{{cite web |last1=Deen |first1=Sam |title=2022 recovery of 2010 RF12? |url=https://groups.io/g/mpml/message/32792 |website=Minor Planet Mailing List |date=October 17, 2017 |access-date=2024-01-26}}</ref> {{As of|2024|1}}, the risk of the 2095 impact was put at 1 in 10, still the highest, with a Palermo Scale rating of −2.98.<ref name="Current_Impact_Risks"/>

=== Projects to minimize the threat ===
{{main|Astronomy:Asteroid impact avoidance}}

{{multiple image
 |direction = vertical
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 |image1    = NEA by survey.png
 |image2    = NEA 1 km or more.png
 |footer    = Annual NEA discoveries by survey: all NEAs ''(top)'' and NEAs &gt; 1&nbsp;km ''(bottom)''
}}
[[File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|thumb|300px|left|{{center|NEOWISE – first four years of data starting in December 2013, with gren dots showing NEAs (animated; April 20, 2018)}}]]
The first astronomical program dedicated to the discovery of near-Earth asteroids was the [[Astronomy:Palomar Planet-Crossing Asteroid Survey|Palomar Planet-Crossing Asteroid Survey]]. The link to impact hazard, the need for dedicated survey telescopes and options to head off an eventual impact were first discussed at a 1981 interdisciplinary conference in Snowmass, Colorado.<ref name="Chapman1998"/> Plans for a more comprehensive survey, named the Spaceguard Survey, were developed by NASA from 1992, under a mandate from the United States Congress.<ref name="Vulcano1995"/><ref name="spaceguard-1998" /> To promote the survey on an international level, the International Astronomical Union (IAU) organised a workshop at [[Earth:Vulcano|Vulcano]], Italy in 1995,<ref name="Vulcano1995">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Vulcano Workshop. Beginning the Spaceguard Survey |location=Vulcano, Italy |publisher=IAU |date=September 1995 |url=http://spaceguard.rm.iasf.cnr.it/SGF/Vulcano/booklet.ps |access-date=2018-03-13 |url-status=dead |archive-url=https://web.archive.org/web/20131031042442/http://spaceguard.rm.iasf.cnr.it/SGF/Vulcano/booklet.ps |archive-date=2013-10-31}}</ref> and set up the Spaceguard Foundation also in Italy a year later.<ref name="spaceguard-2004" /> In 1998, the United States Congress gave NASA a mandate to detect 90% of near-earth asteroids over {{convert|1|km|mi|abbr=on}} diameter (that threaten global devastation) by 2008.<ref name="spaceguard-1998">{{cite web |author=Clark R. Chapman |title=Statement on The Threat of Impact by Near-Earth Asteroids before the Subcommittee on Space and Aeronautics of the Committee on Science of the U.S. House of Representatives at its hearings on "Asteroids: Perils and Opportunities" |publisher=Southwest Research Institute |date=May 21, 1998 |url=http://www.boulder.swri.edu/clark/hr.html |access-date=2024-01-26}}</ref><ref name="shigad" />

Several surveys have undertaken "[[Astronomy:Spaceguard|Spaceguard]]" activities (an umbrella term), including [[Astronomy:Lincoln Near-Earth Asteroid Research|Lincoln Near-Earth Asteroid Research]] (LINEAR), [[Astronomy:Spacewatch|Spacewatch]], [[Organization:Near-Earth Asteroid Tracking|Near-Earth Asteroid Tracking]] (NEAT), [[Astronomy:Lowell Observatory Near-Earth-Object Search|Lowell Observatory Near-Earth-Object Search]] (LONEOS), [[Astronomy:Catalina Sky Survey|Catalina Sky Survey]] (CSS), [[Astronomy:Campo Imperatore Near-Earth Object Survey|Campo Imperatore Near-Earth Object Survey]] (CINEOS), Japanese Spaceguard Association, [[Astronomy:Asiago-DLR Asteroid Survey|Asiago-DLR Asteroid Survey]] (ADAS) and [[Astronomy:Wide-field Infrared Survey Explorer#NEOWISE (pre-hibernation)|Near-Earth Object WISE]] (NEOWISE). As a result, the ratio of the known and the estimated total number of near-Earth asteroids larger than 1&nbsp;km in diameter rose from about 20% in 1998 to 65% in 2004,<ref name="spaceguard-2004">{{cite web |title=NASA on the Prowl for Near-Earth Objects |publisher=NASA/JPL |date=May 26, 2004 |url=https://www.nasa.gov/vision/universe/watchtheskies/near_earth052104.html |access-date=2018-03-06 |url-status=dead |archive-url=https://web.archive.org/web/20220813164957/https://www.nasa.gov/vision/universe/watchtheskies/near_earth052104.html |archive-date=2022-08-13}}</ref> 80% in 2006,<ref name="shigad">{{cite news |last=Shiga |first=David |title=New telescope will hunt dangerous asteroids |work=New Scientist |date=June 27, 2006 |url=https://www.newscientist.com/article/dn9403 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20150626133029/http://www.newscientist.com/article/dn9403 |archive-date=2015-06-26 }}</ref> and 93% in 2011. The original Spaceguard goal has thus been met, only three years late.<ref name="pia14734"/><ref name="WISE-asteroid-census"/> {{As of|2024|1}}, 861 NEAs larger than 1&nbsp;km have been discovered.<ref name="neo-jpl-stats" />

In 2005, the original USA Spaceguard mandate was extended by the George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90% of NEOs with diameters of {{convert|140|m|ft|abbr=on}} or greater, by 2020.<ref name="law-109-155">{{cite web |url=http://www.gpo.gov/fdsys/pkg/PLAW-109publ155/pdf/PLAW-109publ155.pdf |title=Public Law 109–155–DEC.30, 2005 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20171201223922/https://www.gpo.gov/fdsys/pkg/PLAW-109publ155/pdf/PLAW-109publ155.pdf |archive-date=2017-12-01 }}</ref> In January 2020, it was estimated that less than half of these have been found, but objects of this size hit the earth only about once in 2000 years.<ref>{{cite news |first=Leah |last=Crane |title=Inside the mission to stop killer asteroids from smashing into Earth |work=New Scientist |date=Jan 22, 2020 |url=https://www.newscientist.com/article/mg24532661-800-inside-the-mission-to-stop-killer-asteroids-from-smashing-into-earth/ |access-date=2024-01-24}} See especially [https://images.newscientist.com/wp-content/uploads/2020/01/22111519/g_asteroids_realweb2.jpg?width=800 this figure].</ref> In December 2023, the ratio of discovered NEOs with diameters of {{convert|140|m|ft|abbr=on}} or greater was estimated at 38%.<ref name="Grav2023"/> The Chile-based [[Astronomy:Vera C. Rubin Observatory|Vera C. Rubin Observatory]], which will survey the southern sky for transient events from 2025, is expected to increase the number of known asteroids by a factor of 10 to 100 and increase the ratio of known NEOs with diameters of {{convert|140|m|ft|abbr=on}} or greater to at least 60%,<ref>{{cite web |title=Science Goals. What's in our Solar System? |publisher=Vera C. Rubin Observatory |url=https://rubinobservatory.org/explore/science-goals/solar-system |access-date=2024-01-24}}</ref><ref name="Spacecom240123"/> while the [[Astronomy:NEO Surveyor|NEO Surveyor]] satellite, to be launched in 2027, is expected to push the ratio to 76%.<ref name="Grav2023">{{cite journal |last1=Grav |first1=Tommy |last2=Mainzer |first2=Amy K. |title=The NEO Surveyor Near-Earth Asteroid Known Object Model |journal=[[Astronomy:The Planetary Science Journal|The Planetary Science Journal]] |volume=4 |issue=12 |at=part 228 |date=December 5, 2023 |doi=10.3847/PSJ/ad072e|doi-access=free |arxiv=2310.20149 |bibcode=2023PSJ.....4..228G }}</ref>

In January 2016, NASA announced the creation of the [[Astronomy:Planetary Defense Coordination Office|Planetary Defense Coordination Office]] (PDCO) to track NEOs larger than about {{convert|30–50|m|ft|abbr=on}} in diameter and coordinate an effective threat response and mitigation effort.<ref name="ETech">{{cite news |title=NASA is opening a new office for planetary defense |url=http://www.extremetech.com/extreme/220745-nasa-is-opening-a-new-office-for-planetary-defense |author=Graham Templeton |date=January 12, 2016 |access-date=2024-01-26 |work=[[ExtremeTech]] |url-status=live |archive-url=https://web.archive.org/web/20170706223602/https://www.extremetech.com/extreme/220745-nasa-is-opening-a-new-office-for-planetary-defense |archive-date=2017-07-06 }}</ref><ref>{{cite web |title=Planetary Defense Coordination Office |publisher=NASA |url=https://science.nasa.gov/planetary-defense-overview |access-date=2024-01-25 }}</ref>

Survey programs aim to identify threats years in advance, giving humanity time to prepare a space mission to avert the threat.

{{blockquote|REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ...<br /> DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ...|author=Rep. Chris Stewart (R, UT) and Dr. Michael F. A'Hearn, April 10, 2013 |source=United States Congress<ref name="US-Congress-20130410">{{cite web |author=U.S.Congress |title=Threats From Space: a Review of U.S. Government Efforts to Track and mitigate Asteroids and Meteors (Part I and Part II) – Hearing Before the Committee on Science, Space, and Technology House of Representatives One Hundred Thirteenth Congress First Session |url=http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |date=March 19, 2013 |page=147 |publisher=United States Congress |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20170310212940/https://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf |archive-date=2017-03-10 }}</ref>}}

The [[Astronomy:Asteroid Terrestrial-impact Last Alert System|ATLAS]] project, by contrast, aims to find impacting asteroids shortly before impact, much too late for deflection maneuvers but still in time to evacuate and otherwise prepare the affected Earth region.<ref>{{cite news |author=University of Hawaii at Manoa's Institute for Astronomy |title=ATLAS: The Asteroid Terrestrial-impact Last Alert System |date=February 18, 2013 |url=https://www.astronomy.com/science/atlas-the-asteroid-terrestrial-impact-last-alert-system/ |work=[[Astronomy:Astronomy (magazine)|Astronomy]] |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20230604015435/https://www.astronomy.com/science/atlas-the-asteroid-terrestrial-impact-last-alert-system/ |archive-date=2023-06-04 }}</ref> Another project, the [[Organization:Zwicky Transient Facility|Zwicky Transient Facility]] (ZTF), which surveys for objects that change their brightness rapidly,<ref name="AT-20180207">{{cite news |last=Kulkarni |first= S.R. |display-authors=0 |title=The Zwicky Transient Facility (ZTF) begins |url=http://www.astronomerstelegram.org/?read=11266 |date=February 7, 2018 |work=[[Astronomy:The Astronomer's Telegram|The Astronomer's Telegram]] |number=11266 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20180209062936/http://www.astronomerstelegram.org/?read=11266 |archive-date=2018-02-09 }}</ref> also detects asteroids passing close to Earth.<ref name="AT-20180208">{{cite news |last=Ye |first= Quan-Zhi|display-authors=0|title=First Discovery of a Small Near Earth Asteroid with ZTF (2018 CL) |url=http://www.astronomerstelegram.org/?read=11274 |date=February 8, 2018 |work=The Astronomer's Telegram |number=11274 |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20180209002703/http://www.astronomerstelegram.org/?read=11274 |archive-date=2018-02-09 }}</ref>

{{further|Astronomy:List of near-Earth object observation projects}}

Scientists involved in NEO research have also considered options for actively averting the threat if an object is found to be on a collision course with Earth.<ref name="Chapman1998"/> All viable methods aim to deflect rather than destroy the threatening NEO, because the fragments would still cause widespread destruction.<ref name="TaskForceReport"/> Deflection, which means a change in the object's orbit months to years prior to the [[Astronomy:Asteroid impact prediction|predicted impact]], also requires orders of magnitude less energy.<ref name="TaskForceReport"/>

== Number and classification ==
<!---[[File:NearEarthAsteroids-Discovered-19980701-20180525.gif|thumb|Near-Earth asteroids discovered by size, 1998–2018]]-- GIF too large to animate? sometimes gifs at non-native size don't run--->
[[File:Known NEAs.png|thumb|Cumulative discoveries of near-Earth asteroids known by size, 1980–{{#time:Y}}|400px]]
Near-Earth objects are classified as [[Astronomy:Meteoroid|meteoroid]]s, [[Astronomy:Asteroid|asteroid]]s, or [[Astronomy:Comet|comet]]s depending on size, composition, and orbit. Those which are asteroids can additionally be members of an [[Astronomy:Asteroid family|asteroid family]], and comets create meteoroid streams that can generate [[Astronomy:Meteor shower|meteor shower]]s.

