View source for Astronomy:Minimum orbit intersection distance

{{short description|Measure of close approach or collision risk in astronomy}}
[[File:4953 DYK.svg|right|thumb|230px|The orbit of [[Astronomy:(4953) 1990 MU|(4953) 1990 MU]], which, with a MOID of 0.0263 AU, is classified as a potentially hazardous object]]
'''Minimum orbit intersection distance''' ('''MOID''') is a measure used in [[Astronomy:Astronomy|astronomy]] to assess potential close approaches and collision risks between astronomical objects.<ref name=Koehn/><ref>[https://solarsystem.nasa.gov/basics/chapter1-3 Basics of Space Flight: The Solar System, p. 3], NASA Science, retrieved 14 May 2009 (from JPL site), [https://web.archive.org/web/20210917185720/https://solarsystem.nasa.gov/basics/chapter1-3/ archived] 17 September 2021.</ref>  It is defined as the distance between the closest points of the [[Astronomy:Osculating orbit|osculating orbit]]s of two bodies.  Of greatest interest is the risk of a collision with [[Astronomy:Earth|Earth]].  Earth MOID is often listed on [[Astronomy:Comet|comet]] and [[Astronomy:Asteroid|asteroid]] databases such as the [[Astronomy:JPL Small-Body Database|JPL Small-Body Database]].  MOID values are also defined with respect to other bodies as well: [[Astronomy:Jupiter|Jupiter]] MOID, [[Astronomy:Venus|Venus]] MOID and so on.

An object is classified as a [[Astronomy:Potentially hazardous object|potentially hazardous object]] (PHO) – that is, posing a possible risk to Earth – if, among other conditions, its Earth MOID is less than 0.05 [[Astronomy:Astronomical unit|AU]].  For more massive bodies than Earth, there is a potentially notable close approach with a larger MOID; for instance, Jupiter MOIDs less than 1 AU are considered noteworthy since Jupiter is the most massive planet.<ref name=Koehn>Bruce Koehn, "[http://www.lowell.edu/users/elgb/moid.html Minimum Orbital Intersection Distance]", Lowell Observatory, retrieved online 14 May 2009, [https://web.archive.org/web/20150715151635/https://lowell.edu/users/elgb/moid.html archived] 15 July 2015.</ref>

A low MOID does not mean that a collision is inevitable as the planets frequently perturb the orbit of small bodies. It is also necessary that the two bodies reach that point in their orbits at the same time before the smaller body is perturbed into a different orbit with a different MOID value. Two objects gravitationally locked in orbital resonance may never approach one another. [[Numerical integration]]s become increasingly divergent as trajectories are projected further forward in time, especially beyond times where the smaller body is repeatedly perturbed by other planets.  MOID has the convenience that it is obtained directly from the [[Astronomy:Orbital elements|orbital elements]] of the body and no numerical integration into the future is used.<ref>Brian G. Marsden, [http://www.cfa.harvard.edu/iau/pressinfo/100PHAs.html "Press Information Sheet:Potentially Hazardous Asteroids"], Harvard-Smithsonian Center for Astrophysics, retrieved online 3 May 2009, [https://web.archive.org/web/20091122010157/http://www.cfa.harvard.edu/iau/pressinfo/100PHAs.html archived] 22 November 2009.</ref>

The only object that has ever been rated at 4 on the Torino Scale (since downgraded), the [[Astronomy:Aten asteroid|Aten asteroid]] (99942) Apophis, has an Earth MOID of {{convert|0.00026|AU|km mi|abbr=on|lk=on}}.  This is not the smallest Earth MOID in the catalogues; many bodies with a small Earth MOID are not classed as PHO's because the objects are less than roughly 140 meters in diameter (or [[Astronomy:Absolute magnitude|absolute magnitude]], ''H'' > 22). Earth MOID values are generally more practical for asteroids less than 140 meters in diameter as those asteroids are very dim and often have a short [[Astronomy:Observation arc|observation arc]] with a poorly determined orbit. As of September 2023, there have been [[Astronomy:Asteroid impact prediction#List of successfully predicted asteroid impacts|seven objects]] detected and their Earth-MOID calculated before the Earth impact.<ref>[https://newton.spacedys.com/neodys/index.php?pc=4.5 List of Prior Impacts], NEODyS, retrieved 23 September 2023.</ref> The first two objects that were detected and had their Earth-MOID calculated before Earth impact were the small asteroids {{mpl|2008 TC|3}} and [[Astronomy:2014 AA|2014 AA]]. 2014 AA is listed with a MOID of {{convert|0.00000045|AU|km mi|abbr=on}},<ref>[https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2014AA JPL SBDB: 2014 AA] (Earth impactor on 1 January 2014)</ref> and is the second smallest MOID calculated for an [[Astronomy:Apollo asteroid|Apollo asteroid]] after {{mpl|2020 QY|2}} with an Earth-MOID of {{convert|0.00000039|AU|km mi|abbr=on}}.<ref>[https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=2020QY2 JPL SBDB: 2020 QY2] (Near-Earth asteroid roughly 2–meters in diameter)</ref>

