View source for Astronomy:99942 Apophis

{{Short description|Potentially hazardous near-Earth asteroid}}
{{Infobox planet
| background         = #FFC2E0
| discoverer         = {{Ubl
 | [[Biography:Roy A. Tucker|Roy A. Tucker]]
 | [[Biography:David J. Tholen|David J. Tholen]]
 | Fabrizio Bernardi
 }}
| discovery_site     = [[Astronomy:Kitt Peak National Observatory|Kitt Peak]]<ref name=jpldata/>
| minorplanet        = yes
| name               = 99942 Apophis
| image              = 99942 Apophis shape.png
| image_scale         =
| caption            = Model of 99942 Apophis's shape, assuming the entire surface is of a similar composition.
| discovery_ref      = <ref name=jpldata/>
| discovered         = June 19, 2004
| earliest_precovery_date = March 15, 2004
| mpc_name           = (99942) Apophis
| pronounced         = {{IPAc-en|ə|ˈ|p|ɒ|f|ə|s}}, (trad.) {{IPAc-en|ˈ|æ|p|ə|f|ə|s}}
| adjective          = Apophidian {{IPAc-en|æ|p|ə|ˈ|f|ɪ|d|i|ə|n}} (Latin ''Apŏpidis'')
| alt_names          = {{mp|2004 MN|4}}
| named_after        = [[Unsolved:Apep|Ἄποφις]] ''Apophis''
| mp_category        = {{Hlist
 | Aten
 | [[Astronomy:Near-Earth object|NEO]]
 | [[Astronomy:Potentially hazardous object|PHA]]<ref name=jpldata/>
 }}
| orbit_ref          = <ref name=jpldata/>
| epoch              = 13&nbsp;September 2023<br/>([[Astronomy:Julian day|JD]]&nbsp;2453300.5)
| uncertainty        = 0
| observation_arc    = 6267 days (17.16&nbsp;yr)
| aphelion           = {{Convert|1.0994|AU|e6km|abbr=unit|lk=on}}
| period             = 0.89&nbsp;[[Astronomy:Julian year|yr]] (323.7&nbsp;[[Astronomy:Julian year|d]])
| semimajor          = {{Convert|0.9227|AU|e6km|abbr=unit}}
| perihelion         = {{Convert|0.7461|AU|e6km|abbr=unit}}
| eccentricity       = 0.19144
| inclination        = 3.339[[Degree (angle)|°]]
| asc_node           = 203.96°
| arg_peri           = 126.60°
| avg_speed          = 30.73&nbsp;km/[[Second|s]]
| mean_anomaly       = 142.9°
| mean_motion        = 1.11198°/day
| moid               = {{Convert|0.00026|AU|e3km|abbr=unit}}
| jupiter_moid       = {{Convert|4.12|AU|e6km|abbr=unit}}
| tisserand          = 6.464
| abs_magnitude      = {{Ubl
 | {{val|19.7|0.4}}<ref name=jpldata/><ref name=phys/>
 | {{val|19.09|0.19}}<ref name="Brozovic2018"/>
 | {{val|18.95|0.15}}<ref name="Neodys_eq1"/>
 }}
| dimensions         = {{Ubl
 | {{Convert|0.370|km|abbr=on}}
 | 0.45&thinsp;×&thinsp;0.17&nbsp;km<ref name="Brozovic2018"/>
 }}
| mean_radius        = {{Ubl
 | {{Convert|0.185|km|abbr=on}}
 | {{val|0.17|0.02|u=km}}<ref name="Brozovic2018"/>
 }}
| density            = {{Ubl
 | ~3.2&nbsp;g/cm{{sup|3}}<ref name=Binzel2007/>
 | 2.6&nbsp;g/cm{{sup|3}} (assumed)<ref name=riskneo/>
 }}
| mass               = {{Val|6.1e10|u=kg}} (assumed)<ref name=riskneo/>
| rotation           = {{Convert|30.4|h|d|abbr=on|lk=on}}<ref name=jpldata/><ref name=phys/><br />30.55±0.12&nbsp;h<ref name="Reddy2022"/><br />30.67±0.06&nbsp;h<ref name="IAWN2021-Gallery"/><br />Tumbling:<ref name=tumblingspinstate/><br />27.38±0.07&nbsp;h (precession period),<ref name=tumblingspinstate/> 263±6&nbsp;h (rotation period),<ref name=tumblingspinstate/> 30.56±0.01&nbsp;h (period of harmonic with strongest lightcurve amplitude)<ref name=tumblingspinstate/>
| spectral_type      = Sq<ref name=phys/>
| albedo             = {{Ubl
 | 0.23<ref name=esanews1/>
 | {{val|0.35|0.10}}<ref name="Brozovic2018"/>
 }}
| single_temperature = 270 [[Kelvin|K]]
}}

'''99942 Apophis''' (provisional designation '''{{mp|2004 MN|4}}''') is a near-Earth asteroid and a [[Astronomy:Potentially hazardous object|potentially hazardous object]] with a diameter of {{convert|370|m|ft|abbr=off}}<ref name=riskneo/> that caused a brief period of concern in December 2004 when initial observations indicated a probability up to 2.7% that it would hit [[Astronomy:Earth|Earth]] on April&nbsp;13, 2029. Additional observations provided improved predictions that eliminated the possibility of an [[Astronomy:Impact event|impact on Earth]] in 2029. Until 2006, a small possibility nevertheless remained that, during its 2029 close encounter with Earth, Apophis would pass through a [[Astronomy:Gravitational keyhole|gravitational keyhole]] of no more than about {{convert|800|km|mi|round=10}} in diameter,<ref name=keyhole/><ref name="Neil deGrasse Tyson - Death By Giant Meteor"/> which would have set up a future impact exactly seven years later on April&nbsp;13, 2036. This possibility kept it at Level&nbsp;1 on the [[Astronomy:Torino scale|Torino impact hazard scale]] until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis's rating on the Torino scale was lowered to zero. By 2008, the keyhole had been determined to be less than 1&nbsp;km wide.<ref name=keyhole/> During the short time when it had been of greatest concern, Apophis set the record for highest rating ever on the Torino scale, reaching level&nbsp;4 on December 27, 2004.<ref name=nasanews146/>

Preliminary observations by [[Astronomy:Goldstone Deep Space Communications Complex|Goldstone]] radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036.<ref name="NASA2013-017"/> By May 6, 2013 (April 15, 2013, observation arc), the possibility of an impact on April 13, 2036 had been eliminated altogether.<ref name=riskneo/> In 2036, Apophis will approach the Earth at a third the distance of the Sun in both March and December,<ref name=jpldata/> but this is about the distance of the planet [[Astronomy:Venus|Venus]] when it overtakes Earth every 1.6 years. On April 12, 2068, the nominal trajectory has Apophis {{convert|1.87|AU|e6km|abbr=unit}} from Earth.<ref name="NEODyS2068"/> Entering March 2021, six asteroids each had a more notable cumulative [[Palermo Technical Impact Hazard Scale]] rating than Apophis, and none of those has a Torino level above 0.<ref name="risk-table"/>{{efn|name=palermo}} On average, an asteroid the size of Apophis (370 metres) is expected to impact Earth once in about 80,000 years.<ref name="Earth-impact"/> Observations in 2020 by the Subaru telescope confirmed David Vokrouhlický's 2015 [[Yarkovsky effect]] predictions.<ref name="S&T2020"/> The Goldstone radar observed Apophis March 3–11, 2021, helping to refine the orbit again,<ref name="Goldstone2021"/> and on March 25, 2021, the [[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] announced that Apophis has no chance of impacting Earth in the next 100 years.<ref>{{cite web|url=https://www.jpl.nasa.gov/news/nasa-analysis-earth-is-safe-from-asteroid-apophis-for-100-plus-years|title=NASA Analysis: Earth Is Safe From Asteroid Apophis for 100-Plus Years|date=Mar 25, 2021|publisher=Jet Propulsion Laboratory}}</ref><ref>{{Cite web|last=Hurst|first=Luke|date=2021-03-28|title=Asteroid Apophis won't hit Earth for at least 100 years, says NASA|url=https://www.euronews.com/2021/03/28/good-news-asteroid-apophis-won-t-hit-earth-for-at-least-100-years-says-nasa|access-date=2021-04-02|website=euronews|language=en}}</ref> The uncertainty in the 2029 approach distance has been reduced from hundreds of kilometers to now just a couple of kilometers,<ref name="Horizons2029"/> greatly enhancing predictions of future approaches.