{{As of|2024|1|20|df=US}} and according to statistics maintained by CNEOS, 34,266 NEOs have been discovered. Only 123 (0.36%) of them are comets, whilst 32,957 (99.64%) are asteroids. 2,396 of those NEOs are classified as potentially hazardous asteroids (PHAs).<ref name="neo-jpl-stats" />

{{As of|2024|1|24|df=US}}, 1,704 NEAs appear on the [[Astronomy:Sentry (monitoring system)|Sentry impact risk page]] at the [[Organization:NASA|NASA]] website.<ref name="Current_Impact_Risks"/> All but 105 of these NEAs are less than 50 meters in diameter and none of the listed objects are placed even in the "green zone" (Torino Scale 1), meaning that none warrant the attention of the general public.<ref name="torino"/>

=== Observational biases ===
The main problem with estimating the number of NEOs is that the probability of detecting one is influenced by a number of  aspects of the NEO, starting naturally with its size but also including the characteristics of its orbit and the reflectivity of its surface.<ref name="science.sciencemag.org">{{Cite journal |last1=Bottke |first1=W. F. Jr. |title=Understanding the Distribution of Near-Earth Asteroids |journal=Science |volume=288 |issue=5474 |pages=2190–2194 |year=2000 |pmid=10864864 |doi=10.1126/science.288.5474.2190 |bibcode=2000Sci...288.2190B}}</ref>  What is easily detected will be more counted, and these observational biases need to be compensated when trying to calculate the number of bodies in a population from the list of its detected members.<ref name="science.sciencemag.org"/>
[[File:Artist%E2%80%99s_impression_of_an_asteroid_that_orbits_closer_to_the_Sun_than_Earth%E2%80%99s_orbit.jpg|thumb|right|Artist's impression of an asteroid that orbits closer to the Sun than Earth's orbit]]

Bigger asteroids reflect more light, and the two biggest Near-Earth objects, [[Astronomy:433 Eros|433 Eros]] and [[Astronomy:1036 Ganymed|1036 Ganymed]], were naturally also among the first to be detected.<ref name="Browne">{{Cite news |url=https://www.nytimes.com/1996/04/25/us/mathematicians-say-asteroid-may-hit-earth-in-a-million-years.html  |title=Mathematicians Say Asteroid May Hit Earth in a Million Years |last=Browne |first=Malcolm W. |work=The New York Times |date=April 25, 1996 |access-date=2024-01-26 |language=en}}</ref> 1036 Ganymed is about {{convert|35|km|mi|abbr=on}} in diameter and 433 Eros is about {{convert|17|km|mi|abbr=on}} in diameter.<ref name="Browne"/> Meanwhile, the apparent brightness of objects that are closer is higher, introducing a bias that favours the discovery of NEOs of a given size that get closer to Earth.<ref name="LuuJewitt1989"/>

Earth-based astronomy requires dark skies and hence nighttime observations, and even space-based telescopes avoid looking into directions close to the Sun, thus most NEO surveys are blind towards objects passing Earth on the side of the Sun.<ref name="LuuJewitt1989"/><ref name="NEOS-orbit"/> This bias is further enhanced by the effect of [[Astronomy:Planetary phase|phase]]: the narrower the angle of the asteroid and the Sun from the observer, the lesser part of the observed side of the asteroid will be illuminated.<ref name="LuuJewitt1989"/> Another bias results from the different surface brightness or albedo of the objects, which can make a large but low-albedo object as bright as a small but high-albedo object.<ref name="LuuJewitt1989"/><ref name="NEOS-why-infrared"/> In addition, the reflexivity of asteroid surfaces is not uniform but increases towards the direction opposite of illumination, resulting in the phenomenon of phase darkening, which makes asteroids even brighter when the Earth is close to the axis of sunlight.<ref name="LuuJewitt1989"/> An asteroid's observed albedo usually has a strong peak or [[Physics:Opposition surge|opposition surge]] very close to the direction opposite of the Sun.<ref name="LuuJewitt1989"/> Different surfaces display different levels of phase darkening, and research showed that, on top of albedo bias, this favours the discovery of silicon-rich [[Astronomy:S-type asteroid|S-type asteroids]] over carbon-rich [[Astronomy:C-type asteroid|C types]], for example.<ref name="LuuJewitt1989">{{cite journal |first1=Jane |last1=Luu |first2=David |last2=Jewitt |title=On the Relative Numbers of C Types and S Types among Near-Earth Asteroids |journal=The Astronomical Journal |volume=98 |issue=5 |pages=1905–1911 |date=November 1989 |url=https://articles.adsabs.harvard.edu/pdf/1989AJ.....98.1905L |access-date=2024-01-26 |doi=10.1086/115267 |bibcode=1989AJ.....98.1905L}}</ref> As a result of these observational biases, in Earth-based surveys, NEOs tended to be discovered when they were in opposition, that is, opposite from the Sun when viewed from the Earth.<ref name="Grav2023"/>

The most practical way around many of these biases is to use thermal infrared telescopes in space that observe their thermal emissions instead of the light they reflect, with a sensitivity that is almost independent of the illumination.<ref name="Grav2023"/><ref name="NEOS-why-infrared">{{cite web |title=Why Infrared? |url=https://neos.arizona.edu/mission/why-infrared |publisher=[[Organization:Lunar and Planetary Laboratory|UA LPL]] |access-date=2024-01-25 }}</ref> In addition, space-based telescopes in an orbit around the Sun in the shadow of the Earth can make observations as close as 45 degrees to the direction of the Sun.<ref name="NEOS-orbit">{{cite web |title=Mission Orbit and Timeline |url=https://neos.arizona.edu/mission/orbit |publisher=UA LPL |access-date=2024-01-26 }}</ref>

Further observational biases favour objects that have more frequent encounters with the Earth, which makes the detection of [[Astronomy:Aten asteroid|Atens]] more likely than that of [[Astronomy:Apollo asteroid|Apollos]]; and objects that move slower when encountering the Earth, which makes the detection of NEAs with low eccentricities more likely.<ref>{{cite journal |first1=William F. Jr. |last1=Bottke |first2=Michale C. |last2=Nolan |first3= H. Jay |last3=Melosh |first4=Ann M. |last4=Vickery |first5=Richard |last5=Greenberg |title=Origin of the Spacewatch Small Earth-Approaching Asteroids |journal=Icarus |date=August 1996 |volume=122 |issue=2 |pages=406–427 |url=https://www.boulder.swri.edu/~bottke/Reprints/Bottke_1996_Icarus_122_406_Origin_Spacewatch_NEOs.pdf |access-date=2024-01-25 |doi=10.1006/icar.1996.0133 |bibcode=1996Icar..122..406B }}</ref>

Such observational biases must be identified and quantified to determine NEO populations, as studies of asteroid populations then take those known observational selection biases into account to make a more accurate assessment.<ref>{{cite journal|author1=B. Zellner|author2=E. Bowell |title=2. Asteroid Compositional Types and their Distributions |journal=International Astronomical Union Colloquium|year=1977|volume=39 |pages=185–197 |doi=10.1017/S0252921100070093|s2cid=128650102 |doi-access=free}}</ref> In the year 2000 and taking into account all known observational biases, it was estimated that there are approximately 900 near-Earth asteroids of at least kilometer size, or technically and more accurately, with an [[Astronomy:Absolute magnitude#Solar System bodies (H)|absolute magnitude]] brighter than 17.75.<ref name="science.sciencemag.org"/>

=== Near-Earth asteroids (NEAs){{anchor|Near-Earth_asteroids}}<!--'Near-Earth asteroid' and 'Near-Earth asteroids' redirect here--> ===
[[File:ESO-Asteroid Toutatis-phot-28c-04-normal.jpg|thumb|Asteroid Toutatis from Paranal]]

These are asteroids in a near-Earth orbit without the tail or coma of a comet. {{As of|2024|1}}, 34,143 '''near-Earth asteroids'''<!--boldface per WP:R#PLA--> are known, 2,396 of which are both sufficiently large and may come sufficiently close to Earth to be classified as potentially hazardous.<ref name="neo-jpl-stats" />

NEAs survive in their orbits for just a few million years.<ref name="MorbidelliAstIII">{{cite journal |url=http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |title=Origin and Evolution of Near-Earth Objects |first1=Alessandro |last1=Morbidelli |first2=William F. Jr. |last2=Bottke |first3=Christiane |last3=Froeschlé |first4=Patrick |last4=Michel |journal=Asteroids III |editor=W. F. Bottke Jr. |editor2=A. Cellino |editor3=P. Paolicchi |editor4=R. P. Binzel |pages=409–422 |date=January 2002 |doi=10.2307/j.ctv1v7zdn4.33 |bibcode=2002aste.book..409M |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20170809014123/http://www.boulder.swri.edu/%7Ebottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |archive-date=2017-08-09 }}</ref> They are eventually eliminated by planetary [[Astronomy:Perturbation|perturbations]], causing ejection from the Solar System or a [[Astronomy:Impact event|collision]] with the Sun, a planet, or other celestial body.<ref name="MorbidelliAstIII"/> With orbital lifetimes short compared to the age of the Solar System, new asteroids must be constantly moved into near-Earth orbits to explain the observed asteroids. The accepted origin of these asteroids is that [[Astronomy:Asteroid belt|main-belt asteroids]] are moved into the inner Solar System through orbital resonances with [[Astronomy:Jupiter|Jupiter]].<ref name="MorbidelliAstIII"/> The interaction with Jupiter through the resonance [[Astronomy:Perturbation|perturb]]s the asteroid's orbit and it comes into the inner Solar System. The asteroid belt has gaps, known as [[Astronomy:Kirkwood gap|Kirkwood gap]]s, where these resonances occur as the asteroids in these resonances have been moved onto other orbits. New asteroids migrate into these resonances, due to the [[Yarkovsky effect]] that provides a continuing supply of near-Earth asteroids.<ref>{{cite journal |title=The Yarkovsky-driven origin of near-Earth asteroids |author=A. Morbidelli |author2=D. Vokrouhlický |journal=Icarus |volume=163 |issue=1 |pages=120–134 |date=May 2003 |doi=10.1016/S0019-1035(03)00047-2 |bibcode=2003Icar..163..120M |citeseerx=10.1.1.603.7624}}</ref> Compared to the entire mass of the asteroid belt, the mass loss necessary to sustain the NEA population is relatively small; totalling less than 6% over the past 3.5 billion years.<ref name="MorbidelliAstIII"/> The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of [[Astronomy:Asteroid spectral types|asteroid spectral types]].<ref>{{cite journal |title=On the Origins of Earth-Approaching Asteroids |author=D.F. Lupishko |author2=T.A. Lupishko |name-list-style=amp |journal=[[Astronomy:Solar System Research|Solar System Research]] |volume=35 |issue=3 |pages=227–233 |date=May 2001 |doi=10.1023/A:1010431023010 |bibcode = 2001SoSyR..35..227L|s2cid=117912062 }}</ref>

A small number of NEAs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with [[Astronomy:Jupiter|Jupiter]].<ref name = "MorbidelliAstIII" /><ref>{{cite journal |title=What the physical properties of near-Earth asteroids tell us about sources of their origin? |author=D.F. Lupishko |author2=M. di Martino |author3=T.A. Lupishko |name-list-style=amp |journal=Kinematika I Fizika Nebesnykh Tel Supplimen |volume=3 |issue=3 |pages=213–216 |date=September 2000 |bibcode=2000KFNTS...3..213L}}</ref>

Many asteroids have [[Astronomy:Natural satellite|natural satellite]]s ([[Astronomy:Minor-planet moon|minor-planet moon]]s). {{As of|2024|01|df=US}}, 97 NEAs were known to have at least one moon, including three known to have two moons.<ref>{{cite web |title=Asteroids with Satellites |publisher=Johnston's Archive |url=http://www.johnstonsarchive.net/astro/asteroidmoons.html |access-date=2024-01-24}}</ref> The asteroid [[Astronomy:3122 Florence|3122 Florence]], one of the largest PHAs<ref name="MPC-PHA-list"/> with a diameter of {{convert|4.5|km|mi|abbr=on}}, has two moons measuring {{convert|100–300|m|ft|abbr=on}} across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.<ref name="Florence-moons">{{cite news |author1=Lance Benner |author2=Shantanu Naidu |author3=Marina Brozovic |author4=Paul Chodas |title=Radar Reveals Two Moons Orbiting Asteroid Florence |work=News |publisher=NASA/JPL CNEOS |date=September 1, 2017 |url=https://cneos.jpl.nasa.gov/news/news199.html |access-date=2024-01-26 |url-status=live |archive-url=https://web.archive.org/web/20170903060914/https://cneos.jpl.nasa.gov/news/news199.html |archive-date=2017-09-03 }}</ref>

In May 2022, an algorithm known as Tracklet-less Heliocentric Orbit Recovery or THOR and developed by University of Washington researchers to discover asteroids in the solar system was announced as a success.<ref>{{Cite news |title=UW-developed, cloud-based astrodynamics platform to discover and track asteroids |date=May 31, 2022 |publisher=University of Washington |work=UW News |url=https://www.washington.edu/news/2022/05/31/asteroid-discovery/ |access-date=2024-01-26}}</ref> The International Astronomical Union's Minor Planet Center confirmed a series of first candidate asteroids identified by the algorithm.<ref>{{Cite press release |title=Asteroid Institute Uses Revolutionary Cloud-Based Astrodynamics Platform to Discover and Track Asteroids |date=May 31, 2022 |publisher=B612 Foundation |work=PR Newswire |url=https://www.prnewswire.com/news-releases/asteroid-institute-uses-revolutionary-cloud-based-astrodynamics-platform-to-discover-and-track-asteroids-301557487.html |access-date=2024-01-26}}</ref>

==== Size distribution ====
[[File:NEA by size.png|400px|thumb|Known near-Earth asteroids by size]]

