Temporary satellite

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A temporary satellite is an object which has been captured by the gravitational field of a planet and thus has become the planet's natural satellite, but, unlike irregular moons of the larger outer planets of the Solar System, will eventually either leave its orbit around the planet or collide with the planet. The only observed examples are 2006 RH120, a temporary satellite of Earth for nine months in 2006 and 2007, and 2020 CD3, which was discovered in 2020.[1][2] Some defunct space probes or rockets have also been observed on temporary satellite orbits.[3]

In astrophysics, a temporary satellite is any body that enters the Hill sphere of a planet at a sufficiently low velocity such that it becomes gravitationally bound to the planet for some period of time.[4]

Capture of asteroids[edit]

The dynamics of the capture of asteroids by Earth was explored in simulations conducted on a supercomputer,[5] with results published in 2012.[6] Of 10 million virtual near-Earth asteroids, 18,000 have been temporarily captured.[6] Earth has at least one temporary satellite 1 m (3.3 ft) across at any given time, but they are too faint to detect by current surveys.[5]

According to the simulations, temporary satellites are typically caught and released when they pass one of two gravitational equilibrium points of the Sun and the planet along the line connecting the two, the L1 and L2 Lagrangian points.[5] The captured asteroids typically have orbits very similar to the planet's (co-orbital configuration) and are captured most often when the planet is closest to the Sun (in the case of the Earth, in January) or furthest from the Sun (Earth: in July).[5]

In strict sense, only bodies that complete a full orbit around a planet are considered temporary satellites, also called temporarily captured orbiters (TCO). However, asteroids not in a tight co-orbital configuration with a planet can be temporarily captured for less than a full orbit; such objects have been named temporarily-captured fly-bys (TCF).[7] In a 2017 follow-up to the 2012 simulation study which also considered an improved model of near-Earth asteroid populations, 40% of captured objects were TCF. The combined number of TCO/TCF was found to be smaller than in the previous study, the maximum size of objects which can be expected to be orbiting Earth at any given moment was 0.8 m (2.6 ft).[7] In another 2017 study based on simulations with one million virtual co-orbital asteroids, 0.36% have been temporarily captured.[8]


As of February 2020, two objects have been observed at the time when they were temporary satellites: 2006 RH120[1][9][10] and 2020 CD3.[11] According to orbital calculations, on its solar orbit, 2006 RH120 passes Earth at low speed every 20 to 21 years,[10] at which point it can become a temporary satellite again.

As of March 2018, there is one confirmed example of a temporarily captured asteroid that didn't complete a full orbit, 1991 VG.[8] This asteroid was observed for a month after its discovery in November 1991, then again in April 1992, after which it wasn't seen until May 2017.[12] After the recovery, orbital calculations confirmed that 1991 VG was a temporary satellite of Earth in February 1992.[8]

List of known and suspected Satellites, Quasi-satellites, Trojans and Horsehoe orbit objects
Name Eccentricity Diameter
Discoverer Year of Discovery Type Current Type
Moon 0.055 1737400 ? ? Natural satellite Natural satellite
1913 Great Meteor Procession ? ? ? 1913 February 9 Possible Temporary satellite Destroyed
3753 Cruithne 0.515 5000 Duncan Waldron 1986 October 10 Quasi-satellite Horseshoe orbit
1991 VG 0.053 5–12 Spacewatch 1991 November 6 Temporary satellite Apollo asteroid
(85770) 1998 UP1 0.345 210–470 Lincoln Lab's ETS 1998 October 18 Horseshoe orbit Horseshoe orbit
54509 YORP 0.230 124 Lincoln Lab's ETS 2000 August 3 Horseshoe orbit Horseshoe orbit
2001 GO2 0.168 35–85 Lincoln Lab's ETS 2001 April 13 Possible Horseshoe orbit Possible Horseshoe orbit
2002 AA29 0.013 20–100 LINEAR 2002 January 9 Quasi-satellite Horseshoe orbit
2003 YN107 0.014 10–30 LINEAR 2003 December 20 Quasi-satellite Horseshoe orbit
(164207) 2004 GU9 0.136 160–360 LINEAR 2004 April 13 Quasi-satellite Quasi-satellite
(277810) 2006 FV35 0.377 140–320 Spacewatch 2006 March 29 Quasi-satellite Quasi-satellite
2006 JY26 0.083 6–13 Catalina Sky Survey 2006 May 6 Horseshoe orbit Horseshoe orbit
2006 RH120 0.024 2–3 Catalina Sky Survey 2006 September 14 Temporary satellite Apollo asteroid
(419624) 2010 SO16 0.075 357 WISE 2010 September 17 Horseshoe orbit Horseshoe orbit
2010 TK7 0.191 150–500 WISE 2010 October 1 Earth trojan Earth trojan
2013 BS45 0.083 20–40 Spacewatch 2013 January 20 Horseshoe orbit Horseshoe orbit
2013 LX28 0.452 130–300 Pan-STARRS 2013 June 12 Quasi-satellite temporary Quasi-satellite temporary
2014 OL339 0.461 170 EURONEAR 2014 July 29 Quasi-satellite temporary Quasi-satellite temporary
2015 SO2 0.108 50–111 Črni Vrh Observatory 2015 September 21 Quasi-satellite Horseshoe orbit temporary
2015 XX169 0.184 9–22 Mount Lemmon Survey 2015 December 9 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YA 0.279 9–22 Catalina Sky Survey 2015 December 16 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YQ1 0.404 7–16 Mount Lemmon Survey 2015 December 19 Horseshoe orbit temporary Horseshoe orbit temporary
469219 Kamoʻoalewa 0.104 41-100 Pan-STARRS 2016 April 27 Quasi-satellite stable Quasi-satellite stable
DN16082203 ? ? ? 2016 August 22 Possible Temporary satellite Destroyed
2020 CD3 0.017 1–3 Mount Lemmon Survey 2020 February 15 Temporary satellite Apollo asteroid