== Discovery and naming ==
[[File:PIA23195-AsteroidApophis-ClosestApproachToEarth-20190429.webm|thumb|center|600px|<div align="center">Asteroid Apophis{{Snd}} closest approach to Earth on April 13, 2029<ref name="NASA-20190429"/><br /><small>(00:20; VideoFile; April 29, 2019) (turquoise dots = artificial satellites; pink = International Space Station)</small></div>]]
Apophis was discovered on June 19, 2004, by [[Biography:Roy A. Tucker|Roy A. Tucker]], [[Biography:David J. Tholen|David J. Tholen]], and Fabrizio Bernardi at the [[Astronomy:Kitt Peak National Observatory|Kitt Peak National Observatory]].<ref name=jpldata/> On December 21, 2004, Apophis passed {{convert|0.0964|AU|e6km e6mi|abbr=unit|lk=on}} from Earth.<ref name=jpldata/> [[Astronomy:Precovery|Precovery]] observations from March 15, 2004, were identified on December&nbsp;27, and an improved orbit solution was computed.<ref name="MPEC2004-Y70"/><ref name="nasanews148"/> [[Astronomy:Radar astronomy|Radar]] astrometry in January 2005 further refined its orbit solution.<ref name=Arecibo/><ref name="nasanews149"/> The discovery was notable in that it was at a very low [[Astronomy:Elongation|solar elongation]] (56°) and at very long range (1.1&nbsp;AU).{{citationneeded|date=January 2024}}

When first discovered, the object received the provisional designation {{mp|2004 MN|4}}, and early news and scientific articles naturally referred to it by that name. Once its orbit was sufficiently well calculated, it received the permanent number 99942 (on June&nbsp;24, 2005). Receiving a permanent number made it eligible for naming by its discoverers, and they chose the name "Apophis" on July&nbsp;19, 2005.<ref name=naming/> Apophis is the [[Social:Greek language|Greek]] name of [[Unsolved:Apep|Apep]], an enemy of the Ancient Egyptian sun-god [[Religion:Ra|Ra]]. He is the Uncreator, an evil serpent that dwells in the eternal darkness of the [[Unsolved:Duat|Duat]] and tries to swallow Ra during his nightly passage. Apep is held at bay by [[Religion:Set (deity)|Set]], the Ancient Egyptian god of storms and the desert.<ref>{{Cite web|url=https://ancientegyptonline.co.uk/apep/|title=Apep (Apophis)|publisher=Ancient Egypt Online|author=Hill, J.|year=2010|access-date=24 July 2021}}</ref>

Tholen and Tucker, two of the co-discoverers of the asteroid, are reportedly fans of the television series ''Stargate SG-1''. One of the show's persistent villains is an alien named Apophis. He is one of the principal threats to the existence of civilization on Earth through the first few seasons, thus likely why the asteroid was named after him. In the fictional world of the show, the alien's backstory was that he had lived on Earth during ancient times and had posed as a god, thereby giving rise to the myth of the Egyptian god of the same name.<ref name=naming/>

The mythological creature Apophis is pronounced with the accent on the first syllable (/ˈæpəfɪs/). In contrast, the asteroid's name is generally accented on the second syllable (/əˈpɒfɪs/) as the name was pronounced in the TV series.

== Physical characteristics ==

[[File:Apophis model comparison.png|thumb|Comparison between the best-fit convex and nonconvex shape models, and some of the available radar images of (99942) Apophis]]

[[File:Size of Apophis asteroid.png|left|thumb|Comparison of possible size of Apophis asteroid to Eiffel Tower and Empire State Building]]

Based upon the observed brightness, Apophis's diameter was initially estimated at {{convert|450|m}}; a more refined estimate based on spectroscopic observations at [[Organization:NASA|NASA]]'s Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is {{convert|350|m}}. NASA's impact risk page lists the diameter at {{convert|330|m}} and lists a mass of 4{{e|10}} kg based on an assumed density of 2.6&nbsp;g/cm{{sup|3}}.<ref name=riskneo/> The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three.<ref name=riskneo/> Apophis's surface composition probably matches that of LL chondrites.<ref name=Reddy2018/>

Based on Goldstone and Arecibo radar images taken in 2012–2013, Brozović et al. have estimated that Apophis is an elongated object 450&thinsp;×&thinsp;170 metres in size, and that it is bilobed (possibly a [[Astronomy:Contact binary (small Solar System body)|contact binary]]) with a relatively bright surface albedo of {{val|0.35|0.10}}. Its rotation axis has an [[Astronomy:Obliquity|obliquity]] of −59° against the [[Astronomy:Ecliptic|ecliptic]], which means that Apophis is a retrograde rotator.<ref name="Brozovic2018"/>

During the 2029 approach, Apophis's brightness will peak at [[Astronomy:Apparent magnitude|magnitude]]&nbsp;3.1,<ref name="NEODyS2029"/> easily visible to the naked eye, with a maximum angular speed of 42° per hour. The maximum apparent [[Astronomy:Angular diameter|angular diameter]] will be approximately 2&nbsp;[[Arcsecond|arcsecond]]s. This is roughly equivalent to the [[Astronomy:Angular diameter|angular diameter]] of [[Astronomy:Neptune|Neptune]] from earth. Therefore, the asteroid will be barely [[Physics:Optical resolution|resolved]] by ground-based telescopes not equipped with [[Astronomy:Adaptive optics|adaptive optics]] but very well resolved by those that are.<ref name = NeptuneVLTAdaptiveOptics/> Because the approach will be so close, [[Astronomy:Tide|tidal forces]] are likely to alter Apophis's rotation axis. A partial resurfacing of the asteroid is possible, which might change its spectral class from a [[Astronomy:Space weathering|weathered]] Sq- to an unweathered [[Astronomy:Q-type asteroid|Q-type]].<ref name="Brozovic2018"/><ref name=Reddy2018/>

== Orbit ==
Apophis has a low inclination orbit (3.3°) that varies from just outside the orbit of [[Astronomy:Venus|Venus]] (0.746 AU) to just outside the orbit of [[Astronomy:Earth|Earth]] (1.099 AU).<ref name=jpldata/> After the 2029 Earth approach, the orbit will vary from just inside of Earth's to just inside of Mars's.

{| class="wikitable sortable" style="font-size: 0.9em;"
|+Position uncertainty and increasing divergence<ref name=jpldata/>
! Date
! JPL SBDB<br />[[Curve fitting|nominal]] geocentric<br />distance ([[Astronomy:Astronomical unit|AU]])
!class=unsortable| uncertainty<br />region<br />(3-sigma)
|-
|2004-12-21 ||{{Convert|0.09638|AU|e6km|abbr=unit}} || n/a 
|-
|2013-01-09 ||{{Convert|0.09666|AU|e6km|abbr=unit}} || n/a 
|-
|2029-04-13 ||{{Convert|0.000254128|AU|km|abbr=unit}} || ±3.4&nbsp;km<ref name="Horizons2029"/>
|-
|2036-03-27 ||{{Convert|0.309756|AU|e6km|abbr=unit}} || ±130 thousand km<ref name="Horizons2036"/>
|-
|2051-04-20 ||{{Convert|0.041455|AU|e6km|abbr=unit}} || ±250 thousand km
|-
|2066-09-16 ||{{Convert|0.069562|AU|e6km|abbr=unit}} || ±910 thousand km
|-
|2116-04-12 ||{{Convert|0.019462|AU|e6km|abbr=unit}} || ±13 million km<ref name="Horizons2116"/>{{efn|name=Apophis2116}}
|-
|2117-10-07 ||{{Convert|0.48|AU|e6km|abbr=unit}} || ±37 million km<ref name="Horizons2117"/>
|}

=== 2029 close approach ===
The closest known approach of Apophis occurs at April&nbsp;13, 2029 21:46 UT, when Apophis will pass Earth closer than geosynchronous communication satellites, but will come no closer than {{convert|31600|km|lk=on}} above Earth's surface.<ref name=surface>2029-Apr-13 approach: {{Convert|0.000254093|AU|km|abbr=on|lk=on}}. [https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=Apophis&view=OPC 38012km "geocentric distance"] – 6378km "[[Astronomy:Earth radius|Earth radius]]" = 31634km</ref><ref name=blog2023>{{Cite web|url=https://blogs.nasa.gov/osiris-rex/2023/09/24/osiris-rex-spacecraft-departs-for-new-mission/ |title=OSIRIS-REx Spacecraft Departs for New Mission|date=September 24, 2023 }}</ref> Using the June 2021 orbit solution which includes the [[Yarkovsky effect]], the 3-sigma uncertainty region in the 2029 approach distance is about ±3.4&nbsp;km.<ref name="Horizons2029"/><ref name=jpldata/> The distance, a hair's breadth in astronomical terms, is five times the [[Astronomy:Earth radius|radius of the Earth]], ten times closer than the [[Astronomy:Moon|Moon]],<ref name=blog2023/> and closer than the ring of [[Astronomy:Geostationary|geostationary]] satellites currently orbiting the Earth.<ref>{{Cite web|url=https://www.satsig.net/sslist.htm|title=List of satellites in geostationary orbit|website=satsig.net}}</ref><ref name="earthsky-preparing"/> It will be the closest asteroid of its size in recorded history. On that date, it will become as bright as magnitude&nbsp;3.1<ref name="NEODyS2029"/> (visible to the [[Astronomy:Naked eye|naked eye]] from rural as well as darker suburban areas, visible with binoculars from most locations).<ref name="mag-scale"/> The close approach will be visible from [[Earth:Europe|Europe]], [[Earth:Africa|Africa]], and western [[Earth:Asia|Asia]]. During the approach, Earth will [[Astronomy:Perturbation|perturb]] Apophis from an [[Astronomy:Aten asteroid|Aten]]-class orbit with a semi-major axis of 0.92&nbsp;[[Astronomy:Astronomical unit|AU]] to an [[Astronomy:Apollo asteroid|Apollo]]-class orbit with a semi-major axis of 1.1&nbsp;AU.<ref name="Orbit2029"/> Perihelion will lift from 0.746&nbsp;AU to 0.895&nbsp;AU and aphelion will lift from 1.10&nbsp;AU to 1.31&nbsp;AU.<ref name="Orbit2029"/>