While the size of a very small fraction of these asteroids is known to better than 1%, from [[Engineering:Radar|radar]] observations, from images of the asteroid surface, or from [[Astronomy:Occultation#Occultations by asteroids|stellar occultations]], the diameter of the vast majority of near Earth asteroids has only been estimated on the basis of their brightness and a representative asteroid surface reflectivity or [[Earth:Albedo|albedo]], which is commonly assumed to be 14%.<ref name="neo-jpl-intro">{{cite web |title=Discovery Statistics. Introduction |publisher=NASA/JPL CNEOS |url=https://cneos.jpl.nasa.gov/stats/ |date=2012 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240126214444/https://cneos.jpl.nasa.gov/stats/ |archive-date=2024-01-26 }}</ref> Such indirect size estimates are uncertain by over a factor of 2 for individual asteroids, since asteroid albedos can range at least as low as 5% and as high as 30%. This makes the volume of those asteroids uncertain by a factor of 8, and their mass by at least as much, since their assumed density also has its own uncertainty. Using this crude method, an [[Astronomy:Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of 17.75 roughly corresponds to a diameter of {{convert|1|km|mi|abbr=on|lk=off}}<ref name="neo-jpl-intro"/> and an absolute magnitude of 22.0 to a diameter of {{convert|140|m|ft|abbr=on|lk=off}}.<ref name="CNEOS-NEO-groups"/>  Diameters of intermediate precision, better than from an assumed albedo but not nearly as precise as good direct measurements, can be obtained from the combination of reflected light and thermal infrared emission, using a thermal model of the asteroid to estimate both its diameter and its albedo. In May 2016, technologist Nathan Myhrvold questioned the precision of such asteroid diameter estimates arising from thermal modeling of measurements by the [[Astronomy:Wide-field Infrared Survey Explorer|Wide-field Infrared Survey Explorer]] and NEOWISE missions.<ref name="NYT-20160523">{{cite news |last=Chang |first=Kenneth |title=How Big Are Those Killer Asteroids? A Critic Says NASA Doesn't Know. |url=https://www.nytimes.com/2016/05/24/science/asteroids-nathan-myhrvold-nasa.html |date=May 23, 2016 |work=The New York Times |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170828235110/https://www.nytimes.com/2016/05/24/science/asteroids-nathan-myhrvold-nasa.html |archive-date=2017-08-28 }}</ref><ref name="ARX-20160523">{{cite journal |last=Myhrvold |first=Nathan |title=Asteroid thermal modeling in the presence of reflected sunlight with an application to WISE/NEOWISE observational data |journal=Icarus |volume=303 |pages=91–113 |date=May 23, 2016 |arxiv=1605.06490 |doi=10.1016/j.icarus.2017.12.024 |bibcode=2018Icar..303...91M |s2cid=118511665 }}</ref><ref name="SA-20160527">{{cite news |last=Billings |first=Lee |title=For Asteroid-Hunting Astronomers, Nathan Myhrvold Says the Sky Is Falling |url=http://www.scientificamerican.com/article/for-asteroid-hunting-astronomers-nathan-myhrvold-says-the-sky-is-falling1/ |date=May 27, 2016 |work=Scientific American |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170829043702/https://www.scientificamerican.com/article/for-asteroid-hunting-astronomers-nathan-myhrvold-says-the-sky-is-falling1/ |archive-date=2017-08-29 }}</ref> The original version of his criticism itself faced criticism for its methodology<ref>{{cite news |url=http://www.slate.com/blogs/bad_astronomy/2016/05/27/nathan_myhrvold_claims_nasa_scientists_asteroid_calculations_are_all_wrong.html |first=Phil |last=Plait |title=A Physics Outsider Says NASA Asteroid Scientists Are All Wrong. Is He Right? (Spoiler: No) |date=May 27, 2016 |work=Slate |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170814110232/http://www.slate.com/blogs/bad_astronomy/2016/05/27/nathan_myhrvold_claims_nasa_scientists_asteroid_calculations_are_all_wrong.html |archive-date=2017-08-14 }}</ref> and did not pass peer review,<ref name="ARX-20160523"/><ref name="NASA-20160525">{{cite news |author=NASA Content Administrator |title=NASA Response to Recent Paper on NEOWISE Asteroid Size Results |url=https://www.jpl.nasa.gov/news/nasa-response-to-recent-paper-on-neowise-asteroid-size-results |date=May 25, 2016 |work=News |publisher=NASA/JPL |access-date=2017-11-10 |url-status=live |archive-url=https://web.archive.org/web/20210714161518/https://www.jpl.nasa.gov/news/nasa-response-to-recent-paper-on-neowise-asteroid-size-results |archive-date=2021-07-14 }}</ref> but a revised version was subsequently published.<ref name="ICARUS-220180522">{{cite journal |last=Myhrvold |first=Nathan |author-link=Nathan Myhrvold |title=An empirical examination of WISE/NEOWISE asteroid analysis and results |date=May 22, 2018 |journal=Icarus |volume=314 |pages=64–97 |doi=10.1016/j.icarus.2018.05.004 |bibcode=2018Icar..314...64M |doi-access=free }}</ref><ref name="NYT-20180614">{{cite news |last=Chang |first=Kenneth |title=Asteroids and Adversaries: Challenging What NASA Knows About Space Rocks - Two years ago, NASA dismissed and mocked an amateur's criticisms of its asteroids database. Now Nathan Myhrvold is back, and his papers have passed peer review. |url=https://www.nytimes.com/2018/06/14/science/asteroids-nasa-nathan-myhrvold.html |date=June 14, 2018 |work=The New York Times |access-date=2024-01-27 }}</ref>

In 2000, NASA reduced from 1,000–2,000 to 500–1,000 its estimate of the number of existing near-Earth asteroids over one kilometer in diameter, or more exactly brighter than an absolute magnitude of 17.75.<ref>{{cite news |url=https://www.jpl.nasa.gov/news/asteroid-population-count-slashed |title=Asteroid Population Count Slashed |date=January 12, 2000 |author=Jane Platt |work=Press Releases |publisher=NASA/JPL |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240127123244/https://www.jpl.nasa.gov/news/asteroid-population-count-slashed |archive-date=2024-01-27 }}</ref><ref name="Rabinowitzetal00">{{cite journal |title=A reduced estimate of the number of kilometer-sized near-Earth asteroids |journal=Nature |author=David Rabinowitz |author2=Eleanor Helin |author3=Kenneth Lawrence |author4=Steven Pravdo |name-list-style=amp |date=January 13, 2000 |volume=403 |pages=165–166 |doi=10.1038/35003128 |pmid=10646594|issue = 6766 |bibcode=2000Natur.403..165R|s2cid=4303533 }}</ref> Shortly thereafter,  the LINEAR survey provided an alternative estimate  of {{val|1227|+170|-90|fmt=commas}}.<ref name="LINEAR-asteroid-census">{{cite journal |title=A Near-Earth Asteroid Population Estimate from the LINEAR Survey |journal=Science |author=J. S. Stuart |date=November 23, 2001 |volume=294 |issue=5547 |pages=1691–1693 |doi=10.1126/science.1065318 |pmid=11721048 |bibcode=2001Sci...294.1691S |s2cid=37849062 }}</ref> In 2011, on the basis of NEOWISE observations, the estimated number of one-kilometer NEAs was narrowed to {{val|981|19}} (of which 93% had been discovered at the time), while the number of NEAs larger than 140 meters across was estimated at {{val|13200|1900|fmt=commas}}.<ref name="pia14734"/><ref name="WISE-asteroid-census">{{cite journal |title=NEOWISE Observations of Near-Earth Objects: Preliminary Results |display-authors=3|journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |author=A. Mainzer |author2=T. Grav |author3=J. Bauer |author4=J. Masiero |author5=R. S. McMillan |author6=R. M. Cutri |author7=R. Walker |author8=E. Wright |author9=P. Eisenhardt |author10=D. J. Tholen |author11=T. Spahr |author12=R. Jedicke |author13=L. Denneau |author14=E. DeBaun |author15=D. Elsbury |author16=T. Gautier |author17=S. Gomillion |author18=E. Hand |author19=W. Mo |author20=J. Watkins |author21=A. Wilkins |author22=G. L. Bryngelson |author23=A. Del Pino Molina |author24=S. Desai |author25=M. Go'mez Camus |author26=S. L. Hidalgo |author27=I. Konstantopoulos |author28=J. A. Larsen |author29=C. Maleszewski |author30=M. A. Malkan |author31=J.-C. Mauduit |author32=B. L. Mullan |author33=E. W. Olszewski |author34=J. Pforr |author35=A. Saro |author36=J. V. Scotti |author37=L. H. Wasserman |date=December 20, 2011 |volume=743 |issue=2 |page=156 |arxiv = 1109.6400 |bibcode = 2011ApJ...743..156M |doi = 10.1088/0004-637X/743/2/156 |s2cid=239991}}</ref> The NEOWISE estimate differed from other estimates primarily in assuming a slightly lower average asteroid albedo, which produces larger estimated diameters for the same asteroid brightness. This resulted in 911 then known asteroids at least 1&nbsp;km across, as opposed to the 830 then listed by CNEOS from the same inputs but assuming a slightly higher albedo.<ref>{{cite news |author=Kelly Beatty |title=WISE's Survey of Near-Earth Asteroids |work=Sky & Telescope |date=September 30, 2011 |url=http://www.skyandtelescope.com/astronomy-news/wises-survey-of-near-earth-asteroids/ |access-date=2024-01-27 }}</ref> In 2017,  two studies using an improved statistical method reduced the estimated number of NEAs brighter than absolute magnitude 17.75 (approximately over one kilometer in diameter) slightly to {{val|921|20}}.<ref name="NEA1km-est-2017">{{cite news |url=https://www.universetoday.com/137583/good-news-everyone-less-deadly-undiscovered-asteroids-thought/ |title=Good News Everyone! There are Fewer Deadly Undiscovered Asteroids than we Thought |author=Matt Williams |work=Universe Today |date=October 20, 2017 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20171104064609/https://www.universetoday.com/137583/good-news-everyone-less-deadly-undiscovered-asteroids-thought/ |archive-date=2017-11-04 }}</ref><ref name="Tricario"/> The estimated number of near-Earth asteroids brighter than absolute magnitude of 22.0 (approximately over 140&nbsp;m across) rose to {{val|27100|2200|fmt=commas}}, double the WISE estimate, of which about a fourth were known at the time.<ref name="Tricario">{{cite journal |last=Tricarico |first=Pasquale |title=The near-Earth asteroid population from two decades of observations |journal=Icarus |volume=284 |pages=416–423 |date=March 1, 2017 |url=http://orbit.psi.edu/~tricaric/pdf/Tricarico_NEA_population_Icarus_2017.pdf |access-date=2018-03-09 |archive-date=2018-03-10 |archive-url=https://web.archive.org/web/20180310073945/http://orbit.psi.edu/~tricaric/pdf/Tricarico_NEA_population_Icarus_2017.pdf |url-status=dead |doi=10.1016/j.icarus.2016.12.008 |arxiv=1604.06328 |bibcode=2017Icar..284..416T |s2cid=85440139 }}</ref> The number of asteroids brighter than {{nowrap|H {{=}} 25}}, which corresponds to about {{convert|40|m|ft|abbr=on}} in diameter, is estimated at {{val|840000|23000|fmt=commas}}—of which about 1.3 percent had been discovered by February 2016; the number of asteroids brighter than {{nowrap|H {{=}} 30}} (larger than {{convert|3.5|m|ft|abbr=on}}) is estimated at {{val|400|100}} million—of which about 0.003 percent had been discovered by February 2016.<ref name="Tricario"/>

{{As of|2024|1|20|df=US}}, and using diameters mostly estimated crudely from a measured absolute magnitude and an assumed albedo, 861 NEAs listed by CNEOS, including 153 PHAs, measure at least 1&nbsp;km in diameter, and 10,767 known NEAs, including 2,396 PHAs, are larger than 140&nbsp;m in diameter.<ref name="neo-jpl-stats" />

The smallest known near-Earth asteroid is {{mp|2022 WJ|1}} with an absolute magnitude of 33.58,<ref name="JPL-SSD-NEA"/> corresponding to an estimated diameter of about {{convert|0.7|m|ft|abbr=on}}.<ref name="h" /> The largest such object is [[Astronomy:1036 Ganymed|1036 Ganymed]],<ref name="JPL-SSD-NEA"/> with an absolute magnitude of 9.26 and directly measured irregular dimensions which are equivalent to a diameter of about {{convert|38|km|mi|abbr=on}}.<ref>{{cite web |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1036 |title=1036 Ganymed (A924 UB) |publisher=NASA/JPL |date=January 23, 2024 |access-date=2024-01-25}}</ref>

==== Orbital classification ====
[[File:Neo orbit types.jpg|thumb|upright=1.4|NEA orbital groups (NASA/JPL)]]
Near-Earth asteroids are divided into groups based on their semi-major axis (a), [[Astronomy:Apsis|perihelion]] distance (q), and [[Astronomy:Apsis|aphelion]] distance (Q):<ref name="CNEOS-NEO-groups"/><ref name="NEOCC-DA"/>
* The ''[[Astronomy:Atira asteroid|Atiras]]'' or ''Apoheles'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983&nbsp;AU). That is, {{nowrap|Q < 0.983 AU}}, which implies that the asteroid's semi-major axis is also less than 0.983 AU.<ref name="atiras">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=August 1, 2019 |title=Understanding the evolution of Atira-class asteroid 2019 AQ<sub>3</sub>, a major step towards the future discovery of the Vatira population |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume= 487 |issue= 2 |pages= 2742–2752 |arxiv=1905.08695 |bibcode=2019MNRAS.487.2742D |doi=10.1093/mnras/stz1437 |s2cid=160009327 }}</ref> This group includes asteroids on orbits that never get close to Earth, including the sub-group of [[Astronomy:Atira asteroid#ꞌAylóꞌchaxnim asteroids|ꞌAylóꞌchaxnims]], which orbit the Sun entirely within the orbit of Venus,<ref>{{cite journal |last1=Bolin |first1=Bryce T. |display-authors=et al |date=November 2022 |title=The discovery and characterization of (594913) 'Ayló'chaxnim, a kilometre sized asteroid inside the orbit of Venus |url=https://authors.library.caltech.edu/records/4fewd-rff56/files/slac089.pdf?download=1 |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=517 |issue=1 |pages=L49–L54 |doi=10.1093/mnrasl/slac089 |access-date=2024-01-25 |doi-access=free }}</ref> and which include the hypothetical sub-group of [[Astronomy:Vulcanoid|Vulcanoids]], which have orbits entirely within the orbit of [[Astronomy:Mercury (planet)|Mercury]].<ref>{{cite journal |last1=Beech |first1=Martin |last2=Peltier |first2=Lowell |title=The Vulcanoid Asteroids: Past, Present and Future |date=August 25, 2017 |journal=American Journal of Astronomy and Astrophysics |volume=5 |issue=3 |pages=28–41 |doi=10.11648/j.ajaa.20170503.12 |bibcode=2017AmJAA...5...28B |doi-access=free }}</ref>

* The ''[[Astronomy:Aten asteroid|Atens]]'' have a semi-major axis of less than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a < 1.0 AU}} and {{nowrap|Q > 0.983 AU}}. (0.983 AU is Earth's perihelion distance.)

* The ''[[Astronomy:Apollo asteroid|Apollos]]'' have a semi-major axis of more than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a > 1.0 AU}} and {{nowrap|q < 1.017 AU}}. (1.017&nbsp;AU is Earth's aphelion distance.)

* The ''[[Astronomy:Amor asteroid|Amors]]'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017&nbsp;AU). Amor asteroids are also near-earth objects so {{nowrap|q < 1.3 AU}}. In summary, {{nowrap|1.017 AU < q < 1.3 AU}}. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017&nbsp;AU.) Some Amor asteroid orbits cross the orbit of Mars.