Artificial objects on temporary satellite orbits[edit]

The Earth can also temporarily capture defunct space probes or rockets travelling on solar orbits, in which case astronomers cannot always immediately determine whether the object is artificial or natural. The possibility of an artificial origin has been considered for both 2006 RH120[1] and 1991 VG.[8]

The artificial origin has been confirmed in other cases. In September 2002, astronomers found an object designated 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 Saturn V rocket that carried Apollo 12 to the Moon.[13][3] In 2006, an object designated 6Q0B44E was discovered on a temporary satellite orbit, later its artificial nature was confirmed, but its identity is unknown.[3] Another confirmed artificial temporary satellite with unidentified origin is 2013 QW1.[3]

See also[edit]


  1. ^ a b c "2006 RH120 ( = 6R10DB9) (A second moon for the Earth?)". Great Shefford Observatory. September 14, 2017. Archived from the original on 2015-02-06. Retrieved 2017-11-13. CS1 maint: discouraged parameter (link)
  2. ^ "MPEC 2020-D104 : 2020 CD3: Temporarily Captured Object". Minor Planet Electronic Circular. Minor Planet Center. 25 February 2020. Retrieved 25 February 2020. CS1 maint: discouraged parameter (link)
  3. ^ a b c d Azriel, Merryl (September 25, 2013). "Rocket or Rock? NEO Confusion Abounds". Space Safety Magazine. Archived from the original on 2017-11-15. Retrieved 2017-11-14. CS1 maint: discouraged parameter (link)
  4. ^ Lissauer, Jack J.; de Pater, Imke (2019). Fundamental Planetary Sciences : physics, chemistry, and habitability. New York, NY, USA: Cambridge University Press. p. 34. ISBN 9781108411981. Comets or other bodies that enter the Hill sphere of a planet at very low velocity can remain gravitationally bound to the planet for some time as temporary satellites.
  5. ^ a b c d Camille M. Carlisle (December 30, 2011). "Pseudo-moons Orbit Earth". Sky & Telescope.
  6. ^ a b "Earth Usually Has More than One Moon, Study Suggests". Space.com. April 4, 2012.
  7. ^ a b Fedorets, Grigori; Granvik, Mikael; Jedicke, Robert (March 15, 2017). "Orbit and size distributions for asteroids temporarily captured by the Earth-Moon system". Icarus. 285: 83–94. Bibcode:2017Icar..285...83F. doi:10.1016/j.icarus.2016.12.022.
  8. ^ a b c d de la Fuente Marcos, C.; de la Fuente Marcos, R. (January 21, 2018). "Dynamical evolution of near-Earth asteroid 1991 VG". Monthly Notices of the Royal Astronomical Society. 473 (3): 2939–2948. arXiv:1709.09533. Bibcode:2018MNRAS.473.2939D. doi:10.1093/mnras/stx2545.
  9. ^ Roger W. Sinnott (April 17, 2007). "Earth's "Other Moon"". Sky & Telescope. Archived from the original on 2012-08-27. Retrieved 2017-11-13. CS1 maint: discouraged parameter (link)
  10. ^ a b "2006 RH120. Close-Approach Data". JPL Small-Body Database Browser. NASA/JPL. Archived from the original on February 11, 2017. Retrieved 2017-11-13. CS1 maint: discouraged parameter (link)
  11. ^ "MPEC 2020-D104 : 2020 CD3: Temporarily Captured Object". Minor Planet Electronic Circular. Minor Planet Center. 25 February 2020. Retrieved 25 February 2020. CS1 maint: discouraged parameter (link)
  12. ^ "1991 VG Orbit". Minor Planet Center. Retrieved 2018-03-12. CS1 maint: discouraged parameter (link)
  13. ^ Chesley, Steve; Chodas, Paul (October 9, 2002). "J002E3: An Update". News. NASA. Archived from the original on 2003-05-03. Retrieved 2017-11-14. CS1 maint: discouraged parameter (link)