{| class=wikitable style="font-size: 0.9em;"
|+Orbital elements for 2029 (pre-flyby) and 2030 (post-flyby)<ref name="Orbit2029"/>
|- valign=top
! Parameter
! [[Astronomy:Epoch|Epoch]]
! Orbit<br />type
! [[Astronomy:Orbital period|Orbital<br/>period]]
! Semi-major<br />axis
! [[Astronomy:Apsis|Perihelion]]
! [[Astronomy:Apsis|Aphelion]]
! [[Astronomy:Orbital inclination|Inclination]]
! [[Astronomy:Orbital eccentricity|Eccentricity]]
|- valign=top
! Units
!
!
!
!colspan=3|[[Astronomy:Astronomical unit|AU]]
!(°)
!
|- align=center
| Pre-flyby
| 2029
| [[Astronomy:Aten asteroid|Aten]]
| {{convert|323.6|days|years|abbr=off|sigfig=2|order=flip}}
| 0.922
| 0.746
| 1.10
| 3.34°
| 0.191
|- align=center
| Post-flyby
| 2030
| [[Astronomy:Apollo asteroid|Apollo]]
| {{convert|423.1|days|years|abbr=off|sigfig=3|order=flip}}
| 1.103
| 0.895
| 1.31
| 2.22°
| 0.189
|}

{{multiple image
| align = left
| direction = horizontal
| width = 300
| image1 = Animation of 99942 Apophis orbit around Sun.gif
| caption1 = Around Sun
| image2 = Animation of 99942 Apophis orbit around Earth.gif
| caption2 = Around Earth
| footer ={{legend2| yellow| Sun}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2| Magenta| 99942 Apophis }}{{·}}{{legend2| Gold| Moon}}
| header = Animation of 99942 Apophis orbit in 2028–2029
}}
{{Clear}}


=== 2036 approaches ===
In 2036, Apophis will pass the Earth at a third the distance of the Sun in both March and December.<ref name=jpldata/> Using the 2021 orbit solution, the Earth approach on March 27, 2036, will be no closer than {{convert|0.3089|AU|e6km e6mi LD|abbr=unit|lk=in}}, but more likely about {{convert|0.3097|AU|e6km e6mi|abbr=unit}}.<ref name=jpldata/> For comparison, the planet Venus will be closer to Earth at {{convert|0.2883|AU|e6km e6mi LD|abbr=unit|lk=off}} on May 30, 2036.<ref name=Venus>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27299%27&START_TIME=%272036-05-30%2002:10%27&STOP_TIME=%272036-05-30%2002:30%27&STEP_SIZE=%271%20min%27&QUANTITIES=%2720%27|title=Venus 2036-May-30 @ JPL Horizons}}</ref>{{efn|name=Venus2022}} On 31 December 2036 Apophis will be a little bit further away than the March approach at about {{convert|0.33|AU|e6km e6mi|abbr=unit}}.

=== 2051 approach ===
Around April 19–20, 2051, Apophis will pass about {{convert|0.04|AU|e6km e6mi|abbr=unit|lk=off}} from Earth and it will be the first time since 2029 that Apophis will pass within 10 million km of Earth.<ref name=jpldata/>

=== 2066 and 2068 ===

In the 2060s, Apophis will approach Earth in September 2066,<ref name=jpldata/><ref name=NEODys/> and then from February 2067 to December 2071, Apophis will remain farther from Earth than the Sun is.<ref name="NEODyS2060s"/> On April 12, 2068, [[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]] calculates that Apophis will be about {{convert|1.864|+/-|0.003|AU|e6km|abbr=unit|lk=off}} from Earth,<ref name="Horizons2068"/><ref name="NEODyS2068"/> making the asteroid much further than the Sun.

By 2116, the [[Astronomy:JPL Small-Body Database|JPL Small-Body Database]] and [[Astronomy:NEODyS|NEODyS]] close approach data start to become [[Chaos theory|divergent]].<ref name=jpldata/><ref name=NEODys/> In April 2116, Apophis is expected to pass about {{convert|0.02|AU|e6km LD|abbr=unit|sigfig=1}} from Earth, but could pass as close as {{convert|0.001|AU|e3km LD|abbr=unit}} or could pass as far as {{convert|0.1|AU|e6km LD|abbr=unit}}.<ref name=jpldata/>

=== Refinement of close approach predictions ===
Six months after discovery, and shortly after a close approach to Earth on December 21, 2004, the improved orbital estimates led to the prediction of a very close approach on April 13, 2029, by both NASA's automatic [[Astronomy:Sentry (monitoring system)|Sentry]] system and [[Astronomy:NEODyS|NEODyS]], a similar automatic program run by the [[Organization:University of Pisa|University of Pisa]] and the [[Organization:University of Valladolid|University of Valladolid]]. Subsequent observations decreased the uncertainty in Apophis's trajectory. The probability of an impact event in 2029 temporarily climbed, peaking at 2.7% (1 in 37) on December 27, 2004,<ref name="nasanews164"/><ref name="mirror"/> when the uncertainty region had shrunk to 83,000&nbsp;km.<ref name=VI2029/> This probability, combined with its size, caused Apophis to be assessed at level 4 on the [[Astronomy:Torino scale|Torino scale]]<ref name="nasanews146"/> and 1.10 on the [[Palermo Technical Impact Hazard Scale]], scales scientists use to represent how dangerous a given asteroid is to Earth. These are the highest values for which any object has been rated on either scale. The chance that there would be an impact in 2029 was eliminated by late December 27, 2004, as a result of a [[Astronomy:Precovery|precovery]] image that extended the observation arc back to March 2004.<ref name="nasanews148"/> The danger of a 2036 passage was lowered to level 0 on the Torino scale in August 2006.<ref name="Wayback2006-08"/> With a cumulative Palermo Scale rating of −3.22,<ref name="riskneo"/> the risk of impact from Apophis is less than one thousandth the background hazard level.<ref name="riskneo"/>

==== 2005 and 2011 observations ====
In July 2005, former [[Astronomy:Apollo program|Apollo]] astronaut [[Biography:Rusty Schweickart|Rusty Schweickart]], as chairman of the [[Organization:B612 Foundation|B612 Foundation]], formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be in orbital resonance with Earth, which would increase the probability of future impacts. Schweickart also asked NASA to investigate whether a [[Engineering:Transponder|transponder]] should be placed on the asteroid to enable more accurate tracking of how its orbit is affected by the [[Yarkovsky effect]].<ref name="Schweickart"/> On January 31, 2011, astronomers took the first new images of Apophis in more than 3 years.<ref name="Tholen"/>

[[File:Apophis ellipse.svg|thumb|right|Illustration of a common trend where progressively reduced uncertainty regions result in an asteroid impact probability increasing followed by a sharp decrease]]

==== 2013 refinement ====
The close approach in 2029 will substantially alter the object's orbit, prompting Jon Giorgini of JPL to say in 2011, "If we get radar ranging in 2013 [the next good opportunity], we should be able to predict the location of {{mp|2004 MN|4}} out to at least 2070."<ref name="Science"/> Apophis passed within {{convert|0.0966|AU|e6km e6mi|abbr=unit|lk=off}} of Earth in 2013, allowing astronomers to refine the trajectory for future close passes.<ref name=esanews1/><ref name=NEODys/><ref name="refine2013"/> Just after the closest approach on January 9, 2013,<ref name=NEODys/> the asteroid peaked at an [[Astronomy:Apparent magnitude|apparent magnitude]] of about 15.6.<ref name="NEODyS2013"/> The [[Astronomy:Goldstone Deep Space Communications Complex|Goldstone]] radar observed Apophis during that approach from January 3 through January 17.<ref name="Goldstone"/> The [[Astronomy:Arecibo Observatory|Arecibo Observatory]] observed Apophis once it entered Arecibo's declination window after February 13, 2013.<ref name="Goldstone"/> The 2013 observations basically ruled out any chance of a 2036 impact.