(Note: Some authors define Atens differently: they define it as being all the asteroids with a semi-major axis of less than 1&nbsp;AU.<ref>{{cite web |title=Unusual Minor Planets |publisher=IAU/MPC |url=https://minorplanetcenter.net/iau/Unusual.html |access-date=2024-01-27}}</ref><ref name="galache"/> That is, they consider the Atiras to be part of the Atens.<ref name="galache">{{cite web |first=J. L. |last=Galache |title=Asteroid Classification I – Dynamics |publisher=IAU/MPC |date=March 5, 2011 |url=http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |access-date=2018-03-09 |url-status=dead |archive-url=https://web.archive.org/web/20160303235814/http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |archive-date=2016-03-03}}</ref> Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.)

Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.<ref name="MorbidelliAstIII"/><ref>{{Cite journal |last1=Ribeiro |first1=A. O. |last2=Roig |first2=F.|last3=De Prá |first3=M. N. |last4=Carvano |first4=J. M. |last5=DeSouza |first5=S. R. |date=March 17, 2016 |title=Dynamical study of the Atira group of asteroids |url=https://academic.oup.com/mnras/article-pdf/458/4/4471/8143283/stw642.pdf |access-date=2024-01-27 |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=4 |pages=4471–4476 |doi=10.1093/mnras/stw642 |issn=0035-8711 |doi-access=free}}</ref>

{{As of|2024|1|20|df=US}}, 34 Atiras, 2,692 Atens, 19,323 Apollos and 12,094 Amors have been discovered and cataloged.<ref name="neo-jpl-stats" />

==== Co-orbital asteroids ====
[[File:Lagrange Horseshoe Orbit.jpg|400px|thumb|The five Lagrangian points relative to Earth and possible orbits along gravitational contours]]

NEAs on a [[Astronomy:Co-orbital configuration|co-orbital configuration]] have the same orbital period as the Earth. All co-orbital asteroids have special orbits that are relatively stable and, paradoxically, can prevent them from getting close to Earth:
* ''Trojans'': Near the orbit of a planet, there are five gravitational equilibrium points, the [[Astronomy:Lagrangian point|Lagrangian point]]s, in which an asteroid would orbit the Sun in fixed formation with the planet. Two of these, 60 degrees ahead and behind the planet along its orbit (designated L4 and L5 respectively) are stable; that is, an asteroid near these points would stay there for millions of years even if lightly perturbed by other planets and by non-gravitational forces. {{As of|2022|02}}, Earth has two confirmed Trojans: {{mpl|2010 TK|7}} and {{mpl|2020 XL|5}}, both circling Earth's L4 point.<ref name="WISE">{{cite press release |url=https://www.prnewswire.com/news-releases/nasas-wise-mission-finds-first-trojan-asteroid-sharing-earths-orbit-126277963.html |title=NASA's WISE mission finds first Trojan asteroid sharing Earth's orbit |date=July 27, 2011 |work=PR Newswire |publisher=[[Organization:NASA|NASA]] |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240127131203/https://www.prnewswire.com/news-releases/nasas-wise-mission-finds-first-trojan-asteroid-sharing-earths-orbit-126277963.html |archive-date=2024-01-27 }}</ref><ref name="Trojan2">{{cite news |url=https://www.space.com/earth-extra-moon-trojan-asteroid-2020-xl5-discovery |title=Earth has an extra companion, a Trojan asteroid that will hang around for 4,000 years |date=February 1, 2022 |work=Space.com |access-date=2024-01-24}}</ref>
* ''[[Astronomy:Horseshoe orbit|Horseshoe librators]]'': The region of stability around L4 and L5 also includes orbits for co-orbital asteroids that run around both L4 and L5. Relative to the Earth and Sun, the orbit can resemble the circumference of a horseshoe, or may consist of annual loops that wander back and forth ([[Astronomy:Libration|librate]]) in a horseshoe-shaped area. In both cases, the Sun is at the horseshoe's center of gravity, Earth is in the gap of the horseshoe, and L4 and L5 are inside the ends of the horseshoe. By 2016, 12 horseshoe librators of Earth have been discovered.<ref name="Fuentes-horseshoe">{{cite journal |last1=de&nbsp;la&nbsp;Fuente Marcos |first1=C. |last2=de&nbsp;la&nbsp;Fuente Marcos |first2=R. |title=A trio of horseshoes: Past, present, and future dynamical evolution of Earth co-orbital asteroids {{mp|2015 XX|169}}, {{mpl|2015 YA}} and {{mp|2015 YQ|1}} |journal=[[Physics:Astrophysics and Space Science|Astrophysics and Space Science]] |volume=361 |issue=4 |pages=121–133 |date=April 2016 |doi=10.1007/s10509-016-2711-6 |arxiv=1603.02415 |bibcode=2016Ap&SS.361..121D |s2cid=119222384 }}</ref> The most-studied and, at about {{convert|5|km|mi|abbr=on}}, largest is [[Astronomy:3753 Cruithne|3753 Cruithne]], which travels along bean-shaped annual loops and completes its horseshoe libration cycle every 770–780&nbsp;years.<ref>{{cite journal |title=An asteroidal companion to the Earth |type=letter |last1=Wiegert |first1=Paul A. |last2=Innanen |first2=Kimmo A. |last3=Mikkola |first3=Seppo |journal=Nature |date=June 12, 1997 |volume=387 |issue=6634 |pages=685–686 |doi=10.1038/42662 |bibcode=1997Natur.387..685W |s2cid=4305272 |url=http://www.astro.uwo.ca/~wiegert/papers/1997Nature.387.685.pdf |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20160626032239/http://www.astro.uwo.ca/~wiegert/papers/1997Nature.387.685.pdf |archive-date=2016-06-26 }}</ref><ref>{{cite web |url=https://astro101.wwu.edu/a101_cruithne.html |first=Brad |last=Snowder |title=Cruithne |publisher=Western Washington University Planetarium |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240101150817/https://astro101.wwu.edu/a101_cruithne.html |archive-date=2024-01-01 }}</ref> {{mpl|419624|2010 SO|16}} is an asteroid on a relatively stable circumference-of-a-horseshoe orbit, with a horseshoe [[Astronomy:Libration|libration]] period of about 350 years.<ref>{{cite journal |last1=Christou |first1=A.A. |last2=Asher |first2=D.J. |title=A long-lived horseshoe companion to the Earth |date=July 11, 2011 |journal=Monthly Notices of the Royal Astronomical Society |volume=414 |issue=4 |pages=2965–2969 |doi=10.1111/j.1365-2966.2011.18595.x |arxiv=1104.0036 |bibcode=2011MNRAS.414.2965C |s2cid=13832179 |url=https://academic.oup.com/mnras/article-pdf/414/4/2965/18700374/mnras0414-2965.pdf |access-date=2024-01-27 }}</ref>
* ''[[Astronomy:Quasi-satellite|Quasi-satellite]]s'': Quasi-satellites are co-orbital asteroids on a normal elliptic orbit with a higher eccentricity than Earth's, which they travel in a way synchronised with Earth's motion. Since the asteroid orbits the Sun slower than Earth when further away and faster than Earth when closer to the Sun, when observed from Earth, the quasi-satellite appears to orbit Earth in a retrograde direction in one year, even though it is not bound gravitationally. By 2016, five asteroids were known to be a quasi-satellite of Earth. [[Astronomy:469219 Kamoʻoalewa|469219 Kamoʻoalewa]] is Earth's closest quasi-satellite, in an orbit that has been stable for almost a century.<ref name="Fuentes-2016HO3">{{cite journal |last1=de&nbsp;la&nbsp;Fuente Marcos |first1=C. |last2=de&nbsp;la&nbsp;Fuente Marcos |first2=R. |title=Asteroid {{mp|469219|2016 HO|3}}, the smallest and closest Earth quasi-satellite |journal=Monthly Notices of the Royal Astronomical Society |volume=462 |issue=4 |pages=3441–3456 |date=November 11, 2016 |doi=10.1093/mnras/stw1972 |arxiv=1608.01518 |bibcode=2016MNRAS.462.3441D |s2cid=118580771 }}</ref> This asteroid is thought to be a piece of the Moon ejected during an impact.<ref>{{cite news |url=https://www.space.com/moon-rock-near-earth-asteroid-study |title=Moon rocks blasted off the lunar surface could become near-Earth asteroids |date=October 24, 2023 |work=Space.com |access-date=2024-01-24}}</ref> Orbit calculations until 2016 showed that all quasi-satellites and four of the horseshoe librators then known repeatedly transfer between horseshoe and quasi-satellite orbits.<ref name="Fuentes-2016HO3"/> One of these objects, {{mpl|2003 YN|107}}, was observed during its transition from a quasi-satellite orbit to a horseshoe orbit in 2006; it is expected to transfer back to a quasi-satellite orbit sometime around year 2066.<ref>{{cite news |url=http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm |title=Corkscrew asteroid |first=Tony |last=Phillips |date=June 9, 2006 |work=Science@NASA |publisher=[[Organization:NASA|NASA]] |access-date=2017-11-13 |url-status=dead |archive-url=https://web.archive.org/web/20060929155325/http://science.nasa.gov/headlines/y2006/09jun_moonlets.htm |archive-date=2006-09-29}}</ref> A quasi-satellite discovered in 2023 but then found in old photographs back to 2012, {{mpl|2023 FW|13}}, was found to have an orbit that is stable for about 4,000 years, from 100 BC to AD 3700.<ref>{{cite news |title=Astronomers have discovered an asteroid that orbits the Sun with Earth, earning it the moniker "quasi-moon." |work=Sky & Telescope |first=David L. |last=Chandler |date=April 7, 2023 |url=https://skyandtelescope.org/astronomy-news/does-earth-have-new-quasi-moon/ |access-date=2024-01-24}}</ref>
* ''[[Astronomy:Temporary satellite|Temporary satellite]]s'': NEAs can also transfer between solar orbits and distant Earth orbits, becoming gravitationally bound temporary satellites. According to simulations, temporary satellites are typically caught when they pass the L1 or L2 Lagrangian points, and Earth typically has at least one temporary satellite {{convert|1|m|ft|abbr=on}} across at any given time, but they are too faint to detect by current surveys.<ref name="ST111230">{{cite news |first=Camille M. |last=Carlisle |title=Pseudo-moons orbit Earth |work=Sky & Telescope |date=December 30, 2011 |url=https://skyandtelescope.org/astronomy-news/pseudo-moons-orbit-earth/ |accessdate=2024-01-25}}</ref> {{As of|2021|11}}, the only observed transitions were those of asteroids {{mpl|2006 RH|120}} and {{mpl|2020 CD|3}}, which were temporary satellites of Earth for at least a year since their capture dates.<ref>{{cite news |title=Earth's "other moon" |work=Sky & Telescope |first=Roger W. |last=Sinnott |date=April 17, 2007 |url=https://skyandtelescope.org/astronomy-news/earths-other-moon/ |access-date=2024-01-25}}</ref><ref name="Naidu2020">{{cite news |title=Tiny Object Discovered in Distant Orbit Around the Earth |url=https://cneos.jpl.nasa.gov/news/news205.html |last1=Naidu |first1=Shantanu |last2=Farnocchia |first2=Davide |publisher=NASA/JPL CNEOS |access-date=2024-01-25}}</ref>

=== Meteoroids ===

In 1961, the IAU defined [[Astronomy:Meteoroid|meteoroid]]s as a class of solid interplanetary objects distinct from asteroids by their considerably smaller size.<ref name="Rubin2010"/> This definition was useful at the time because, with the exception of the [[Physics:Tunguska event|Tunguska event]], all historically observed meteors were produced by objects significantly smaller than the smallest asteroids then observable by telescopes.<ref name="Rubin2010"/> As the distinction began to blur with the discovery of ever smaller asteroids and a greater variety of observed NEO impacts, revised definitions with size limits have been proposed from the 1990s.<ref name="Rubin2010">{{cite journal |last1=Rubin |first1=Alan E. |last2=Grossman |first2=Jeffrey N. |title=Meteorite and meteoroid: New comprehensive definitions |journal=Meteoritics & Planetary Science |volume=45 |issue=1 |pages=114–122 |date=January 2010 |doi=10.1111/j.1945-5100.2009.01009.x |bibcode=2010M&PS...45..114R |s2cid=129972426 }}</ref> In April 2017, the IAU adopted a revised definition that generally limits meteoroids to a size between 30&nbsp;µm and 1&nbsp;m in diameter, but permits the use of the term for any object of any size that caused a meteor, thus leaving the distinction between asteroid and meteoroid blurred.<ref name="met-def-2017">{{cite news |first=Vincent |last=Perlerin |title=Definitions of terms in meteor astronomy (IAU) |date=September 26, 2017 |work=News |publisher=[[Organization:International Meteor Organization|International Meteor Organization]] |url=https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau/ |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20180123072313/https://www.imo.net/definitions-of-terms-in-meteor-astronomy-iau/ |archive-date=2018-01-23 }}</ref>

=== Near-Earth comets<!--'Near-Earth comet' and 'Near-Earth comets' redirect here--> ===

[[File:Halley's Comet - May 29 1910.jpg|thumb|Halley's Comet during its 0.10&nbsp;AU<ref>{{cite web |first=Donald K. |last=Yeomans |title=Great Comets in History |date=April 2007 |publisher=NASA/JPL |url=https://ssd.jpl.nasa.gov/sb/great_comets.html |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240126111145/https://ssd.jpl.nasa.gov/sb/great_comets.html |archive-date=2024-01-26 }}</ref> approach of Earth in May 1910]]

'''Near-Earth comets'''<!--boldface per WP:R#PLA--> (NECs) are objects in a near-Earth orbit with a tail or coma. Comet nuclei are typically less dense than asteroids but they pass Earth at higher relative speeds, thus the impact energy of a comet nucleus is slightly larger than that of a similar-sized asteroid.<ref name="NEOSDT2003"/> NECs may pose an additional hazard due to fragmentation: the meteoroid streams which produce meteor showers may include large inactive fragments, effectively NEAs.<ref name="Jenniksens2005">{{Cite journal |last=Jenniksens |first=Peter |title=Meteor Showers from Broken Comets |date=September 2005 |journal=Workshop on Dust in Planetary Systems (ESA SP-643) |volume=643 |pages=3–6 |bibcode=2007ESASP.643....3J}}</ref> Although no impact of a comet in Earth's history has been conclusively confirmed, the [[Physics:Tunguska event|Tunguska event]] may have been caused by a fragment of [[Astronomy:Comet Encke|Comet Encke]].<ref>{{cite journal |last=Kresak| first=L'.l |title=The Tunguska object – A fragment of Comet Encke |bibcode=1978BAICz..29..129K |volume=29 |date=1978 |pages=129 |journal=Bulletin of the Astronomical Institutes of Czechoslovakia}}</ref>

