A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet (or dwarf planet) where the object stays close to that planet over many orbital periods.

Diagram of generic quasi-satellite orbit

A quasi-satellite's orbit around the Sun takes the same time as the planet's, but has a different eccentricity (usually greater), as shown in the diagram. When viewed from the perspective of the planet by an observer facing the Sun, the quasi-satellite will appear to travel in an oblong retrograde loop around the planet. (See Analemma § Of quasi-satellites).

In contrast to true satellites, quasi-satellite orbits lie outside the planet's Hill sphere, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighborhood, then possibly later move back to a quasi-satellite orbit, etc.

Other types of orbit in a 1:1 resonance with the planet include horseshoe orbits and tadpole orbits around the Lagrangian points, but objects in these orbits do not stay near the planet's longitude over many revolutions about the star. Objects in horseshoe orbits are known to sometimes periodically transfer to a relatively short-lived quasi-satellite orbit,[1] and are sometimes confused with them. An example of such an object is 2002 AA29.

A quasi-satellite is similar to an object in a distant retrograde orbit, in a different context. The latter term is usually used for a space probe or artificial satellite in a retrograde orbit around a moon, and the period may be much shorter than that of the moon, whereas the term "quasi-satellite" usually refers to an object like an asteroid whose period is similar to that of the planet of which it is considered to be a quasi-satellite. But in both cases, the object (asteroid, space probe) viewed in a reference frame that rotates with the two main objects (once a year for Sun-Earth, once a month for Earth-Moon) appears to move retrograde compared to that rotation, thus lengthening its sidereal period. So a quasi-satellite (with low inclination) tends to stay in certain constellations rather than going through the whole zodiac. Quasi-satellites with high eccentricity can get quite far from their planet, more than an astronomical unit for quasi-satellites of Earth such as 2014 OL339.

The word "geosynchronous" is sometimes used to describe quasi-satellites of the Earth, because their motion around the Sun is synchronized with Earth's. However, this usage is unconventional and confusing. Conventionally, geosynchronous satellites revolve in the prograde sense around the Earth, with orbital periods that are synchronized to the Earth's rotation.





Venus has one known quasi-satellite, 524522 Zoozve. This asteroid is also a Mercury- and Earth-crosser; it seems to have been a "companion" to Venus for approximately the last 7,000 years only, and is destined to be ejected from this orbital arrangement about 500 years from now.[2]


The oscillating path of asteroid 469219 Kamoʻoalewa viewed from Earth's perspective as it orbits around the Sun. The traced path of Kamoʻoalewa makes it appear as a constant companion of the Earth.

As of 2023, Earth had seven known quasi-satellites:

On the longer term, asteroids can transfer between quasi-satellite orbits and horseshoe orbits, which circulate around Lagrangian points L4 and L5. By 2016, orbital calculations showed that all five of Earth's then known quasi-satellites repeatedly transfer between horseshoe and quasi-satellite orbits.[8] 3753 Cruithne,[9] 2002 AA29,[1] 2003 YN107 and 2015 SO2[5] are minor planets in horseshoe orbits that might evolve into a quasi-satellite orbit. The time spent in the quasi-satellite phase differs from asteroid to asteroid. Quasi-satellite 2016 HO3 is predicted to be stable in this orbital state for several hundred years, in contrast to 2003 YN107 which was a quasi-satellite from 1996 to 2006 but then departed Earth's vicinity on a horseshoe orbit.[8][10]

469219 Kamoʻoalewa (2016 HO3) is thought to be one of the most stable quasi-satellites found yet of Earth. It stays between 38 and 100 lunar distances from the Earth.[10]