A NASA assessment as of February 21, 2013, that did not use the January and February 2013 radar measurements gave an impact probability of 2.3 in a million for 2068.<ref name=news178/> As of May 6, 2013, using observations through April 15, 2013, the odds of an impact on April 12, 2068, as calculated by the JPL Sentry risk table had increased slightly to 3.9 in a million (1 in 256,000).<ref name=riskneo/>

==== 2015 observations ====
As of January 2019, Apophis had not been observed since 2015, mostly because its orbit kept it very near the Sun from the perspective of Earth. It was not further than 60 degrees from the Sun between April 2014 and December 2019. With the early 2015 observations, the April 12, 2068, impact probability was 6.7 in a million (1 in 150,000), and the asteroid had a cumulative 9 in a million (1 in 110,000) chance of impacting Earth before 2106.<ref name="Sentry_Mar2015"/>

==== 2020–21 observations ====
[[File:Apophis February 2021 NBO.webm|thumb|right|Apophis in February 2021]]
No observations of Apophis were made between January 2015 and February 2019, and then observations started occurring regularly in January 2020.<ref name="mpc"/> In March 2020, astronomers David Tholen and Davide Farnocchia measured the acceleration of Apophis due to the [[Yarkovsky effect]] for the first time, significantly improving the prediction of its orbit past the 2029 flyby. Tholen and Farnocchia found that the Yarkovsky effect caused Apophis to drift by about 170 meters per year.<ref name="TholenFarmocchia"/> In late 2020, Apophis approached the Earth again. It passed {{convert|0.11265|AU|e6km LD|abbr=unit}} from Earth on March 6, 2021, brightening to +15 [[Astronomy:Apparent magnitude|mag]] at the time. Radar observations of Apophis were planned at Goldstone in March 2021.<ref name="Goldstone2021"/> The asteroid has been observed by NEOWISE (between December 2020 and April 2021)<ref name="Satpathy2022"/><ref name="FrankfurterRundschau_20210304"/> and by NEOSSat (in January 2021).<ref name="Reddy2022"/><ref name="IAWN2021"/><ref name="IAWN2021-Gallery"/> Apophis was the target of an observing campaign by IAWN, resulting in the collection of light curves, [[Physics:Astronomical spectroscopy|spectra]], and [[Astronomy:Astrometry|astrometry]].<ref name="Reddy2022"/><ref name="IAWN2021"/><ref name="IAWN2021-Gallery"/> The observations were used to practice and coordinate the response to an actual impact threat.<ref name="PlanetaryDefense2021"/>

[[File:Apophis impact risk corridor 2029.jpg|thumb|left|Hypothetical risk corridor for an impact on 13 April 2029, based on the 2020–21 planetary defense exercise]]

On February 21, 2021, Apophis was removed from the Sentry Risk Table, as an impact in the next 100 years was finally ruled out.<ref name=removed>[https://cneos.jpl.nasa.gov/sentry/removed.html Removed Objects] from Sentry Risk Table</ref>

Several occultations of bright stars ([[Astronomy:Apparent magnitude|apparent magnitude]] 8–11) by Apophis occurred in March and April 2021.<ref name="JOA2021_3"/><ref name="Gaia_20210329"/><ref name="Tanga2021"/><ref name="Occultation award"/> A total of five separate occultations were observed successfully, marking the first time that an asteroid as small as Apophis was observed using the occultation method (beating the previous record set in 2019 by asteroid 3200 Phaeton by more than a factor of ten).<ref name="Gaia_20210329"/> The first event, on March 7, was successfully observed from the United States by multiple observers.<ref name="SETI2021"/><ref name="Occultation_prelim"/><ref name="JOA2021_3"/> The next potential occultation, which occurred on March 11, was predicted to be visible from central Europe.<ref name="Tanga2021"/> This event was missed, mainly because of bad weather (two negative observations were recorded from Greece).<ref name="Gaia_20210329"/> On March 22, another occultation was observed only by a single observer from the United States, [[Astronomy:Amateur astronomy|amateur astronomer]] Roger Venable. Larger-than-expected [[Errors and residuals|residuals]] in the March 7 data had caused the majority of observers to be deployed outside of the actual path for the March 22 occultation.<ref name="JOA2021_3"/> This single detection then allowed the prediction of several more events that would have been unobservable otherwise, including an occultation on April 4, which was observed from New Mexico, again by Venable, alongside others.<ref name="Occultation award"/><ref name="JOA2021_3"/> Two more occultations, observable on April 10 and April 11 from {{wipe|Japan}} and New Mexico, respectively, were seen by several observers each.<ref name="JOA2021_3"/>

On March 9, 2021, using radar observations from Goldstone taken on March 3–8 and three positive detections of the stellar occultation on March 7, 2021,<ref name="vvs.be"/> Apophis became the asteroid with the most precisely measured Yarkovsky effect of all asteroids, at a [[Physics:Signal-to-noise ratio|signal-to-noise ratio]] (SNR) of 186.4,<ref name="JPLdata_20210309"/>{{efn|name=Yarkovsky_20210309}} surpassing [[Astronomy:101955 Bennu|101955 Bennu]] (SNR=181.6).<ref name="YarkovskySNR"/>

The 2021 apparition was the last opportunity to observe Apophis before its 2029 flyby.<ref name=jpldata/>
[[File:PIA24168-Asteroid-99942Apophis-RadarImages-20210326.jpg|thumb|center|600px|<div align="center">Asteroid 99942 Apophis – radar observations March 8–10, 2021 (March 26, 2021)</div>]]