Comets are commonly divided between short-period and long-period comets. Short-period comets, with an orbital period of less than 200 years, originate in the [[Astronomy:Kuiper belt|Kuiper belt]], beyond the orbit of [[Astronomy:Neptune|Neptune]]; while long-period comets originate in the Oort Cloud, in the outer reaches of the Solar System.<ref name="TaskForceReport"/> The orbital period distinction is of importance in the evaluation of the risk from near-Earth comets because short-period NECs are likely to have been observed during multiple apparitions and thus their orbits can be determined with some precision, while long-period NECs can be assumed to have been seen for the first and last time when they appeared since the start of precise observations, thus their approaches cannot be predicted well in advance.<ref name="TaskForceReport">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Report of the Task Force on potentially hazardous Near Earth Objects |location=London |publisher=British National Space Centre |date=September 2000 |url=https://spaceguardcentre.com/wp-content/uploads/2014/04/full_report.pdf |access-date=2024-01-27 }}</ref> Since the threat from long-period NECs is estimated to be at most 1% of the threat from NEAs, and long-period comets are very faint and thus difficult to detect at large distances from the Sun, Spaceguard efforts have consistently focused on asteroids and short-period comets.<ref name="Vulcano1995"/><ref name="NEOSDT2003">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters |publisher=NASA |date=August 22, 2003 |url=https://cneos.jpl.nasa.gov/doc/neoreport030825.pdf |access-date=2024-01-27 }}</ref> Both NASA's CNEOS<ref name="CNEOS-NEO-groups"/> and ESA's restrict their definition of NECs to short-period comets. {{As of|2024|1|20|df=US|lc=y}}, 123 such objects have been discovered.<ref name="neo-jpl-stats" />

{{As of|2024|1}}, only 23 comets have been observed to pass within {{convert|0.1|AU|km mi|abbr=on|lk=off}} of Earth, including 10 which are or have been short-period comets.<ref name="closest-NEC"/> Two of these comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches.<ref name="closest-NEC"/> The closest observed approach was 0.0151&nbsp;AU (5.88&nbsp;LD) for [[Astronomy:Lexell's Comet|Lexell's Comet]] on July 1, 1770.<ref name="closest-NEC" /> After an orbit change due to a close approach of Jupiter in 1779, this object is no longer a NEC. The closest approach ever observed for a current short-period NEC is 0.0229&nbsp;AU (8.92&nbsp;LD) for Comet Tempel–Tuttle in 1366.<ref name="closest-NEC"/> This comet is the parent body of the [[Astronomy:Leonids|Leonid meteor shower]], which also produced the Great Meteor Storm of 1833.<ref>{{cite journal |last=Mason |first=John W. |title=The Leonid meteors and comet 55P/Tempel-Tuttle |journal=[[Organization:Journal of the British Astronomical Association|Journal of the British Astronomical Association]] |volume=105 |issue=5 |pages=219–235 |date=1995 |bibcode=1995JBAA..105..219M }}</ref> Orbital calculations show that P/1999 J6 (SOHO), a faint sungrazing comet and confirmed short-period NEC observed only during its close approaches to the Sun,<ref>{{cite journal |last1=Sekanina |first1=Zdenek |last2=Chodas |first2=Paul W. |title=Origin of the Marsden and Kracht Groups of Sunskirting Comets. I. Association with Comet 96P/Machholz and Its Interplanetary Complex |journal=The Astrophysical Journal Supplement Series |volume=151 |issue=2 |pages=551–586 |date=December 2005 |doi=10.1086/497374 |url=https://ui.adsabs.harvard.edu/link_gateway/2005ApJS..161..551S/PUB_PDF |access-date=2024-01-27 |bibcode=2005ApJS..161..551S |s2cid=85442034 }}</ref> passed Earth undetected at a distance of 0.0120&nbsp;AU (4.65&nbsp;LD) on June 12, 1999.<ref>{{cite web |url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=1999%20J6&view=OPC |title=Small-Body Database Lookup. P/1999 J6 (SOHO) |publisher=NASA/JPL |date=April 16, 2021 |access-date=2024-01-27 }}</ref>

Comet 109P/Swift–Tuttle, which is also the source of the [[Astronomy:Perseids|Perseid meteor shower]] every year in August, has a roughly 130-year orbit that passes close to the Earth. During the comet's September 1992 recovery, when only the two previous returns in 1862 and 1737 had been identified, calculations showed that the comet would pass close to Earth during its next return in 2126, with an impact within the range of uncertainty. By 1993, even earlier returns (back to at least 188 AD) have been identified, and the longer observation arc eliminated the impact risk. The comet will pass Earth in 2126 at a distance of 23 million kilometers. In 3044, the comet is expected to pass Earth at less than 1.6 million kilometers.<ref>{{cite web |url=https://astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |title=What about the comet that's supposed to hit the Earth in 130 years? |author=Sally Stephens |work=Cosmic Collisions |publisher=[[Organization:Astronomical Society of the Pacific|Astronomical Society of the Pacific]] |date=1993 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20231003194758/https://www.astrosociety.org/file_download/inline/245c66de-59dd-49e9-8773-16ef08de09ff |archive-date=2023-10-03 }}</ref>

=== Artificial near-Earth objects ===
[[Image:J002e3 animated.gif|thumb|J002E3 discovery images taken on September 3, 2002. J002E3 is in the circle]]
Defunct [[Engineering:Uncrewed spacecraft|space probes]] and [[Astronomy:Multistage rocket|final stages of rockets]] can end up in near-Earth orbits around the Sun, and be re-discovered by NEO surveys when they return to Earth's vicinity.

In November 1991, astronomers found an object classified as asteroid [[Astronomy:1991 VG|1991 VG]] and could observe it until 1992. Some scientists suspected it to be a returning piece of man-made space debris. After new observations in 2017, a new study found the artificial origin unlikely.<ref name="Fuente-Marcos-2018">{{Cite journal |first1=Carlos |last1=de la Fuente Marcos |first2=Raúl |last2=de la Fuente Marcos |date= January 2018 |title=Dynamical evolution of near-Earth asteroid 1991 VG |journal=Monthly Notices of the Royal Astronomical Society |volume=473 |issue=3 |pages=2939–2948 |bibcode=2018MNRAS.473.2939D |doi=10.1093/mnras/stx2545  |arxiv=1709.09533}}</ref>

In September 2002, astronomers found an object designated [[Astronomy:J002E3|J002E3]]. The object was on a temporary satellite orbit around Earth, leaving for a solar orbit in June 2003. Calculations showed that it was also on a solar orbit before 2002, but was close to Earth in 1971. J002E3 was identified as the third stage of the [[Engineering:Saturn V|Saturn V]] rocket that carried [[Engineering:Apollo 12|Apollo 12]] to the Moon.<ref>{{cite news |first1=Steve |last1=Chesley |first2=Paul |last2=Chodas |title=J002E3: An Update |work=News |publisher=NASA/JPL |date=October 9, 2002 |url=http://neo.jpl.nasa.gov/news/news136.html |access-date=2017-11-14 |url-status=dead |archive-url=https://web.archive.org/web/20030503111617/http://neo.jpl.nasa.gov/news/news136.html |archive-date=2003-05-03}}</ref><ref name="rocket-or-rock"/> In 2006, two more apparent temporary satellites were discovered which were suspected of being artificial.<ref name="rocket-or-rock"/> One of them was eventually confirmed as an asteroid and classified as the temporary satellite {{mp|2006 RH|120}}.<ref name="rocket-or-rock"/> The other, [[Astronomy:6Q0B44E|6Q0B44E]], was confirmed as an artificial object, but its identity is unknown.<ref name="rocket-or-rock"/> Another temporary satellite was discovered in 2013, and was designated {{mp|2013 QW|1}} as a suspected asteroid. It was later found to be an artificial object of unknown origin. {{mp|2013 QW|1}} is no longer listed as an asteroid by the Minor Planet Center.<ref name="rocket-or-rock">{{cite news |last=Azriel |first=Merryl |title=Rocket or Rock? NEO Confusion Abounds |url=http://www.spacesafetymagazine.com/space-debris/satellite-tracking/rocket-or-rock/ |date=September 25, 2013 |work=Space Safety Magazine |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20171115143536/http://www.spacesafetymagazine.com/space-debris/satellite-tracking/rocket-or-rock/ |archive-date=2017-11-15 }}</ref><ref name="MPC_QW1">{{Cite web |url=https://minorplanetcenter.net/db_search/show_object?utf8=✓&object_id=2013+QW1 |title=MPC Database Search. Unknown object: 2013 QW1 |publisher=IAU/MPC |access-date=2024-01-27}}</ref> In September 2020, an object detected on an orbit very similar to that of the Earth was temporarily designated {{mpl|2020 SO|}}. However, orbital calculations and spectral observations confirmed that the object was the [[Engineering:Centaur (rocket stage)|Centaur rocket booster]] of the 1966 [[Astronomy:Surveyor 2|Surveyor 2]] uncrewed lunar lander.<ref>{{cite news |title=Earth May Have Captured a 1960s-Era Rocket Booster |date=November 12, 2020 |work=News |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/earth-may-have-captured-a-1960s-era-rocket-booster |access-date=2024-01-31 }}</ref><ref>{{cite news |title=New Data Confirm 2020 SO to Be the Upper Centaur Rocket Booster From the 1960's  |date=December 2, 2020 |work=News |publisher=NASA/JPL |url=https://www.jpl.nasa.gov/news/new-data-confirm-2020-so-to-be-the-upper-centaur-rocket-booster-from-the-1960s |access-date=2024-01-31 }}</ref>

In some cases, active space probes on solar orbits have been observed by NEO surveys and erroneously catalogued as asteroids before identification. During its 2007 flyby of Earth on its route to a comet, ESA's space probe ''[[Engineering:Rosetta (spacecraft)|Rosetta]]'' was detected unidentified and classified as asteroid {{mp|2007 VN|84}}, with an alert issued due to its close approach.<ref>{{cite news |url=https://www.newscientist.com/article/dn12914-astronomers-defend-asteroid-warning-mix-up/ |title=Astronomers defend asteroid warning mix-up |author=Justin Mullins |date=November 13, 2007 |work=New Scientist |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170307062102/https://www.newscientist.com/article/dn12914-astronomers-defend-asteroid-warning-mix-up/ |archive-date=2017-03-07 }}</ref> The designation {{mp|2015 HP|116}} was similarly removed from asteroid catalogues when the observed object was identified with ''[[Engineering:Gaia (spacecraft)|Gaia]]'', ESA's space observatory for [[Astronomy:Astrometry|astrometry]].<ref>{{cite web |url=http://www.minorplanetcenter.net/mpec/K15/K15HC5.html |title=MPEC 2015-H125: Deletion Of 2015 HP116 |date=April 27, 2015 |work=Minor Planet Electronic Circular |publisher=IAU/MPC |access-date=2024-01-27 }}</ref>

== Impacts ==
{{main|Astronomy:Impact event}}

When a near-Earth object impacts Earth, objects up to a few tens of metres across ordinarily explode in the [[Earth:Mesosphere|upper atmosphere]] (usually harmlessly), with most or all of the solids [[Physics:Evaporation|vaporized]] and only small amounts of meteorites arriving to the Earth surface, while larger objects hit the water surface, forming [[Tsunami|tsunami]] waves, or the solid surface, forming [[Astronomy:Impact crater|impact crater]]s.<ref>{{Cite journal |last1=Chapman |first1= Clark R. |last2=Morrison |first2= David |name-list-style=amp |title=Impacts on the Earth by asteroids and comets: Assessing the hazard |journal=Nature |volume=367 |issue=6458 |pages=33–40 |date=January 6, 1994 |bibcode=1994Natur.367...33C |doi=10.1038/367033a0|s2cid=4305299 |url=https://zenodo.org/records/1233151/files/article.pdf |access-date=2024-01-27 }}</ref>

The frequency of impacts of objects of various sizes is estimated on the basis of orbit simulations of NEO populations, the frequency of impact craters on the Earth and the Moon, and the frequency of close encounters.<ref name="Collins2005">{{cite journal |last1=Collins |first1=Gareth S. |last2=Melosh |first2=H. Jay |last3=Marcus |first3=Robert A. |title=Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth |journal=Meteoritics & Planetary Science |volume=40 |number=6 |pages=817–840 |date=June 2005 |doi=10.1111/j.1945-5100.2005.tb00157.x |url=https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/effects.pdf |access-date=2024-01-27 |bibcode=2005M&PS...40..817C |hdl=10044/1/11554 |s2cid=13891988 |hdl-access=free }}</ref><ref name="Asher2005">{{cite journal |last1=Asher |first1=D. J. |last2=Bailey |first2=M. |last3=Emel'Yanenko |first3=V. |last4=Napier |first4=W. |title=Earth in the Cosmic Shooting Gallery |journal=The Observatory |volume=125 |issue=2 |pages=319–322 |date=October 2005 |url=https://articles.adsabs.harvard.edu/full/2005Obs...125..319A |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20220214090951/https://articles.adsabs.harvard.edu/full/2005Obs...125..319A |archive-date=2022-02-14 |bibcode=2005Obs...125..319A }}</ref> The study of impact craters indicates that impact frequency has been more or less steady for the past 3.5&nbsp;billion years, which requires a steady replenishment of the NEO population from the asteroid main belt.<ref name="MorbidelliAstIII"/> One impact model based on widely accepted NEO population models estimates the average time between the impact of two stony asteroids with a diameter of at least {{convert|4|m|ft|abbr=on}} at about one year; for asteroids {{convert|7|m|ft|abbr=on}} across (which impacts with as much energy as the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT) at five years, for asteroids {{convert|60|m|ft|abbr=on}} across (an impact energy of 10 megatons, comparable to the [[Physics:Tunguska event|Tunguska event]] in 1908) at 1,300 years, for asteroids {{convert|1|km|mi|abbr=on}} across at 440 thousand years, and for asteroids {{convert|5|km|mi|abbr=on}} across at 18 million years.<ref name="Earth-impact"/> Some other models estimate similar impact frequencies,<ref name="MorbidelliAstIII"/> while others calculate higher frequencies.<ref name="Asher2005"/> For Tunguska-sized (10 megaton) impacts, the estimates range from one event every 2,000–3,000 years to one event every 300 years.<ref name="Asher2005"/>