{| class="wikitable sortable"
|+[[Astronomy:Potentially hazardous asteroid|Potentially hazardous asteroid]]s with Earth MOID < 0.0004 AU (~60,000km or ~5 Earth diameters) include:<ref name="jpl-search"/>
|-
! Object !! Earth MOID<br>([[Astronomy:Astronomical unit|AU]]) !! Size ([[Metre|m]])<br>(approximate) !! ([[Astronomy:Absolute magnitude#Solar System bodies (H)|H]])
|-
| {{mpl|2016 FG|60}} || {{convert|0.000076|AU|km mi|abbr=on|lk=on}}<ref name=JPL-16FG60/> ||align="center"|300 ||align="center"|21.1
|-
| {{mpl|(177049) 2003 EE|16}} || {{convert|0.000107|AU|km mi|abbr=on}} ||align="center"|320 ||align="center"|19.8
|-
| {{mpl|2012 HZ|33}} || {{convert|0.000131|AU|km mi|abbr=on}} ||align="center"|260 ||align="center"|20.4
|-
| {{mpl|2010 JE|88}} || {{convert|0.000148|AU|km mi|abbr=on}} ||align="center"|180 ||align="center"|21.5
|-
| {{mpl|(137108) 1999 AN|10}} || {{convert|0.000153|AU|km mi|abbr=on}} ||align="center"|1300 ||align="center"|17.9
|-
| {{mpl|2022 BX|1}} || {{convert|0.000177|AU|km mi|abbr=on}} ||align="center"|170 ||align="center"|21.7
|-
| {{mpl|2003 EG|16}} || {{convert|0.000179|AU|km mi|abbr=on}} ||align="center"|490 ||align="center"|19.4
|-
| {{mpl|2021 NQ|5}} || {{convert|0.000187|AU|km mi|abbr=on}} ||align="center"|210 ||align="center"|21.2
|-
| {{mpl|(442037) 2010 PR|66}} || {{convert|0.000238|AU|km mi|abbr=on}} ||align="center"|695 ||align="center"|19.3
|-
| {{mpl|(216985) 2000 QK|130}} || {{convert|0.000252|AU|km mi|abbr=on}} ||align="center"|200 ||align="center"|21.3
|-
| [[Astronomy:99942 Apophis|99942 Apophis]] || {{convert|0.000257|AU|km mi|abbr=on}} ||align="center"|370 ||align="center"|19.7
|-
| {{mpl|(89958) 2002 LY|45}} || {{convert|0.000261|AU|km mi|abbr=on}} ||align="center"|1300 ||align="center"|17.2
|-
| {{mpl|(35396) 1997 XF|11}} || {{convert|0.000305|AU|km mi|abbr=on}} ||align="center"|704 ||align="center"|17.0
|-
| [[Astronomy:162173 Ryugu|162173 Ryugu]] || {{convert|0.000315|AU|km mi|abbr=on}} ||align="center"|896 ||align="center"|19.6
|-
| {{mpl|(143651) 2003 QO|104}} || {{convert|0.000321|AU|km mi|abbr=on}} ||align="center"|2300 ||align="center"|16.1
|-
| {{mpl|(85236) 1993 KH|}} || {{convert|0.000335|AU|km mi|abbr=on}} ||align="center"|500 ||align="center"|18.8
|-
| {{mpl|(471240) 2011 BT|15}} || {{convert|0.000368|AU|km mi|abbr=on}} ||align="center"|150 ||align="center"|21.4
|}

{| class="wikitable sortable"
|+Numbered periodic comets with Earth MOID < 0.02 AU (~3 million km) include:
|-
! Object !! [[Astronomy:Epoch|Epoch]] !! Earth MOID<br/>([[Astronomy:Astronomical unit|AU]])
|-
| 3D/Biela || 1832 || {{convert|0.0005|AU|km mi LD|abbr=on|lk=on}}
|-
| 109P/Swift-Tuttle || 1995 || {{convert|0.0009|AU|km mi LD|abbr=on}}
|-
| 55P/Tempel–Tuttle || 1998 || {{convert|0.0085|AU|km mi LD|abbr=on}}
|-
| 255P/Levy || 2007 || {{convert|0.0088|AU|km mi LD|abbr=on}}
|-
| 15P/Finlay || 2015 || {{convert|0.0092|AU|km mi LD|abbr=on}}
|-
| 73P–BW || 2022 || {{convert|0.0093|AU|km mi LD|abbr=on}}<ref>[https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?des=73P-BW JPL SBDB: 73P-BW] (Short-lived comet fragment)</ref>
|-
| 252P/LINEAR || 2016 || {{convert|0.0122|AU|km mi LD|abbr=on}}
|-
| 460P/PanSTARRS || 2016 || {{convert|0.0163|AU|km mi LD|abbr=on}}
|-
| 289P/Blanpain || 2019 || {{convert|0.0165|AU|km mi LD|abbr=on}}
|-
| 21P/Giacobini–Zinner || 2017 || {{convert|0.0179|AU|km mi LD|abbr=on}}
|}