=== History of impact estimates ===

{| class="wikitable sortable"
|-
! Date !! Time !! Status
|-
| rowspan=2 style="white-space: nowrap;" | 2004-12-23 ||style="white-space: nowrap;" | || The original NASA report mentioned impact chances of "around 1 in 300" in 2029, which was widely reported in the media.<ref name=nasanews146/> The actual NASA estimates at the time were 1 in 233; these resulted in a Torino scale rating of 2, the first time any asteroid had received a rating above 1.
|-
| || Later that day, based on a total of 64 observations, the estimates were changed to 1 in 62 (1.6%), resulting in an update to the initial report and an upgrade to a Torino scale rating of 4.
|-
| 2004-12-25 || || The chances were first reported as 1 in 42 (2.4%) and later that day (based on 101 observations) as 1 in 45 (2.2%). At the same time, the asteroid's estimated diameter was lowered from 440&nbsp;m to 390&nbsp;m and its mass from 1.2×10<sup>11</sup>&nbsp;kg to 8.3×10<sup>10</sup>&nbsp;kg.
|-
| 2004-12-26|| || Based on a total of 169 observations, the impact probability was still estimated as 1 in 45 (2.2%), the estimates for diameter and mass were lowered to 380&nbsp;m and 7.5×10<sup>10</sup>&nbsp;kg, respectively.
|-
|rowspan=2 | 2004-12-27|| || Based on a total of 176 observations with an [[Astronomy:Observation arc|observation arc]] of 190 days, the impact probability was raised to 1 in 37 (2.7%)<ref name="mirror"/> with a line of variation (LOV) of only 83,000&nbsp;km;<ref name=VI2029>[https://web.archive.org/web/20050314032111/https://astro.uni-bonn.de/~dfischer/mirror/285neo041227.html Virtual Impactor for 2029-04-13] (Stretch LOV = 1.29E+1) * Earth radius of 6,420&nbsp;km = 82,818&nbsp;km.</ref> diameter was increased to 390&nbsp;m, and mass to 7.9×10<sup>10</sup>&nbsp;kg.
|-
| || Later that afternoon, a [[Astronomy:Precovery|precovery]] increased the span of observations to 287 days, which eliminated the 2029 impact threat.<ref name="nasanews148"/> The cumulative impact probability was estimated to be around 0.004%, a risk lower than that of asteroid {{mpl|2004 VD|17}}, which once again became the greatest-risk object. A 2053 approach to Earth still poses a minor risk of impact, and Apophis was still rated at level one on the Torino scale for this orbit.
|-
| 2004-12-28 || style="white-space: nowrap;" | 12:23 GMT || Based on a total of 139 observations, a value of one was given on the Torino scale for 2044-04-13.29 and 2053-04-13.51.
|-
|rowspan=2 | 2004-12-29 || 01:10 GMT || The only pass rated 1 on the Torino scale was for 2053-04-13.51 based on 139 observations spanning 287.71 days (2004-Mar-15.1104 to 2004-Dec-27.8243). ({{As of|2013|2}} the 2053 risk is only 1 in 20 billion.)<ref name=riskneo/>
|-
| 19:18 GMT || This was still the case based upon 147 observations spanning 288.92&nbsp;days (2004-Mar-15.1104 to 2004-Dec-29.02821), though the close encounters have changed and been reduced to 4 in total.
|-
|rowspan=2 | 2004-12-30 || 13:46 GMT || No passes were rated above 0, based upon 157&nbsp;observations spanning 289.33&nbsp;days (2004-Mar-15.1104 to 2004-Dec-29.44434). The most dangerous pass was rated at 1 in 7,143,000.
|-
| 22:34 GMT || 157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). One pass at 1 (Torino scale) 3 other passes.
|-
| 2005-01-02 || 03:57 GMT || Observations spanning 290.97 days (2004-Mar-15.1104 to 2004-Dec-31.07992) One pass at 1 (Torino scale) 19 other passes.
|-
| 2005-01-03 || 14:49 GMT || Observations spanning 292.72 days (2004-Mar-15.1104 to 2005-Jan-01.82787) One pass at 1 (Torino scale) 15 other passes.
|-
| 2005–01 || || Extremely precise [[Engineering:Radar|radar]] observations at [[Astronomy:Arecibo Observatory|Arecibo Observatory]]<ref name=Arecibo/> refine the orbit further and show that the April 2029 close approach will occur at only 5.7&nbsp;Earth radii,<ref name="nasanews149"/> approximately one-half the distance previously estimated.
|-
| 2005-02-06 || || Apophis ({{mp|2004 MN|4}}) had a 1-in-13,000 chance of impacting in April 2036.<ref name="Wayback2005-02"/>
|-
| 2005-08-07 || ||Radar observation<ref name=Arecibo/> refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino scale&nbsp;1 (with a 1-in-5,560 chance of impact).<ref name="Wayback2005-10"/>
|-
| 2005–10 || || It is predicted that Apophis will pass just below the altitude of [[Astronomy:Geosynchronous satellite|geosynchronous satellite]]s, which are at approximately {{convert|35900|km|mi}}.<ref name="Wee2012"/> Such a close approach by an asteroid of that size is estimated to occur every 800 years or so.<ref name="neo"/>
|-
| 2006-05-06 || || Radar observation at Arecibo Observatory<ref name=Arecibo/> slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino scale&nbsp;1<ref name="Wayback2006-07"/> despite the impact probability dropping by a factor of four.
|-
| 2006-08-05 || || Additional observations through 2006 resulted in Apophis being lowered to Torino scale&nbsp;0.<ref name="Wayback2006-08"/> (The impact probability was 1 in 45,000.)<ref name="Wayback2006-08"/>
|-
| 2008-04 || || Nico Marquardt published a research paper in which he calculated the probability of Apophis to collide with a [[Astronomy:Geosynchronous satellite|geosynchronous satellite]] during its flyby on April 13, 2029, and the consequences of this event to the likelihood of an Earth-collision 2036. Afterwards, the German newspaper ''Bild'' published an article stating a 100 times higher probability of an Earth-collision in the year 2036 than Marquardt calculated.<ref name="Bild.de"/> Nearly all international press reported the news with false data caused by the review from ''Bild'' even though Marquardt denied.<ref name="RadioCanada"/> This estimate was allegedly confirmed by ESA and NASA<ref name="Schoolboy"/><ref name="Bild.de"/> but in an official statement,<ref name="NASAkid"/> NASA denied the wrong statement. The release went on to explain that since the angle of Apophis's approach to the Earth's equator means the asteroid will not travel through the belt of current equatorial geosynchronous satellites, there is currently no risk of collision; and the effect on Apophis's orbit of any such impact would be insignificant.
|-
| 2008-04-16 || || NASA News Release 08-103 reaffirmed that its estimation of a 1-in-45,000 chance of impact in 2036 remained valid.<ref name="NASAkid"/>
|-
| 2009-04-29 || || An animation is released<ref name="ApophisBias"/> that shows how unmeasured physical parameters of Apophis bias the entire statistical uncertainty region. If Apophis is a ''retrograde'' rotator on the small, less-massive end of what is possible, the measurement uncertainty region will get pushed back such that the center of the distribution encounters Earth's orbit. This would result in an impact probability much higher than computed with the Standard Dynamical Model. Conversely, if Apophis is a small, less-massive ''prograde'' rotator, the uncertainty region is advanced along the orbit. Only the remote tails of the probability distribution could encounter Earth, producing a negligible impact probability.
|-
| 2009-10-07 || || Refinements to the [[Astronomy:Precovery|precovery]] images of Apophis by the University of Hawaii's [[Organization:Institute for Astronomy (Hawaii)|Institute for Astronomy]], the 90-inch [[Astronomy:Bok Telescope|Bok Telescope]], and the [[Astronomy:Arecibo Observatory|Arecibo Observatory]] have generated a refined path that reduces the odds of an April 13, 2036, impact to about 1 in 250,000.<ref name=Brown2009/>
|-
| || || Criticism of older published impact probabilities rests on the fact that important physical parameters such as mass and spin that affect its precise trajectory have not yet been accurately measured and hence there are no associated probability distributions. The Standard Dynamical Model used for making predictions simplifies calculations by assuming Earth is a point mass; this can introduce up to 2.9 Earth radii of prediction error for the 2036 approach, and Earth's oblateness must be considered for the 2029 passage to predict a potential impact reliably.<ref name="neo"/> Additional factors that can greatly influence the predicted motion in ways that depend on unknown details, are the spin of the asteroid,<ref name="TrajectoryChange"/> its precise mass, the way it reflects and absorbs sunlight, radiates heat, and the gravitational pull of other asteroids passing nearby.<ref name="neo"/> Small uncertainties in the masses and positions of the [[Astronomy:Planet|planet]]s and [[Astronomy:Sun|Sun]] can cause up to 23 Earth radii of prediction error for Apophis by 2036.<ref name="neo"/>
|-
| 2013-01 || || A statistical impact risk analysis of the data up to this point calculated that the odds of the 2036 impact at 7.07 in a billion, effectively ruling it out. The same study looked at the odds of an impact in 2068, which were calculated at 2.27 in a million.<ref name="Farnocchia2013_Icarus"/> First appearance of Sentry virtual impactors that also include mid-October dates.<ref name="Wayback2013-01"/>
|-
| 2013-01-09 || || The [[Organization:European Space Agency|European Space Agency]] (ESA) announced the [[Astronomy:Herschel Space Observatory|Herschel Space Observatory]] made new thermal infrared observations of the asteroid as it approached Earth. The initial data shows the asteroid to be bigger than first estimated because it is now expected to be less reflective than originally thought.<ref name=esanews1/> The Herschel Space Observatory observations increased the diameter estimate by 20% from 270 to 325 metres, which translates into a 75% increase in the estimates of the asteroid's volume or mass.<ref name=esanews1/> Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact.<ref name="NASA2013-017"/><ref name="Beatty2013"/><ref name="Plait2013"/> Apophis will then come no closer than about {{convert|14|e6mi|e6km|order=flip}}—and more likely miss us by something closer to {{convert|35|e6mi|e6km|order=flip}}.<ref name="Beatty2013"/> The radar astrometry is more precise than was expected.<ref name="Beatty2013"/>
|-
| 2016-03-25 || || The Sentry Risk Table assessed Apophis as having a 6.7-in-a-million (1-in-150,000) chance of impacting Earth in 2068, and a 9-in-a-million (1-in-110,000) cumulative chance of impacting Earth by 2105.<ref name=riskneo/>
|-
| 2020-03 || || By taking observations of Apophis with the [[Astronomy:Subaru Telescope|Subaru Telescope]] in January and March 2020, as well as remeasuring older observations using the new Gaia DR2 star catalog, astronomers positively detect the [[Yarkovsky effect]] on Apophis. The asteroid's position is found to shift by 170 meters per year. The Yarkovsky effect is the main source of uncertainty in impact probability estimates for this asteroid.<ref name="TholenFarmocchia"/>
|-
| 2021-01-20 || || The Sentry Risk Table assessed Apophis as having a 2.6-in-a-million (1-in-380,000) chance of impacting Earth in 2068, and a 4.5-in-a-million (1-in-220,000) cumulative chance of impacting Earth by 2107.<ref name=riskneo/> The previous 2068 odds were 1-in-150,000.
|-
| 2021-03-15 || 10:44 || JPL solution #207 using observations in 2020 and 2021 reduced the 3-sigma uncertainty region in the 2029 approach distance from ±700&nbsp;km<ref name=JPLSBDB2020>{{Cite web|url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=99942;cad=1|title=JPL Small-Body Database Browser|date=November 6, 2020|archive-url=https://web.archive.org/web/20201106120010/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=99942;cad=1|archive-date=2020-11-06}}</ref> to about ±3&nbsp;km.<ref name=jpldata/> The June 2021 solution showed the Earth approach on March 27, 2036, will be no closer than {{convert|0.30889|AU|e6km e6mi LD|abbr=unit|lk=in}}.<ref name=jpldata/>
|}