{{wide image|SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg|600px|align-cap=center|Location and impact energy of small asteroids impacting Earth's atmosphere}}

The second-largest observed event after the Tunguska meteor was a 1.1 megaton air blast in 1963 near the Prince Edward Islands between South Africa and Antarctica, which was detected only by [[Physics:Infrasound|infrasound]] sensors.<ref name="David_spacecom"/> However this may not have been a meteor.<ref name=silber>{{cite journal |doi=10.1029/2009JE003334 |title=An estimate of the terrestrial influx of large meteoroids from infrasonic measurements|journal=[[Earth:Journal of Geophysical Research|Journal of Geophysical Research]] |volume=114 |issue=E8|year=2009 |last1=Silber|first1=Elizabeth A.|last2=Revelle|first2=Douglas O.|last3=Brown|first3=Peter G.|last4=Edwards|first4=Wayne N.|bibcode=2009JGRE..114.8006S|doi-access=free}}</ref> The third-largest, but by far best-observed impact, was the [[Physics:Chelyabinsk meteor|Chelyabinsk meteor]] of 15 February 2013. A previously unknown {{convert|20|m|ft|abbr=on}} asteroid exploded above this Russian city with an equivalent blast yield of 400–500 kilotons.<ref name="David_spacecom">{{cite news |url=http://www.space.com/23423-russian-fireball-meteor-airburst-risk.html |title=Russian fireball explosion shows meteor risk greater than thought |last=David |first=Leonard |date=November 1, 2013 |work=Space.com |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170819031019/https://www.space.com/23423-russian-fireball-meteor-airburst-risk.html |archive-date=2017-08-19 }}</ref> The calculated orbit of the pre-impact asteroid is similar to that of Apollo asteroid {{mpl|2011 EO|40}}, making the latter the meteor's possible parent body.<ref>{{cite journal |title=Reconstructing the Chelyabinsk event: Pre-impact orbital evolution |first1=C. |last1=de&nbsp;la&nbsp;Fuente Marcos |first2=R. |last2=de&nbsp;la&nbsp;Fuente Marcos |date=September 1, 2014 |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=443 |issue=1 |pages=L39–L43 |arxiv=1405.7202 |bibcode=2014MNRAS.443L..39D |doi=10.1093/mnrasl/slu078 |s2cid=118417667 }}</ref>

On October 7, 2008, 20 hours after it was first observed and 11 hours after its trajectory has been calculated and announced, {{convert|4|m|ft|abbr=on}} asteroid {{mpl|2008 TC|3}} blew up {{convert|37|km|sigfig=2|abbr=on}} above the Nubian Desert in Sudan. It was the first time that an asteroid was observed and its impact was predicted prior to its entry into the atmosphere as a meteor. 10.7&nbsp;kg of meteorites were recovered after the impact.<ref>{{cite journal |last1=Shaddad |first1=Muawia H. |display-authors=0 |title=The recovery of asteroid {{mp|2008 TC|3}} |journal=Meteoritics & Planetary Science |volume=45 |issue=10–11 |pages=1557–1589 |date=October 2010 |doi=10.1111/j.1945-5100.2010.01116.x |bibcode=2010M&PS...45.1557S |url=http://asima.seti.org/2008TC3/papers/maps1116-1296.pdf |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20160304185539/http://asima.seti.org/2008TC3/papers/maps1116-1296.pdf |archive-date=2016-03-04 |doi-access=free }}</ref>

On January 2, 2014, just 21 hours after it was the first asteroid to be discovered in 2014, {{convert|2|-|4|m|ft|abbr=on}} asteroid {{mpl|2014 AA|}} blew up in Earth's atmosphere above the Atlantic Ocean. Far from any land, the meteor explosion was only observed by three infrasound detectors of the Comprehensive Nuclear-Test-Ban Treaty Organization. This impact was the second to be predicted.<ref name="S&T140102">{{cite news |title=Small asteroid 2014&nbsp;AA hits Earth |magazine=Sky & Telescope |first1=Kelly |last1=Beatty |date=January 2, 2014 |url=http://www.skyandtelescope.com/astronomy-news/small-asteroid-2014-aa-hitsearth/ |access-date=2024-01-27 }}</ref> {{As of|2024|01}}, eight impacts have been predicted, all of them small bodies that produced meteor explosions.<ref name="Spacecom240123">{{cite news |title=Scientists discover near-Earth asteroid hours before it exploded over Berlin |work=Space.com |first1=Kiley |last1=Price |date=January 23, 2024 |url=https://www.space.com/scientists-discover-near-earth-asteroid-hours-before-it-exploded-over-berlin |access-date=2024-01-24}}</ref>

[[Astronomy:Asteroid impact prediction|Asteroid impact prediction]] remains in its infancy and successfully predicted asteroid impacts are rare. The vast majority of impacts recorded by infrasound sensors designed to detect detonation of nuclear devices are not predicted.<ref>{{cite web |title=Fireballs. Fireball and Bolide Data |url=http://cneos.jpl.nasa.gov/fireballs/ |date=December 30, 2023 |archive-url=https://web.archive.org/web/20240120155455/https://cneos.jpl.nasa.gov/fireballs/ |url-status=live |archive-date=2024-01-20 |publisher=NASA/JPL |access-date=2024-01-25}}</ref>

Observed impacts aren't restricted to the surface and atmosphere of Earth. Dust-sized NEOs have impacted man-made spacecraft, including NASA's [[Organization:Long Duration Exposure Facility|Long Duration Exposure Facility]], which collected [[Astronomy:Interplanetary dust cloud|interplanetary dust]] in low Earth orbit for six years from 1984.<ref name="Rubin2010"/> Impacts on the Moon can be observed as flashes of light with a typical duration of a fraction of a second.<ref name="NASA-lunar-impacts"/> The first lunar impacts were recorded during the 1999 Leonid storm.<ref>{{cite encyclopedia |last1=Rubio |first1=Luis R. Bellot |last2=Ortiz |first2=Jose L. |last3=Sada |first3=Pedro V. |title=Observation and Interpretation of Meteoroid Impact Flashes on the Moon |editor1-last=Jenniskens |editor1-first=P. |editor2-last=Rietmeijer |editor2-first=F. |editor3-last=Brosch |editor3-first=N. |editor4-last=Fonda |editor4-first=M. |encyclopedia=Leonid Storm Research |publisher=Springer |location=Dordrecht |year=2000 |pages=575–598 |isbn=978-90-481-5624-5 |doi=10.1007/978-94-017-2071-7_42 |bibcode=2000lsr..book..575B |s2cid=118392496 }}</ref> Subsequently, several continuous monitoring programs were launched.<ref name="NASA-lunar-impacts">{{cite web |title=Lunar Impact Monitoring Program |publisher=NASA |url=https://www.nasa.gov/meteoroid-environment-office/about-lunar-impact-monitoring/ |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240127144250/https://www.nasa.gov/meteoroid-environment-office/about-lunar-impact-monitoring/ |archive-date=2024-01-27 }}</ref><ref name="2013-lunar-impact"/><ref name="ESA-lunar-impacts">{{cite web |title=About the NELIOTA project |publisher=ESA |url=https://neliota.astro.noa.gr/About/Project |access-date=2024-01-27 }}</ref> A lunar impact that was observed on September 11, 2013, lasted 8 seconds, was likely caused by an object {{convert|0.6–1.4|m|ft|abbr=on}} in diameter,<ref name="2013-lunar-impact">{{cite news |first=Michele |last=Catanzaro |title=Largest lunar impact caught by astronomers |date=February 24, 2014 |journal=Nature |url=https://www.nature.com/articles/nature.2014.14773 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20211004133116/https://www.nature.com/articles/nature.2014.14773 |archive-date=2021-10-04 }}</ref> and created a new crater {{convert|40|m|ft|abbr=on}} across, was the largest ever observed {{as of|2019|07}}.<ref>{{cite web |title=MIDAS: Moon Impacts Detection and Analysis System. Main Results |work=Meteoroides.NET |url=http://www.meteoroides.net/e_midas_results.html |access-date=2024-01-27 }}</ref>

== Close approaches ==
{{main|Astronomy:List of asteroid close approaches to Earth}}

[[File:Asteroid 2004 FH.gif|framed|right|Flyby of asteroid 2004 FH (centre dot being followed by the sequence). The other object that flashes by is an artificial satellite]]
Each year, several mostly small NEOs pass Earth closer than the distance of the Moon.<ref name="closest-NEA"/> NASA's catalog of near-Earth objects includes the approach distances of asteroids and comets (expressed in [[Astronomy:Lunar distance|lunar distances]]).<ref name="NEO-close">{{cite web |url=https://cneos.jpl.nasa.gov/ca/ |title=NEO Earth Close Approaches |publisher=NASA/JPL CNEOS |access-date=2024-01-25 |url-status=live |archive-url=https://web.archive.org/web/20240124004749/https://cneos.jpl.nasa.gov/ca/ |archive-date=2024-01-24 }}</ref>

On August 10, 1972, a meteor that became known as the [[Astronomy:1972 Great Daylight Fireball|1972 Great Daylight Fireball]] was witnessed by many people; it moved north over the Rocky Mountains from the U.S. Southwest to Canada. It was an [[Astronomy:Earth-grazing fireball|Earth-grazing meteoroid]] that passed within {{convert|57|km|mi|abbr=on}} of the Earth's surface, and was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera.<ref>{{cite web |url=https://www.youtube.com/watch?v=7M8LQ7_hWtE |title=Grand Teton Meteor (video) |work=[[Company:YouTube|YouTube]] |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170214110154/https://www.youtube.com/watch?v=7M8LQ7_hWtE |archive-date=2017-02-14 }}</ref>

On October 13, 1990, [[Astronomy:Earth-grazing meteoroid of 13 October 1990|Earth-grazing meteoroid EN131090]] was observed above Czechoslovakia and Poland, moving at {{convert|41.74|km/s|mi/s|abbr=on}} along a {{convert|409|km|mi|adj=on|abbr=on}} trajectory from south to north. The closest approach to the Earth was {{convert|98.67|km|mi|abbr=on}} above the surface. It was captured by two all-sky cameras of the [[Organization:European Fireball Network|European Fireball Network]], which for the first time enabled geometric calculations of the orbit of such a body.<ref name="AA">{{cite journal |last1=Borovička |first1=J. |last2=Ceplecha |first2=Z. |title=Earth-grazing fireball of October 13, 1990 |journal=Astronomy & Astrophysics |volume=257 |issue=1 |pages=323–328 |url=https://adsabs.harvard.edu/full/1992A%26A...257..323B |accessdate=2024-01-27 |date=April 1992 |bibcode=1992A&A...257..323B |issn=0004-6361 }}</ref>

On March 18, 2004, LINEAR announced that a {{convert|30|m|ft|abbr=on}} asteroid, [[Astronomy:2004 FH|2004 FH]], would pass the Earth that day at only {{convert|42,600|km|mi|abbr=on}}, about one-tenth the distance to the Moon, and the closest miss ever noticed until then. They estimated that similar-sized asteroids come as close about every two years.<ref>{{cite news |url=http://cneos.jpl.nasa.gov/news/news142.html |title=Recently Discovered Near-Earth Asteroid Makes Record-breaking Approach to Earth |author=Steven R. Chesley |author2=Paul W. Chodas |name-list-style=amp |work=News |publisher=NASA/JPL CNEOS |date=March 17, 2004 |access-date=2017-11-09}}</ref> On March 31, 2004, two weeks after 2004 FH, {{mpl|2004 FU|162}} set a new record for closest recorded approach above the atmosphere, passing Earth's surface only {{convert|6,500|km|mi|abbr=on}} away (about one Earth radius or one-sixtieth of the distance to the Moon). Because it was very small, only {{convert|5–10|m|ft|abbr=on}} across, {{mp|2004 FU|162}} was detected only hours before its closest approach. If it had collided with Earth, it probably would have disintegrated harmlessly in the atmosphere.<ref>{{cite news |title=Asteroid shaves past Earth's atmosphere |first=Jeff |last=Hecht |work=New Scientist |date=August 23, 2004 |url=https://www.newscientist.com/article/dn6307-asteroid-shaves-past-earths-atmosphere/ |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20211130181600/https://www.newscientist.com/article/dn6307-asteroid-shaves-past-earths-atmosphere/ |archive-date=2021-11-30 }}</ref> Its record was surpassed on February 4, 2011, when an asteroid designated {{mpl|2011 CQ|1}}, estimated at {{convert|1|m|ft|abbr=on}} in diameter, passed within {{convert|5,500|km|mi|abbr=on}} of the Earth.<ref name=2011cq1>{{cite news |title=Very Small Asteroid Makes Close Earth Approach on February 4, 2011 |work=News |publisher=NASA/JPL Near-Earth Object Program Office |author=Don Yeomans |author2=Paul Chodas |name-list-style=amp |date=February 4, 2011 |url=http://neo.jpl.nasa.gov/news/news170.html |access-date=2017-11-09 |url-status=dead |archive-url=https://web.archive.org/web/20110902004227/http://neo.jpl.nasa.gov/news/news170.html |archive-date=2011-09-02}}</ref> On November 14, 2020, the ATLAS program detected a {{convert|5|-|11|m|ft|abbr=on}} asteroid receding from Earth, which was designated {{mpl|2020 VT|4}}. Calculations showed that on the day before, it had a close approach at about {{convert|6750|km|mi|abbr=on}} from the Earth's centre, or about {{convert|380|km|mi|abbr=on}} above its surface.<ref name="EarthSky">{{cite news |title=This asteroid just skimmed Earth's atmosphere |url=https://earthsky.org/space/asteroid-2020-vt4-skimmed-atmosphere-fri-nov-13-2020 |first=Eddie |last=Irizarry |work=EarthSky |date= November 16, 2020 |accessdate=2024-01-25 }}</ref> {{As of|2024|1|lc=y}}, this remains the closest approach without impact ever detected.<ref name="NEO-close"/>

