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# Minimum orbit intersection distance

The orbit of (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 to assess potential close approaches and collision risks between astronomical objects.[1][2] It is defined as the distance between the closest points of the osculating orbits of two bodies. Of greatest interest is the risk of a collision with Earth. Earth MOID is often listed on comet and asteroid databases such as the JPL Small-Body Database. MOID values are also defined with respect to other bodies as well: Jupiter MOID, Venus MOID and so on.

An object is classified as a potentially hazardous object (PHO) – that is, posing a possible risk to Earth – if, among other conditions, its Earth MOID is less than 0.05 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.[1]

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 integrations 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 orbital elements of the body and no numerical integration into the future is used.[3]

The only object that has ever been rated at 4 on the Torino Scale (since downgraded), the Aten asteroid (99942) Apophis, has an Earth MOID of 0.000316 AU. 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 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 observation arc with a poorly determined orbit. The only objects that have been detected and had their Earth-MOID calculated before Earth impact were the small asteroids 2008 TC3 and 2014 AA. 2008 TC3 was listed with a MOID of 0.00001 AU in the Minor Planet Center database, and is the smallest MOID calculated for an Apollo asteroid.[4] It is even smaller at the more precise JPL Small Body Database (0.0000078 AU).[5]

Potentially hazardous asteroids with Earth MOID < 0.0004 AU include:[6]
Object Earth MOID
(AU)
Size (m)
(approximate)
(H) Currently on
Sentry Risk Table[7]
(137108) 1999 AN10 0.000003 AU (450 km; 280 mi) 1300 17.9
2004 MX<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">2</span></span> 0.000019 AU (2,800 km; 1,800 mi) 1258[8] 19.3
2017 YZ1 0.000036 AU (5,400 km; 3,300 mi) 260 20.4
(164207) 2004 GU9 0.000050 AU (7,500 km; 4,600 mi) 163[9] 21.1
(442037) 2010 PR<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">66</span></span> 0.000083 AU (12,400 km; 7,700 mi) 488[10] 19.3
(177049) 2003 EE16 0.000103 AU (15,400 km; 9,600 mi) 320 19.9
2016 FG<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">60</span></span> 0.000113 AU (16,900 km; 10,500 mi) 300 21.1
2000 TU<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">28</span></span> 0.000132 AU (19,700 km; 12,300 mi) 200 20.8
(433953) 1997 XR2 0.000133 AU (19,900 km; 12,400 mi) 200 20.7
(292220) 2006 SU49 0.000175 AU (26,200 km; 16,300 mi) 380 19.5
(216985) 2000 QK<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">130</span></span> 0.000212 AU (31,700 km; 19,700 mi) 200 21.0
2011 SM<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">68</span></span> 0.000214 AU (32,000 km; 19,900 mi) 300 19.6
2007 PV<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">27</span></span> 0.000221 AU (33,100 km; 20,500 mi) 300 20.2
(85236) 1993 KH 0.000226 AU (33,800 km; 21,000 mi) 500 18.6
2009 KK 0.000231 AU (34,600 km; 21,500 mi) 280 20.5
99942 Apophis 0.000316 AU (47,300 km; 29,400 mi) 370[7] 19.7 Yes
2008 PK<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">3</span></span> 0.000321 AU (48,000 km; 29,800 mi) 140 22.0
2007 TU24 0.000328 AU (49,100 km; 30,500 mi) 250 20.3
2014 EG<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">45</span></span> 0.000352 AU (52,700 km; 32,700 mi) 140 21.9
2016 CB<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">194</span></span> 0.000370 AU (55,400 km; 34,400 mi) 1300 17.6
2004 UE 0.000378 AU (56,500 km; 35,100 mi) 220 21.0
(297300) 1998 SC<span style="position: relative; top: 0.3em;"><span style="font-size:80%;">15</span></span> 0.000392 AU (58,600 km; 36,400 mi) 320[11] 19.2
(367789) 2011 AG5 0.000392 AU (58,600 km; 36,400 mi) 140 21.8

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## References

1. ^ a b Bruce Koehn, "Minimum Orbital Intersection Distance", Lowell Observatory, retrieved online 14 May 2009, archived 15 July 2015.
2. ^ Basics of Space Flight: The Solar System, p3, NASA JPL, retrieved 14 May 2009, archived 15 July 2015.
3. ^ Brian G. Marsden, "Press Information Sheet:Potentially Hazardous Asteroids", Harvard-Smithsonian Center for Astrophysics, retrieved online 3 May 2009, archived 15 July 2015.
4. ^ List Of Apollo Minor Planets, International Astronomical Union: Minor Planet Center, retrieved 14 May 2009, archived 15 July 2015.
5. ^ 2008 TC3 at JPL Small Body Database. Retrieved 14 May 2009, archived 15 July 2015.
6. ^ "JPL Small-Body Database Search Engine: H <= 22 (mag) and Earth MOID < 0.0004 (AU)". JPL Solar System Dynamics. Retrieved 13 May 2018.
7. ^ a b Sentry: Earth Impact Monitoring, retrieved 13 May 2018.
8. ^ JPL Small-Body Database Browser: 2004 MX2, retrieved 13 May 2018.
9. ^ JPL Small-Body Database Browser: 164207, retrieved 13 May 2018.
10. ^ JPL Small-Body Database Browser: 442037, retrieved 13 May 2018.
11. ^ JPL Small-Body Database Browser: 297300, retrieved 13 May 2018.
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