== Possible impact effects ==
The Sentry Risk Table estimates that Apophis would impact Earth with [[Physics:Kinetic energy|kinetic energy]] equivalent to 1,200&nbsp;[[Physics:TNT equivalent|megatons of TNT]]. 
In comparison, the [[Earth:Chicxulub crater|Chicxulub impact]] which caused the mass extinction event responsible for wiping out the dinosaurs has been estimated to have released about as much energy as 100,000,000 megatons (100 teratons). The exact effects of any impact would vary based on the asteroid's composition, and the location and angle of impact. Any impact would be extremely detrimental to an area of thousands of square kilometres, but would be unlikely to have long-lasting global effects, such as the initiation of an [[Astronomy:Impact winter|impact winter]].{{Citation needed|date=January 2010}} Assuming Apophis is a {{convert|370|m|ft|adj=mid|-wide}} stony asteroid with a density of 3,000&nbsp;kg/m<sup>3</sup>, if it were to impact into [[Earth:Sedimentary rock|sedimentary rock]], Apophis would create a {{convert|5.1|km|ft|adj=mid}} [[Astronomy:Impact crater|impact crater]].<ref name="Earth-impact"/><ref name=riskneo/>

===Expired 2036 path of risk===
In 2008, the [[Organization:B612 Foundation|B612 Foundation]] made estimates of Apophis's path if a 2036 Earth impact were to occur, as part of an effort to develop viable deflection strategies.<ref name="Schweickart2"/> The result was a narrow corridor a few kilometres wide, called the "path of risk", extending across southern {{wipe|Russia}}, across the north Pacific (relatively close to the coastlines of {{wipe|California}} and Mexico), then right between Nicaragua and Costa Rica, crossing northern Colombia and Venezuela, ending in the Atlantic, just before reaching [[Earth:Africa|Africa]].<ref>''Range of Possible Impact Points on April 13, 2036'' in [http://www.aero.org/conferences/planetarydefense/2007papers/S3-4--Gennery-Brief.pdf Scenarios for Dealing with Apophis], by Donald B. Gennery, presented at the Planetary Defense Conference. Washington, DC. March 5–8, 2007 (archived from [https://web.archive.org/web/20120412123940/http://www.aero.org/conferences/planetarydefense/2007papers/S3-4--Gennery-Brief.pdf the original] on 2012-04-12).</ref> Using the computer simulation tool NEOSim, it was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which were in the path of risk, could have more than 10 million casualties.<ref name="Baileya2006"/> A deep-water impact in the Atlantic or Pacific oceans would produce an incoherent short-range [[Tsunami|tsunami]] with a potential destructive radius (inundation height of >2&nbsp;m) of roughly {{convert|1000|km}} for most of North America, Brazil and Africa, {{convert|3000|km|abbr=on}} for Japan and {{convert|4500|km|abbr=on}} for some areas in Hawaii.<ref name="Paine1999"/>

== Exploration ==

=== OSIRIS-APEX post-Earth-encounter rendezvous ===
The ''[[Astronomy:OSIRIS-REx|OSIRIS-REx]]'' spacecraft returned a sample of Bennu to Earth on 24 September 2023.<ref>{{cite report |title=OSIRIS-REx factsheet |date=August 2011 |publisher=[[Organization:NASA|NASA]] |department=Explorers and Heliophysics Projects Division |place=[[Organization:Goddard Space Flight Center|Goddard SFC]] |website=ehpd.gsfc.nasa.gov |url=https://ehpd.gsfc.nasa.gov/documents/552572main_OSIRIS_REx_Factsheet.pdf}} {{PD-notice}}</ref> After ejecting the sample canister, the spacecraft can use its remaining fuel to target another body during an extended mission. Apophis is the only asteroid which the spacecraft could reach for a long-duration rendezvous, rather than a brief [[Engineering:Planetary flyby|flyby]]. In April 2022, the extension was approved, and ''OSIRIS-REx'' will perform a rendezvous with Apophis in April 2029, a few days after the close approach to Earth. It will study the asteroid for 18 months and perform a maneuver similar to the one it made during sample collection at Bennu, by approaching the surface and firing its thrusters. This will expose the asteroid's subsurface and allow mission scientists to learn more about the asteroid's material properties.<ref name="Bartels2021"/><ref name="Lauretta2020"/> For its Apophis mission after the sample return, ''OSIRIS-REx'' was renamed ''OSIRIS-APEX'' (short for OSIRIS-Apophis Explorer).<ref>{{Cite web |date=2022-04-25 |title=NASA gives green light for OSIRIS-REx spacecraft to visit another asteroid |url=https://news.arizona.edu/story/nasa-gives-green-light-osiris-rex-spacecraft-visit-another-asteroid |access-date=2022-04-26 |website=University of Arizona News |first=Mikayla |last=Mace Kelley |language=en}}</ref>

=== Other proposed space missions ===
==== Planetary Society competition ====
In 2007, The Planetary Society, a {{wipe|California}}-based space advocacy group, organized a $50,000 competition to design an uncrewed space probe that would 'shadow' Apophis for almost a year, taking measurements that would "determine whether it will impact Earth, thus helping governments decide whether to mount a deflection mission to alter its orbit". The society received 37&nbsp;entries from 20&nbsp;countries on 6&nbsp;continents.

The commercial competition was won by a design called 'Foresight' created by SpaceWorks Enterprises, Inc.<ref name="competition"/> SpaceWorks proposed a simple orbiter with only two instruments and a radio beacon at a cost of ~US$140&nbsp;million, launched aboard a [[Engineering:Minotaur (rocket family)|Minotaur IV]] between 2012 and 2014, to arrive at Apophis five to ten months later. It would then rendezvous with, observe, and track the asteroid. Foresight would orbit the asteroid to gather data with a multi-spectral imager for one month. It would then leave orbit and fly in formation with Apophis around the Sun at a range of {{Convert|2|km|spell=in|abbr=off}}. The spacecraft would use laser ranging to the asteroid and radio tracking from Earth for ten months to accurately determine the asteroid's orbit and how it might change.

Pharos, the winning student entry, would be an orbiter with four science instruments (a multi-spectral imager, [[Physics:Near-infrared spectroscopy|near-infrared spectrometer]], [[Physics:Laser rangefinder|laser rangefinder]], and [[Physics:Magnetometer|magnetometer]]) that would rendezvous with and track Apophis. Earth-based tracking of the spacecraft would then allow precise tracking of the asteroid. The Pharos spacecraft would also carry four instrumented probes that it would launch individually over the course of two weeks. [[Physics:Accelerometer|Accelerometer]]s and temperature sensors on the probes would measure the seismic effects of successive probe impacts, a creative way to explore the interior structure and dynamics of the asteroid.

Second place, for $10,000, went to a European team led by Deimos Space S.L. of [[Place:Madrid|Madrid]], {{wipe|Spain}}, in cooperation with EADS [[Company:Astrium|Astrium]], Friedrichshafen, Germany; [[Organization:University of Stuttgart|University of Stuttgart]], Germany; and [[Organization:University of Pisa|University of Pisa]], Italy. Juan L. Cano was [[Principal investigator|principal investigator]].

Another European team took home $5,000 for third place. Their team lead was EADS Astrium Ltd, United Kingdom, in conjunction with EADS Astrium SAS, France; IASF-Roma, INAF, Rome, Italy; [[Organization:Open University|Open University]], UK; Rheinisches Institut für Umweltforschung, Germany; Royal Observatory of Belgium; and Telespazio, Italy. The principal investigator was Paolo D'Arrigo.

Two teams tied for second place in the Student Category: Monash University, Clayton Campus, Australia, with Dilani Kahawala as principal investigator; and [[Organization:University of Michigan|University of Michigan]], with Jeremy Hollander as principal investigator. Each second-place team won $2,000. A team from Hong Kong Polytechnic University and Hong Kong University of Science and Technology, under the leadership of Peter Weiss, received an honorable mention and $1,000 for the most innovative student proposal.

==== Don Quijote mission ====
Apophis is one of two asteroids that were considered by the [[Organization:European Space Agency|European Space Agency]] as the target of its [[Engineering:Don Quijote (spacecraft)|Don Quijote mission concept]] to study the effects of impacting an asteroid.<ref name="DonQuixote"/>

==== Cancelled Chinese mission ====
China had planned an encounter with Apophis in 2022, several years prior to the close approach in 2029. This mission, now known as ''[[Astronomy:Tianwen-2|Tianwen-2]]'', would have included exploration and close study of three asteroids including an extended encounter with Apophis for close observation, and land on the asteroid [[Astronomy:(175706) 1996 FG3|1996 FG3]] to conduct in situ sampling analysis on the surface.<ref name="XinhuaNews"/> The launch date is now scheduled for 2024, with a different set of targets.