On November 8, 2011, asteroid {{mpl|(308635) 2005 YU|55}}, relatively large at about {{convert|400|m|ft|abbr=on}} in diameter, passed within {{convert|324930|km|mi|abbr=on}} (0.845 lunar distances) of Earth.<ref>{{cite web |url=https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2005YU55&view=OPC |title=Small-Body Database Lookup. 308635 (2005 YU55) |date=January 7, 2022 |publisher=NASA/JPL |access-date=2024-01-27 }}</ref>

On February 15, 2013, the {{convert|30|m|ft|abbr=on}} asteroid [[Astronomy:367943 Duende|367943 Duende]] ({{mp|2012 DA|14}}) passed approximately {{convert|27700|km|mi|abbr=on}} above the surface of Earth, closer than satellites in geosynchronous orbit.<ref name="Duende-BBC">{{cite news |first=Jason |last=Palmer |title=Asteroid 2012 DA14 in record-breaking Earth pass |work=BBC News |publisher=BBC |date=February 15, 2013 |url=https://www.bbc.com/news/science-environment-21442863 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20180217085054/http://www.bbc.com/news/science-environment-21442863 |archive-date=2018-02-17 }}</ref> The asteroid was not visible to the unaided eye. This was the first close passage of an object discovered during a previous passage, and was thus the first to be predicted well in advance.<ref name="Duende-predict">{{cite news |title=Near-Earth Asteroid {{mp|2012 DA|14}} to Miss Earth on February 15, 2013 |work=News |publisher=NASA/JPL CNEOS |author1=Paul Chodas |author2=Jon Giorgini |author3=Don Yeomans |name-list-style=amp |url=https://cneos.jpl.nasa.gov/news/news174.html |date=March 6, 2012 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20171222113153/https://cneos.jpl.nasa.gov/news/news174.html |archive-date=2017-12-22 }}</ref>

{{wide image|File:Objects_between_earth_and_moon.jpg|2250px|Diagram showing spacecraft and asteroids (past and future) between the Earth and the Moon.}}

== Exploratory missions ==

Some NEOs are of special interest because they can be physically explored with lower mission velocity than is necessary for even the Moon, due to their combination of low velocity with respect to Earth and weak gravity. They may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation.<ref name="USAToday-NEA">{{cite news |url=https://www.usatoday.com/tech/science/space/2007-02-12-asteroid_x.htm |title=Near-Earth asteroids could be 'steppingstones to Mars' |author=Dan Vergano |work=[[Software:USA Today|USA Today]] |date=February 2, 2007 |access-date=2017-11-18 |url-status=live |archive-url=https://web.archive.org/web/20120417034648/http://www.usatoday.com/tech/science/space/2007-02-12-asteroid_x.htm |archive-date=2012-04-17 }}</ref> This makes them an attractive target for exploration.<ref>{{cite journal |title=Design and optimization of trajectory to Near-Earth asteroid for sample return mission using gravity assists |author=Rui Xu |author2=Pingyuan Cui |author3=Dong Qiao |author4=Enjie Luan |name-list-style=amp |journal=Advances in Space Research |volume=40 |issue=2 |pages=200–225 |date=March 18, 2007 |doi=10.1016/j.asr.2007.03.025 |bibcode=2007AdSpR..40..220X}}</ref>

=== Missions to NEAs ===
[[File:PIA02475 Eros' Bland Butterscotch Colors.jpg|thumb|433 Eros as seen by NASA's ''NEAR'' probe]]
[[File:Bennu mosaic OSIRIS-REx (square).png|thumb|Image mosaic of asteroid 101955 Bennu, target of NASA's ''OSIRIS-REx'' probe]]
[[File:DART-impact-SAAO-Lesedi-Mookodi.gif|thumb|DART impact and its corresponding plume as seen by using the Mookodi instrument on the SAAO's 1-m Lesedi telescope]]

The IAU held a minor planets workshop in Tucson, Arizona, in March 1971. At that point, launching a spacecraft to asteroids was considered premature; the workshop only inspired the first astronomical survey specifically aiming for NEAs.<ref name="wired20130323"/> Missions to asteroids were considered again during a workshop at the [[Organization:University of Chicago|University of Chicago]] held by NASA's Office of Space Science in January 1978. Of all of the near-Earth asteroids (NEA) that had been discovered by mid-1977, it was estimated that spacecraft could [[Astronomy:Space rendezvous|rendezvous]] with and return from only about 1 in 10 using less [[Astronomy:Spacecraft propulsion|propulsive energy]] than is necessary to reach [[Astronomy:Mars|Mars]]. It was recognised that due to the low surface gravity of all NEAs, moving around on the surface of an NEA would cost very little energy, and thus space probes could gather multiple samples.<ref name="wired20130323"/> Overall, it was estimated that about one percent of all NEAs might provide opportunities for [[Astronomy:Human spaceflight|human-crewed]] missions, or no more than about ten NEAs known at the time. A five-fold increase in the NEA discovery rate was deemed necessary to make a crewed mission within ten years worthwhile.<ref name="wired20130323"/>

The first near-Earth asteroid to be visited by a spacecraft was {{convert|17|km|mi|abbr=on}} asteroid [[Astronomy:433 Eros|433 Eros]] when [[Organization:NASA|NASA]]'s ''[[Astronomy:NEAR Shoemaker|Near Earth Asteroid Rendezvous]]'' (''NEAR'') probe orbited it from February 2001, landing on the asteroid surface in February 2002.<ref name="Eros-NEAR">{{cite news |url=http://nssdc.gsfc.nasa.gov/planetary/news/near_descent_pr_20010131.html |title=NEAR Mission Completes Main Task, Now Will Go Where No Spacecraft Has Gone Before |author=Donald Savage |author2=Michael Buckley |name-list-style=amp |work=Press Releases |publisher=NASA |date=January 31, 2001 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20160617001709/http://nssdc.gsfc.nasa.gov/planetary/news/near_descent_pr_20010131.html |archive-date=2016-06-17 }}</ref> A second near-Earth asteroid, the {{convert|535|m|ft|abbr=on}} long peanut-shaped [[Astronomy:25143 Itokawa|25143 Itokawa]], was explored from September 2005 to April 2007 by [[Organization:JAXA|JAXA]]'s ''Hayabusa'' mission, which succeeded in taking material samples back to Earth in June 2010.<ref name="Itokawa-hayabusa">{{cite web |url=https://hayabusa.jaxa.jp/e/index.html |title=Hayabusa. The Final Approach. Overview |publisher=JAXA |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20230602050007/https://hayabusa.jaxa.jp/e/index.html |archive-date=2023-06-02 }}</ref> A third near-Earth asteroid, the {{convert|2.26|km|mi|abbr=on}} long elongated [[Astronomy:4179 Toutatis|4179 Toutatis]], was explored by [[Organization:China National Space Administration|CNSA]]'s ''[[Astronomy:Chang'e 2|Chang'e 2]]'' spacecraft during a flyby in December 2012.<ref name="Toutatis-Change">{{cite news |url=http://www.planetary.org/blogs/emily-lakdawalla/2012/12141551-change-2-imaging-of-toutatis.html |title=Chang'e 2 imaging of Toutatis |author=Emily Lakdawalla |work=Blog |publisher=The Planetary Society |date=December 14, 2012 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20170707021230/http://www.planetary.org/blogs/emily-lakdawalla/2012/12141551-change-2-imaging-of-toutatis.html |archive-date=2017-07-07 }}</ref><ref name="IAU-NEOs"/>

The {{convert|980|m|ft|abbr=on}} Apollo asteroid [[Astronomy:162173 Ryugu|162173 Ryugu]] was the target of JAXA's ''[[Astronomy:Hayabusa2|Hayabusa2]]'' mission. The space probe was launched in December 2014, arrived at the asteroid in June 2018, and returned a sample to Earth in December 2020.<ref name="SFN-Hayabusa2">{{cite news |url=https://spaceflightnow.com/2014/12/03/hayabusa-2-launches-on-audacious-asteroid-adventure/ |title=Hayabusa 2 launches on audacious asteroid adventure |author=Stephen Clark |work=Spaceflight Now |date=December 3, 2014 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20160722120740/http://spaceflightnow.com/2014/12/03/hayabusa-2-launches-on-audacious-asteroid-adventure/ |archive-date=2016-07-22 }}</ref> The {{convert|500|m|ft|abbr=on}} Apollo asteroid [[Astronomy:101955 Bennu|101955 Bennu]], which, {{As of|2024|1|lc=y}}, has the highest cumulative Palermo scale rating (−1.59 for several close encounters between 2178 and 2290),<ref name="Current_Impact_Risks"/> is the target of NASA's ''[[Astronomy:OSIRIS-REx|OSIRIS-REx]]'' probe. The [[Astronomy:New Frontiers program|New Frontiers program]] mission was launched in September 2016.<ref name="space-osiris-launch">{{cite news |first=Mike |last=Wall |title='Exactly Perfect'! NASA Hails Asteroid Sample-Return Mission's Launch |url=https://www.space.com/34020-nasa-hails-osiris-rex-asteroid-mission-launch.html |date=September 9, 2016 |work=Space.com |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20231006005439/https://www.space.com/34020-nasa-hails-osiris-rex-asteroid-mission-launch.html |archive-date=2023-10-06 }}</ref> On its two-year journey to Bennu, the probe had searched for Earth's Trojan asteroids,<ref>{{cite news |url=https://www.nasa.gov/feature/goddard/2017/osiris-rex-begins-earth-trojan-asteroid-search |title=NASA's OSIRIS-REx Begins Earth-Trojan Asteroid Search |work=News |publisher=NASA |first1=Erin |last1=Morton |first2=Nancy |last2=Neal-Jones |date=February 9, 2017 |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20180207092541/https://www.nasa.gov/feature/goddard/2017/osiris-rex-begins-earth-trojan-asteroid-search/ |archive-date=2018-02-07 }}</ref>, entered into orbit around Bennu in December 2018, and touched down on its surfce in October 2020.<ref name="space-osiris-overview">{{cite news |first=Nola |last=Taylor Tillman |title=OSIRIS-REx: A complete guide to the asteroid-sampling mission |url=https://www.space.com/33776-osiris-rex.html |date=September 25, 2023 |work=Space.com |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240125154614/https://www.space.com/33776-osiris-rex.html |archive-date=2024-01-25 }}</ref> ''OSIRIS-REx'' returned samples from the asteroid in September 2023.<ref name="space-osiris-sample">{{cite news |first=John |last=Loeffer |title=NASA finally opens OSIRIS-REx asteroid sample canister after freeing stuck lid |url=https://www.space.com/nasa-osiris-rex-asteroid-sample-canister-open |date=January 23, 2024 |work=Space.com |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20240125183741/https://www.space.com/nasa-osiris-rex-asteroid-sample-canister-open |archive-date=2024-01-25 }}</ref> China plans to launch its own sample-return mission, ''[[Astronomy:Tianwen-2|Tianwen-2]]'', in May 2025, targeting Earth quasi-satellite {{mp|469219 Kamoʻoalewa|}} and returning samples to Earth in late 2027.<ref>{{cite news |first=Andrew |last=Jones |url=https://spacenews.com/china-conducts-parachute-tests-for-asteroid-sample-return-mission/ |title=China conducts parachute tests for asteroid sample return mission |work=SpaceNews |date=June 26, 2023 |access-date=2024-01-26}}</ref>

After a ten-month journey to Apollo asteroid {{mpl|65803 Didymos|}}, on September 26, 2022, the ''[[Astronomy:Double Asteroid Redirection Test|DART]]'' spacecraft impacted the asteroid's moon [[Astronomy:Dimorphos|Dimorphos]], in a test of a method of [[Astronomy:Asteroid impact avoidance|planetary defense]] against near-Earth objects.<ref name="NASA220927">{{cite news |first=Roxana |last=Bardan |title=NASA's DART Mission Hits Asteroid in First-Ever Planetary Defense Test |date=September 27, 2022 |url=https://www.nasa.gov/news-release/nasas-dart-mission-hits-asteroid-in-first-ever-planetary-defense-test/ |work=Press Releases |publisher=NASA |access-date=2024-01-26}}</ref> In addition to telescopes on or in orbit around the Earth, the impact was observed by a mini-spacecraft or [[Engineering:CubeSat|CubeSat]], ''Light Italian CubeSat for Imaging of Asteroids'' (''LICIACube''), which separated from ''DART'' 15 days before impact.<ref name="NASA220927"/> The impact shortened the orbital period of Dimorphos around Didymos by 33 minutes, indicating that the moon's momentum change was 3.6 times the momentum of the impacting spacecraft, thus most of the change was due to the ejected material of the moon itself.<ref name="NASA221215">{{cite news |first=Jessica |last=Merzdorf |title=Early Results from NASA's DART Mission |date=December 15, 2022 |url=http://www.nasa.gov/feature/early-results-from-nasa-s-dart-mission |work=Press Releases |publisher=NASA |access-date=2024-01-26}}</ref> In October 2024, ESA plans to launch the spacecraft ''[[Astronomy:Hera (space mission)|Hera]]'', which is to enter orbit around Didymos in December 2026, to study the consequences of the impact.<ref>{{cite web |title=Hera |url=https://www.esa.int/Space_Safety/Hera |publisher=ESA |access-date=2024-01-26}}</ref> China plans to launch its own asteroid deflection probe, targeting {{convert|30|m|ft|abbr=on}} Aten asteroid {{mpl|2019 VL|5}}, in 2025.<ref>{{cite news |first=Andrew |last=Jones |url=https://spacenews.com/china-to-target-asteroid-2019-vl5-for-2025-planetary-defense-test/ |title=China to target asteroid 2019 VL5 for 2025 planetary defense test |work=SpaceNews |date=April 11, 2023 |access-date=2024-01-26}}</ref>