{| class="wikitable sortable"
|+Some well known Main-belt asteroids<br/>with Earth MOID < 1 AU
|-
! Object !! Earth MOID<br>([[Astronomy:Astronomical unit|AU]])
|-
| [[Astronomy:6 Hebe|6 Hebe]] || {{Convert|0.975|AU|e6km e6mi LD|abbr=unit|lk=on}}
|-
| [[Astronomy:7 Iris|7 Iris]] || {{Convert|0.850|AU|e6km e6mi LD|abbr=unit}}
|-
| [[Astronomy:8 Flora|8 Flora]] || {{Convert|0.873|AU|e6km e6mi LD|abbr=unit}}
|-
| [[Astronomy:12 Victoria|12 Victoria]] || {{Convert|0.824|AU|e6km e6mi LD|abbr=unit}}
|-
| [[Astronomy:18 Melpomene|18 Melpomene]] || {{Convert|0.811|AU|e6km e6mi LD|abbr=unit}}
|-
| [[Astronomy:84 Klio|84 Klio]] || {{Convert|0.798|AU|e6km e6mi LD|abbr=unit}}
|-
| [[Astronomy:228 Agathe|228 Agathe]] || {{Convert|0.657|AU|e6km e6mi LD|abbr=unit}}
|}

==See also==
*[[Astronomy:Asteroid impact prediction|Asteroid impact prediction]]
*[[Astronomy:List of Mercury-crossing minor planets|List of Mercury-crossing minor planets]]
*[[Astronomy:List of Venus-crossing minor planets|List of Venus-crossing minor planets]]
*[[Astronomy:List of Earth-crossing minor planets|List of Earth-crossing minor planets]]
*[[Astronomy:List of Mars-crossing minor planets|List of Mars-crossing minor planets]]
*[[Astronomy:List of Jupiter-crossing minor planets|List of Jupiter-crossing minor planets]]
*[[Astronomy:List of Saturn-crossing minor planets|List of Saturn-crossing minor planets]]
*[[Astronomy:List of Uranus-crossing minor planets|List of Uranus-crossing minor planets]]
*[[Astronomy:List of Neptune-crossing minor planets|List of Neptune-crossing minor planets]]

==References==
{{reflist | refs =

<ref name="jpl-search">{{cite web
  |title=JPL Small-Body Database Search Engine: Group: PHA and Orbit Constraint: Earth MOID < 0.0004 (AU)
  |type=currently defined at an [[Astronomy:Epoch|epoch]] of 2023-Sep-13
  |publisher=JPL Solar System Dynamics
  |url=https://ssd.jpl.nasa.gov/tools/sbdb_query.html
  |access-date=2023-09-20}}</ref>

<ref name=JPL-16FG60>{{cite web
  |type=last observation: 2020-06-17; [[Astronomy:Observation arc|arc]]: 4.29 years
  |title=JPL Small-Body Database: (2016 FG60)
  |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2016FG60
  |publisher=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]]
  |archive-url=https://web.archive.org/web/20190528050554/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2016FG60
  |archive-date=2019-05-28
  |url-status=live
  |accessdate=2023-09-23}}</ref>

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

== External links ==
* [http://moid.cbk.waw.pl/orbity/static/MOID.pdf Fast Geometric Method for Calculating Accurate Minimum Orbit Intersection Distances] (PDF) 
* [http://andrew-lowe.ca/ALL_MOID.TXT MOID for all NEOs (Near-Earth Objects) for Mercury to Jupiter (Updated Daily)]
* [http://www.minorplanetcenter.net/iau/lists/Dangerous.html List of the Potentially Hazardous Asteroids (PHAs)]
* [http://www.brera.mi.astro.it/sormano/mbpl.html MBPL - Minor Body Priority List ( PHA Asteroids )]
* [http://www.brera.mi.astro.it/sormano/sael.html SAEL - Small Asteroids Encounters List]

{{Solar System}}
{{Small Solar System bodies}}

[[Category:Near-Earth objects]]
[[Category:Orbits]]
[[Category:Astrodynamics]]

{{Sourceattribution|Minimum orbit intersection distance}}