==== RAMSES ====
Apophis is the target of the [[Organization:European Space Agency|European Space Agency]]'s proposed RAMSES (Rapid Apophis Mission for SEcurity and Safety) mission, with a launch in 2027 and rendezvous with the asteroid in 2029.<ref name=RAMSES>{{cite arXiv |last1=Morelli |first1=Andrea C. |last2=Mannocchi |first2=Alessandra |display-authors=0 |date=September 2023 |title=Initial Trajectory Assessment of the RAMSES Mission to (99942) Apophis |eprint=2309.00435 |class=astro-ph.EP}}</ref>

== Proposed deflection strategies ==
{{further|Astronomy:Asteroid impact avoidance}}
Studies by NASA, ESA,<ref name="Izzo2006"/> and various research groups in addition to the Planetary Society contest teams,<ref name="aero.org"/> have described a number of proposals for deflecting Apophis or similar objects, including gravitational tractor, kinetic impact, and nuclear bomb methods.

On December 30, 2009, Anatoly Perminov, the director of the Russian Federal Space Agency, said in an interview that [[Company:Roscosmos|Roscosmos]] will also study designs for a possible deflection mission to Apophis.<ref name="Isachenkov2009"/>

On August 16, 2011, researchers at China's Tsinghua University proposed launching a mission to knock Apophis onto a safer course using an impactor spacecraft in a retrograde orbit, steered and powered by a solar sail. Instead of moving the asteroid on its potential resonant return to Earth, Shengping Gong and his team believe the secret is shifting the asteroid away from entering the gravitational keyhole in the first place.<ref name="ChinaSail"/>

On February 15, 2016, Sabit Saitgarayev, of the [[Organization:Makeyev Rocket Design Bureau|Makeyev Rocket Design Bureau]], announced intentions to use {{wipe|Russia}}n ICBMs to target relatively small [[Astronomy:Near-Earth object|near-Earth object]]s. Although the report stated that likely targets would be between the 20 to 50 metres in size, it was also stated that 99942 Apophis would be an object subject to tests by the program.<ref name="TASS"/>

In October 2022, a method of mapping the inside of a potentially problematic asteroid, such as 99942 Apophis, in order to determine the best area for impact was proposed.<ref name="WP-20221021">{{cite news |last=Verma |first=Pranshu |title=There's a new tool to help blow up asteroids – Researchers from MIT and Stanford have created a tool that could improve the aim of future planetary defense missions |url=https://www.washingtonpost.com/technology/2022/10/21/asteroid-destruction-tool/ |date=21 October 2022 |newspaper=The Washington Post |accessdate=22 October 2022 }}</ref>

== Popular culture ==
In [[Company:Id Software|Id Software]]'s [[Engineering:Video game|video game]] ''[[Software:Rage (video game)|Rage]]'', the backstory involves the asteroid colliding with Earth on August 23, 2029. The asteroid almost wipes out the human race and ushers in a post-apocalyptic age.<ref name="Gamespot" />

== See also ==
{{div col|colwidth=20em}}
* [[Astronomy:Asteroid capture|Asteroid capture]]
* [[Astronomy:Asteroid impact prediction|Asteroid impact prediction]]
* [[Astronomy:Asteroid Redirect Mission|Asteroid Redirect Mission]]
* [[Astronomy:Earth-grazing fireball|Earth-grazing fireball]]
* [[Earth:List of impact craters on Earth|List of impact craters on Earth]]
* [[Astronomy:NEO Surveyor|NEO Surveyor]]
{{div col end}}

== Notes ==
{{notelist|refs=

{{efn|name=palermo|Of the six asteroids with a riskier [[Palermo Technical Impact Hazard Scale|palermo scale rating]] than Apophis:
* [[Astronomy:(29075) 1950 DA|(29075) 1950 DA]] and [[Astronomy:101955 Bennu|101955 Bennu]] can not impact within the next 100 years.
* [[Astronomy:1979 XB|1979 XB]] and [[Astronomy:2007 FT3|2007 FT3]] are lost [[Astronomy:Observation arc#Short arcs|short-arc]] objects that can not be recovered with targeted observations and await serendipitous [[Astronomy:Astronomical survey|survey]] observations.
* [[Astronomy:2000 SG344|2000 SG344]] and 2009 JF1 are less than 50 meters in diameter.}}

{{efn|name=Yarkovsky_20210309|Using the 9 March 2021 solution, JPL gave the strength of the Yarkovsky effect as <math>A_{2}=-2.877\times10^{-14}\text{ AU/d}^{2}</math>, with an uncertainty of <math>1.543\times10^{-16}\text{ AU/d}^{2}</math>. The SNR, defined as the size of the signal divided by the uncertainty, is <math>2.877\times10^{-14}/1.543\times10^{-16}=186.4</math>. As of the latest orbit solution (29 June 2021), the SNR is <math>2.901\times10^{-14}/1.942\times10^{-16}=149.4</math> (again lower than Bennu's).}}

{{efn|name=Venus2022|On January 8, 2022 Venus was even closer to Earth at {{convert|0.2658|AU|e6km e6mi LD|abbr=unit|lk=off}}.}}

{{efn|name=Apophis2116|The minimum possible Earth approach between 5–20 April 2116 is {{convert|0.001|AU|e3km e3mi LD|abbr=unit|lk=off}}.}}

}} <!-- end of notelist -->

== References ==
{{reflist|colwidth=30em|refs =

<ref name=jpldata>{{cite web
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<ref name=NEODys>{{cite web
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<ref name="mpc">{{cite web
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<ref name=esanews1>{{cite web
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<ref name="Goldstone">{{cite web
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<ref name="Brozovic2018">{{cite journal
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<ref name=phys>{{cite web
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<ref name=keyhole>{{cite magazine
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 <ref name=nasanews146>{{cite news
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 |quote=Today's impact monitoring results indicate that the impact probability for April 13, 2029, has risen to about 1.6%, which for an object of this size corresponds to a rating of 4 on the ten-point Torino Scale.}}</ref>

<ref name="nasanews148">{{cite news
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 |title=Radar Observations Refine the Future Motion of Asteroid 2004 MN4
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<ref name="nasanews164">{{cite news
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<ref name="risk-table">{{cite web
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<ref name=riskneo>{{cite web
 |title=99942 Apophis (2004 MN4) Earth Impact Risk Summary
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 |url=https://cneos.jpl.nasa.gov/sentry/details.html#?des=99942
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<ref name="Earth-impact">{{cite web
 |title=Earth Impact Effects Program
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<ref name=Reddy2018>{{cite journal
 |last1=Reddy
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<ref name="Orbit2029">{{cite web
  |title=Horizons Batch for Apophis Orbital Elements for 2029-01-01 and 2030-01-01
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272029-01-01%27&STOP_TIME=%272030-01-01%27&STEP_SIZE=%271%20year%27&CENTER=%27@Sun%27&OUT_UNITS=%27AU-D%27
  |access-date=2022-07-31}}</ref>

<ref name="Horizons2029">{{cite web
  |title=Horizons Batch for Apophis @ 2029-Apr-13 21:46 showing 3-sigma uncertainty of ±3.4km
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272029-Apr-13%2021:46%27&STOP_TIME=%272029-Apr-14%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |archive-url=https://web.archive.org/web/20220131021056/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272029-Apr-13%2021:46%27&STOP_TIME=%272029-Apr-14%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |archive-date=2022-01-31
  |url-status=live
  |access-date=2021-09-03}}</ref>

<ref name="Horizons2036">{{cite web
  |title=Horizons Batch for Apophis @ 2036-Mar-27 08:30 showing 3-sigma uncertainty of ±129145km
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272036-Mar-27%2008:30%27&STOP_TIME=%272036-Mar-28%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |archive-url=https://web.archive.org/web/20230710121335/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272036-Mar-27%2008:30%27&STOP_TIME=%272036-Mar-28%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |archive-date=2023-07-10
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  |access-date=2023-07-10}}</ref>

<ref name="Horizons2116">{{cite web
  |title=Horizons Batch for Apophis @ 2116-Apr-12 17:50 showing 3-sigma uncertainty of ±13 million km
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272116-Apr-12%2017:50%27&STOP_TIME=%272116-Apr-13%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |access-date=2022-07-31}}</ref>

<ref name="Horizons2117">{{cite web
  |title=Horizons Batch for Apophis @ 2117-Oct-07 16:24 showing 3-sigma uncertainty of ±37 million km
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272117-Oct-07%2016:24%27&STOP_TIME=%272117-Oct-08%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |access-date=2023-09-30}}</ref>

<ref name="Horizons2068">{{cite web
  |title=Horizons Batch for Apophis on 2068-Apr-12
  |publisher=[[Astronomy:JPL Horizons On-Line Ephemeris System|JPL Horizons]]
  |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27Apophis%27&START_TIME=%272068-Apr-12%27&STOP_TIME=%272068-Apr-13%27&STEP_SIZE=%272%20days%27&QUANTITIES=%2720,39%27
  |accessdate=2022-07-31}}</ref>