After completing its mission to Bennu, the probe ''OSIRIS-REx'' was redirected towards 99942 Apophis, which it is planned to orbit from April 2029.<ref name="space-osiris-overview"/> After completing its exploration of 162173 Ryugu, the mission of the ''Hayabusa2'' space probe was extended, to include flybys of [[Astronomy:L-type asteroid|L-type]] Apollo asteroid {{mpl|(98943) 2001 CC|21}} in July 2026 and fast-rotating Apollo asteroid {{mpl|1998 KY|26}} in July 2031.<ref>{{Cite conference |last1=Hirabayashi |first1=Masatoshi |last2=Yoshikawa |first2=Makoto |last3=Mimasu |first3=Yuya |last4=Tanaka |first4=Satoshi |last5=Saiki |first5=Takanao |last6=Nakazawa |first6=Satoru |last7=Tsuda |first7=Yuichi |last8=Tatsumi |first8=Eri |last9=Popescu |first9=Marcel |last10=Pravec |first10=Petr |last11=Urakawa |first11=Seitaro |last12=Yoshida |first12=Fumi |last13=Hirata |first13=Naru |last14=Kamata |first14=Shunichi |last15=Kitazato |first15=Kohei |date=February 15, 2023 |title=Hayabusa2#'s Exploration to Asteroids 2001 CC21 and 1998 KY26 Provides Key Insights Into Planetary Defense |url=https://ntrs.nasa.gov/citations/20230002153 |conference=8th IAA Planetary Defense Conference |location=Vienna, Austria |archive-url=https://web.archive.org/web/20240123201919/https://ntrs.nasa.gov/citations/20230002153 |archive-date=2024-01-23 |url-status=live }}</ref> In 2025, JAXA plans to launch another probe, ''[[Astronomy:DESTINY+|DESTINY+]]'', to explore Apollo asteroid {{mpl|3200 Phaethon|}}, the parent body of the [[Astronomy:Geminids|Geminid meteor shower]], during a flyby.<ref name="space-20231106">{{cite news |first=Andrew |last=Jones |url=https://www.space.com/japan-destiny-mission-asteroid-phaethon-launch-delay |title=Japan's mission to bizarre asteroid Phaethon delayed to 2025 |work=Space.com |date=November 6, 2023 |access-date=2024-01-26}}</ref>

From the 2000s, there were plans for the commercial exploitation of near-Earth asteroids, either through the use of robots or even by sending private commercial astronauts to act as space miners, but few of these plans were pursued.<ref name="Forbes210831">{{Cite web |last=Dorminey |first=Bruce |title=Does Commercial Asteroid Mining Still Have A Future? |url=https://www.forbes.com/sites/brucedorminey/2021/08/31/does-commercial-asteroid-mining-still-have-a-future/ |date=August 31, 2021 |website={{wipe|Forbes}} |access-date=2024-01-27}}</ref> In April 2012, the company [[Company:Planetary Resources|Planetary Resources]] announced its plans to [[Astronomy:Asteroid mining|mine asteroids]] commercially. In a first phase, the company reviewed data and selected potential targets among NEAs. In a second phase, space probes would be sent to the selected NEAs; mining spacecraft would be sent in a third phase.<ref name="ST-AsteroidMining">{{cite news |author=Kelly Beatty |url=http://www.skyandtelescope.com/astronomy-news/asteroid-mining-forfunandprofit/ |title=Asteroid Mining for Fun and Profit |date=April 24, 2012 |work=Sky & Telescope |access-date=2024-01-27}}</ref> Planetary Resources launched two testbed satellites in April 2015<ref name="Arkyd6LeftBuilding"/> and January 2018,<ref>{{cite news |title=Planetary Resources Launches Latest Spacecraft in Advance of Space Resource Exploration Mission |work=News |publisher=Planetary Resources |date=January 12, 2018 |url=https://www.planetaryresources.com/2018/01/planetary-resources-launches-latest-spacecraft-in-advance-of-space-resource-exploration-mission/ |access-date=2018-01-13 |url-status=dead |archive-url=https://web.archive.org/web/20180113093243/https://www.planetaryresources.com/2018/01/planetary-resources-launches-latest-spacecraft-in-advance-of-space-resource-exploration-mission/ |archive-date=2018-01-13 }}</ref> and the first prospecting satellite for the second phase was planned for a 2020 launch prior to the company closing and its assets purchased by ConsenSys Space in 2018.<ref name="Arkyd6LeftBuilding">{{cite news |author=Alan Boyle |url=https://www.geekwire.com/2017/planetary-resources-arkyd-6-prototype-imaging-satellite-left-building/ |title=Planetary Resources' Arkyd-6 prototype imaging satellite has left the building |date=November 13, 2017 |work=GeekWire |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20171114080201/https://www.geekwire.com/2017/planetary-resources-arkyd-6-prototype-imaging-satellite-left-building/ |archive-date=2017-11-14 }}</ref><ref name="oneyearsinceplanetaryresources">{{cite news |author=Alan Boyle |url=https://www.geekwire.com/2019/one-year-planetary-resources-faded-history-space-mining-retains-appeal/ |title=One year after Planetary Resource faded into history, space mining retails its appeal |date=November 4, 2019 |work=GeekWire |access-date=2024-01-27 }}</ref> Another American company established with the goal of space mining, AstroForge, plans to launch the probe ''Odin'' (formerly ''Brokkr-2'') in 2024,<ref>{{cite news |first1=Matt |last1=Gialich |first2=Jose |last2=Acain |url=https://www.astroforge.io/updates/2023-update |title=An update on our progress towards mining in space |work=AstroForge |date=December 11, 2023 |access-date=2024-01-26}}</ref> with the goal of performing a flyby of an as yet undisclosed asteroid to confirm if it is a metal-rich [[Astronomy:M-type asteroid|M-type asteroid]].<ref>{{cite news |first=Jeff |last=Foust |url=https://spacenews.com/asteroid-mining-startup-astroforge-to-launch-first-missions-this-year-2/ |title=Asteroid mining startup AstroForge to launch first missions this year |work=SpaceNews |date=January 30, 2023 |access-date=2024-01-26}}</ref>

=== Missions to NECs ===
[[File:67P Churyumov-Gerasimenko - Rosetta (32755885495).png|thumb|67P/Churyumov–Gerasimenko as seen by ESA's ''Rosetta'' probe]]

The first near-Earth comet visited by a space probe was 21P/Giacobini–Zinner in 1985, when the NASA/ESA probe ''[[Astronomy:International Cometary Explorer|International Cometary Explorer]]'' (''ICE'') passed through its coma. In March 1986, ICE, along with [[Place:Soviet Union|Soviet]] probes ''[[Engineering:Vega 1|Vega 1]]'' and ''[[Astronomy:Vega 2|Vega 2]]'', [[Organization:Institute of Space and Astronautical Science|ISAS]] probes ''[[Astronomy:Sakigake|Sakigake]]'' and ''[[Engineering:Suisei (spacecraft)|Suisei]]'' and ESA probe ''[[Engineering:Giotto (spacecraft)|Giotto]]'' flew by the nucleus of Halley's Comet. In 1992, ''Giotto'' also visited another NEC, 26P/Grigg–Skjellerup.<ref name="TaskForceReport"/>

In November 2010, the NASA probe ''[[Engineering:Deep Impact (spacecraft)|Deep Impact]]'' flew by the near-Earth comet 103P/Hartley. Earlier, in July 2005, this probe flew by the non-near-Earth comet [[Astronomy:Tempel 1|Tempel 1]], hitting it with a large copper mass.<ref name="DeepImpactHartley">{{Cite news |title=Mr. Hartley's Amazing Comet |first=Kelly |last=Beatty |date=November 4, 2010 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-news/mr-hartleys-amazing-comet/ |access-date=2024-01-27 |url-status=live |archive-url=https://web.archive.org/web/20231020025244/https://skyandtelescope.org/astronomy-news/mr-hartleys-amazing-comet/ |archive-date=2023-10-20}}</ref>

In August 2014, ESA probe ''Rosetta'' began orbiting near-Earth comet 67P/Churyumov–Gerasimenko, while its lander ''[[Engineering:Philae (spacecraft)|Philae]]'' landed on its surface in November 2014. After the end of its mission, Rosetta was crashed into the comet's surface in 2016.<ref name="newsci20160930">{{cite news |first=Jacob |last=Aron |title=Rosetta lands on 67P in grand finale to two year comet mission |work=New Scientist |date=September 30, 2016 |url=https://www.newscientist.com/article/2107585-rosetta-lands-on-67p-in-grand-finale-to-two-year-comet-mission/ |access-date=2024-01-27}}</ref>
{{clear}}

== See also ==
{{div col|colwidth=20em}}
* [[Astronomy:Asteroid capture|Asteroid capture]]
* [[Astronomy:Asteroid Day|Asteroid Day]]
* [[Astronomy:Asteroid impact prediction|Asteroid impact prediction]]
* [[Astronomy:Asteroid Redirect Mission|Asteroid Redirect Mission]]
* [[Astronomy:Claimed moons of Earth|Claimed moons of Earth]]
* Euronear
* [[Astronomy:List of Earth-crossing minor planets|List of Earth-crossing minor planets]]
* [[Earth:List of impact craters on Earth|List of impact craters on Earth]]
* Near Earth Object Camera
* NEOShield 
* [[Astronomy:NEODyS|NEODyS]]
* [[Astronomy:Orbit@home|Orbit@home]]
{{div col end}}

== References ==
{{reflist|25em|refs=

<ref name="pia14734">{{cite web
  |date        = September 29, 2011
  |title       = WISE Revises Numbers of Asteroids Near Earth
  |publisher   = NASA/JPL
  |url         = https://www.jpl.nasa.gov/images/pia14734-wise-revises-numbers-of-asteroids-near-earth
  |access-date = 2024-01-27
  |url-status  = live
  |archive-url = https://web.archive.org/web/20240127112654/https://www.jpl.nasa.gov/images/pia14734-wise-revises-numbers-of-asteroids-near-earth
  |archive-date= 2024-01-27}}</ref>

<ref name="JPL-SSD-NEA">{{cite web
  |title      = JPL Small-Body Database Search Engine. Constraints: asteroids and NEOs
  |publisher  = [[Astronomy:JPL Small-Body Database|JPL Small-Body Database]]
  |date       = January 25, 2024
  |url        = https://ssd.jpl.nasa.gov/tools/sbdb_query.html
  |access-date = 2024-01-25
  |url-status  = live
  |archive-url = https://web.archive.org/web/20240123202045/https://ssd.jpl.nasa.gov/tools/sbdb_query.html
  |archive-date = 2024-01-23}}</ref>

<ref name="closest-NEA">{{cite web
  |title      = Closest Approaches to the Earth by Minor Planets
  |publisher  = IAU/MPC
  |url        = http://www.minorplanetcenter.net/iau/Closest.html
  |access-date = 2024-01-24}}</ref>

<ref name="closest-NEC">{{cite web
  |title      = Closest Approaches to the Earth by Comets
  |publisher  = IAU/MPC
  |url        = https://www.minorplanetcenter.net/iau/lists/ClosestComets.html
  |access-date = 2024-01-24}}</ref>

<ref name="Earth-impact">{{cite web
  |date       = 2010
  |title      = Earth Impact Effects Program
  |publisher  = Imperial College London / Purdue University
  |first1     = Robert
  |last1      = Marcus
  |first2     = H. Jay
  |last2      = Melosh
  |first3     = Gareth
  |last3      = Collins
  |name-list-style = amp
  |url        = https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/
  |access-date = 2024-01-25
  |url-status    = live
  |archive-url = https://web.archive.org/web/20240124034305/https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/
  |archive-date= 2024-01-24}} (solution using 2600&nbsp;kg/m^3, 17&nbsp;km/s, 45 degrees)</ref>

<ref name="IAU-NEOs">{{cite web
  |title      = Near Earth Objects
  |url        = https://www.iau.org/public/themes/neo/
  |publisher  = [[Organization:International Astronomical Union|IAU]]
  |access-date = 2024-01-27}}</ref>

<ref name="neo-jpl-stats">{{cite web
  |title      = Discovery Statistics – Cumulative Totals
  |publisher  = NASA/JPL CNEOS
  |url        = https://cneos.jpl.nasa.gov/stats/totals.html
  |date       = January 20, 2024
  |access-date = 2024-01-24}}</ref>

<ref name="Current_Impact_Risks">{{cite web
  |title      = Sentry Risk Table
  |url        = https://cneos.jpl.nasa.gov/sentry/
  |publisher  = NASA/JPL CNEOS
  |access-date = 2024-01-24
  |url-status  = live
  |archive-url = https://web.archive.org/web/20240124041047/https://cneos.jpl.nasa.gov/sentry/
  |archive-date= 2024-01-24}}</ref>

<ref name="h">{{cite web
  |title      = Asteroid Size Estimator
  |publisher  = NASA/JPL CNEOS
  |url        = https://cneos.jpl.nasa.gov/tools/ast_size_est.html
  |access-date = 2024-01-25}}</ref>

}} <!-- end of reflist -->

==External links==
* [http://cneos.jpl.nasa.gov/ Center for Near Earth Object Studies (CNEOS)] – [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]], [[Organization:NASA|NASA]]
* [http://www.brera.mi.astro.it/sormano/teca.html Table of Asteroids Next Closest Approaches to the Earth] – Sormano Astronomical Observatory
* [http://smallbodies.ru/en/ Catalogue of the Solar System Small Bodies Orbital Evolution] – Samara State Technical University
* [https://web.archive.org/web/20170513205505/http://szyzyg.arm.ac.uk/~spm/neo_map.html Current Map Of The Solar System] – [[Organization:Armagh Observatory|Armagh Observatory]]

; Minor Planet Center
* [http://www.minorplanetcenter.net/iau/NEO/toconfirm_tabular.html The NEO Confirmation Page]
* {{YouTube|id=Ar_h-zAccsY|title=Minor Planet Center: Asteroid Hazards, Part 2: ''The Challenge of Detection''}} {{small|(min. 7:14)}}
* {{YouTube|id=8M_27st1ZCI|title=Minor Planet Center: Asteroid Hazards, Part 3: ''Finding the Path''}} {{small|(min. 5:38)}}

{{Asteroids}}
{{Modern impact events}}
{{Planetary defense}}
{{Small Solar System bodies}}
{{Solar System}}

{{DEFAULTSORT:Near-Earth Object}}
[[Category:Near-Earth objects| ]]

{{Sourceattribution|Near-Earth object}}