<ref name="NEODyS2029">{{cite web
 |title=(99942) Apophis Ephemerides for 13 Apr 2029
 |publisher=NEODyS (Near Earth Objects{{Snd}} Dynamic Site)
 |url=https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2029&m0=4&d0=13&h0=18&mi0=0&y1=2029&m1=4&d1=14&h1=0&mi1=0&ti=10&tiu=minutes
 |access-date=19 August 2018}}</ref>

<ref name="NEODyS2068">{{cite web
 |title=99942 Apophis Ephemerides for April 2068
 |publisher=NEODyS (Near Earth Objects{{Snd}} Dynamic Site)
 |url=https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2068&m0=4&d0=1&h0=0&mi0=0&y1=2068&m1=4&d1=30&h1=0&mi1=0&ti=1.0&tiu=days
 |archive-url=https://web.archive.org/web/20210310182811/https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2068&m0=4&d0=1&h0=0&mi0=0&y1=2068&m1=4&d1=30&h1=0&mi1=0&ti=1.0&tiu=days
 |archive-date=10 March 2021
 |url-status=live
 |access-date=9 May 2019}}</ref>

<ref name="NEODyS2060s">{{cite web
 |title=99942 Apophis Ephemerides for 2067–2071
 |publisher=NEODyS (Near Earth Objects{{Snd}} Dynamic Site)
 |url=https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2067&m0=2&d0=1&h0=0&mi0=0&y1=2071&m1=12&d1=31&h1=0&mi1=0&ti=10.0&tiu=days
 |archive-url=https://web.archive.org/web/20210311112715/https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2067&m0=2&d0=1&h0=0&mi0=0&y1=2071&m1=12&d1=31&h1=0&mi1=0&ti=10.0&tiu=days
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<ref name="MPEC2004-Y70">{{cite web
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<ref name=Arecibo>{{cite web
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<ref name="Schweickart">{{cite web
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 <ref name=Binzel2007>{{cite web
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<ref name="Tholen">{{cite magazine
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 |access-date=10 March 2011}}</ref>

<ref name="Science">{{cite web
 |title=Asteroid 2004 MN4 will come scarily close to Earth on April 13, 2029, but it will not hit
 |publisher=Science@NASA
 |author=Morrison, D.
 |url=https://science.nasa.gov/science-news/science-at-nasa/2005/13may_2004mn4/
 |date=6 April 2011}}</ref>

<ref name="refine2013">{{cite web
 |title=Apophis asteroid encounter in 2013 should help answer impact worries
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<ref name="S&T2020">{{cite web
 |title=The subtle effect of sunlight may turn the near-Earth asteroid Apophis toward Earth in 2068 . . . but chances for impact remain small.
 |publisher=Sky & Telescope
 |first=G.
 |last=Schilling
 |url=https://skyandtelescope.org/astronomy-news/apophis-impact-small-chance-2068/
 |date=27 October 2020
 |access-date=1 November 2020}}</ref>

<ref name="NEODyS2013">{{cite web
 |title=99942 Apophis Ephemerides for 9 Jan 2013
 |publisher=NEODyS (Near Earth Objects{{Snd}} Dynamic Site)
 |url=https://newton.spacedys.com/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2013&m0=01&d0=07&h0=0&mi0=0&y1=2013&m1=01&d1=25&h1=0&mi1=0&ti=3.0&tiu=hours
 |access-date=14 March 2021}}</ref>

<ref name="Neil deGrasse Tyson - Death By Giant Meteor">{{cite web
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<ref name="mirror">{{cite web
 |title=2004 MN4 Earth Impact Risk Summary (computed on Dec 27, 2004)
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 |access-date=4 November 2011}}</ref>

<ref name="SETI2021">{{cite web
 |title=We Got it! The Story of Astronomers and Citizen Astronomers Catching Hazardous Apophis Asteroid over Colorado and Louisiana
 |publisher=[[Organization:SETI Institute|SETI Institute]]
 |first=F.
 |last=Marchis
 |url=https://www.seti.org/we-got-it
 |archive-url=https://web.archive.org/web/20210310222038/https://www.seti.org/we-got-it
 |date=10 March 2021
 |archive-date=10 March 2021
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<ref name="Neodys_eq1">{{cite web
 |title=99942 Apophis dynamical parameters
 |url=https://newton.spacedys.com/~neodys2/epoch//99942.eq1
 |publisher=NEODyS-2
 |date=14 March 2021
 |access-date=14 March 2021}}</ref>

<!-- <ref name="www.worldhistory.org">{{cite encyclopedia
 |title=Apophis
 |url=https://www.worldhistory.org/Apophis/
 |access-date=21 July 2020
 |encyclopedia=[[Organization:World History Encyclopedia|World History Encyclopedia]]}}</ref>-->

 <ref name="Goldstone2021">{{cite web
 |title=Goldstone Radar Observations Planning: 99942 Apophis in 2021
 |url=https://echo.jpl.nasa.gov/asteroids/Apophis/apophis.2021.goldstone.planning.html
 |publisher=Jet Propulsion Laboratory
 |date=9 March 2021
 |access-date=19 March 2021}}</ref>

 <ref name="NASA-20190429">{{cite news
 |last1=Brown
 |first1=D.
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 <ref name="Wayback2006-08">{{cite web
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}}

== External links ==
{{Commons}}
* {{cite journal|arxiv=1404.5847|doi=10.1051/0004-6361/201423841|bibcode=2014A&A...566A..22M|title=Thermal infrared observations of asteroid (99942) Apophis with ''Herschel''|year=2014|last1=Müller|first1=T. G.|last2=Kiss|first2=C.|last3=Scheirich|first3=P.|last4=Pravec|first4=P.|last5=o'Rourke|first5=L.|last6=Vilenius|first6=E.|last7=Altieri|first7=B.|journal=Astronomy & Astrophysics|volume=566|pages=A22|s2cid=119282830|display-authors=4}}
* [https://web.archive.org/web/20130317183119/http://apophisasteroid.net/ Apophis Asteroid]
* [http://meetingorganizer.copernicus.org/EPSC-DPS2011/EPSC-DPS2011-1212.pdf Asteroid Apophis orbit from recent observations], EPSC Abstracts Vol. 6, EPSC-DPS2011-1212, 2011, EPSC-DPS Joint Meeting 2011
* [http://www.brera.mi.astro.it/sormano/Apophis/Sormano_Apophis.html Diagrams and orbits of Apophis] (Sormano Astronomical Observatory)
* [https://web.archive.org/web/20200611222254/https://gravitysimulator.org/solar-system/apophis-2029-earth-encounter/ Interactive 3D gravity simulation of Apophis's 2029 Earth flyby]

'''Risk assessment'''
* [https://cneos.jpl.nasa.gov/doc/apophis/ Apophis Orbital Prediction Page at NASA JPL]
* [https://newton.spacedys.com/neodys/index.php?pc=1.1.0&n=99942 99942 Apophis page] from [[Astronomy:NEODyS|NEODyS]]
* [http://www.brera.mi.astro.it/sormano/mbpl.html MBPL – Minor Body Priority List (technical List) at Sormano Observatory]
* [http://www.brera.mi.astro.it/sormano/teca.html TECA – Table of Asteroids Next Closest Approaches to the Earth at Sormano Observatory]

'''NASA'''
* [https://web.archive.org/web/20041228072731/http://neo.jpl.nasa.gov/news/news148.html Possibility of an Earth Impact in 2029 Ruled Out for Asteroid {{mp|2004 MN|4}}] (JPL)
* [https://cneos.jpl.nasa.gov/news/news149.html Radar Observations Refine the Future Motion of Asteroid {{mp|2004 MN|4}}] (JPL)
* [https://web.archive.org/web/20041102064506/http://neo.jpl.nasa.gov/gif/mea-orbit-big.gif Animation explaining how impact risk is determined] from [https://web.archive.org/web/20160106045208/http://neo.jpl.nasa.gov/cgi-bin/ip?1 Impact Probability]
* {{JPL small body|id=99942}}

{{s-start}}
{{s-bef|before={{mpl|(153814) 2001 WN|5}}}}
{{s-ttl|title=Large [[Astronomy:Near-Earth object|NEO]] Earth close approach<br />(inside the orbit of the Moon)|years=13 April 2029}}
{{s-aft|after={{mpl|2012 UE|34}}}}
{{end}}

{{Planetary defense}}
{{Small Solar System bodies}}

{{DEFAULTSORT:Apophis}}
[[Category:Aten asteroids|099942]]
[[Category:Named minor planets]]
[[Category:Potentially hazardous asteroids|099942]]
[[Category:Radar-imaged asteroids|099942]]

{{Sourceattribution|99942 Apophis}}