Known and suspected companions of Earth
Name Eccentricity Diameter
Discoverer Date of Discovery Type Current Type
Moon 0.055 3474800 ? Prehistory Natural satellite Natural satellite
1913 Great Meteor Procession ? ? ? 1913-02-09 Possible Temporary satellite Destroyed
3753 Cruithne 0.515 5000 Duncan Waldron 1986-10-10 Quasi-satellite Horseshoe orbit
1991 VG 0.053 5–12 Spacewatch 1991-11-06 Temporary satellite Apollo asteroid
(85770) 1998 UP1 0.345 210–470 Lincoln Lab's ETS 1998-10-18 Horseshoe orbit Horseshoe orbit
54509 YORP 0.230 124 Lincoln Lab's ETS 2000-08-03 Horseshoe orbit Horseshoe orbit
2001 GO2 0.168 35–85 Lincoln Lab's ETS 2001-04-13 Possible Horseshoe orbit Possible Horseshoe orbit
2002 AA29 0.013 20–100 LINEAR 2002-01-09 Quasi-satellite Horseshoe orbit
2003 YN107 0.014 10–30 LINEAR 2003-12-20 Quasi-satellite Horseshoe orbit
(164207) 2004 GU9 0.136 160–360 LINEAR 2004-04-13 Quasi-satellite Quasi-satellite
(277810) 2006 FV35 0.377 140–320 Spacewatch 2006-03-29 Quasi-satellite Quasi-satellite
2006 JY26 0.083 6–13 Catalina Sky Survey 2006-05-06 Horseshoe orbit Horseshoe orbit
2006 RH120 0.024 2–3 Catalina Sky Survey 2006-09-13 Temporary satellite Apollo asteroid
(419624) 2010 SO16 0.075 357 WISE 2010-09-17 Horseshoe orbit Horseshoe orbit
(706765) 2010 TK7 0.191 150–500 WISE 2010-10-01 Earth trojan Earth trojan
2013 BS45 0.083 20–40 Spacewatch 2010-01-20 Horseshoe orbit Horseshoe orbit
2013 LX28 0.452 130–300 Pan-STARRS 2013-06-12 Quasi-satellite temporary Quasi-satellite temporary
2014 OL339 0.461 70–160 EURONEAR 2014-07-29 Quasi-satellite temporary Quasi-satellite temporary
2015 SO2 0.108 50–110 Črni Vrh Observatory 2015-09-21 Quasi-satellite Horseshoe orbit temporary
2015 XX169 0.184 9–22 Mount Lemmon Survey 2015-12-09 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YA 0.279 9–22 Catalina Sky Survey 2015-12-16 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YQ1 0.404 7–16 Mount Lemmon Survey 2015-12-19 Horseshoe orbit temporary Horseshoe orbit temporary
469219 Kamoʻoalewa 0.104 40-100 Pan-STARRS 2016-04-27 Quasi-satellite stable Quasi-satellite stable
DN16082203 ? ? ? 2016-08-22 Possible Temporary satellite Destroyed
2020 CD3 0.017 1–6 Mount Lemmon Survey 2020-02-15 Temporary satellite Temporary satellite
2020 PN1 0.127 10–50 ATLAS-HKO 2020-08-12 Horseshoe orbit temporary Horseshoe orbit temporary
2020 PP1 0.074 10–20 Pan-STARRS 2020-08-12 Quasi-satellite stable Quasi-satellite stable
(614689) 2020 XL5 0.387 1100-1260 Pan-STARRS 2020-12-12 Earth trojan Earth trojan
2022 NX1 0.025 5-15 Moonbase South Observatory 2020-07-02 Temporary satellite Apollo asteroid
2023 FW13 0.177 10-20 Pan-STARRS 2023-03-28 Quasi-satellite Quasi-satellite Ceres edit

The dwarf-planet asteroid 1 Ceres is believed to have a quasi-satellite, the as-yet-unnamed (76146) 2000 EU16.



(309239) 2007 RW10 is a temporary quasi-satellite of Neptune.[11] The object has been a quasi-satellite of Neptune for about 12,500 years and it will remain in that dynamical state for another 12,500 years.[11]

Other planets


Based on simulations, it is believed that Uranus and Neptune could potentially hold quasi-satellites for up to the age of the Solar System (about 4.5 billion years),[12] but a quasi-satellite's orbit would remain stable for only 10 million years near Jupiter and 100,000 years near Saturn. Jupiter and Saturn are known to have quasi-satellites.[clarification needed] 2015 OL106, a co-orbital to Jupiter, intermittently becomes a quasi satellite of the planet, and will next become one between 2380 and 2480.

Artificial quasi-satellites


In early 1989, the Soviet Phobos 2 spacecraft was injected into a quasi-satellite orbit around the Martian moon Phobos, with a mean orbital radius of about 100 kilometres (62 mi) from Phobos.[13] According to computations, it could have then stayed trapped in the vicinity of Phobos for many months. The spacecraft was lost due to a malfunction of the on-board control system.

Accidental quasi-satellites


Some objects are known to be accidental quasi-satellites, which means that they are not forced into the configuration by the gravitational influence of the body of which they are quasi-satellites.[14] The dwarf planets Ceres and Pluto are known to have accidental quasi-satellites.[14] In the case of Pluto, the known accidental quasi-satellite, 15810 Arawn, is, like Pluto, a plutino, and is forced into this configuration by the gravitational influence of Neptune.[14] This dynamical behavior is recurrent where Arawn becomes a quasi-satellite of Pluto every 2.4 Myr and remains in that configuration for nearly 350,000 years.[14][15][16]

See also



  1. ^ a b Connors, Martin; Chodas, Paul; Mikkola, Seppo; Wiegert, Paul; Veillet, Christian; Innanen, Kimmo (2002). "Discovery of an asteroid and quasi-satellite in an Earth-like horseshoe orbit". Meteoritics & Planetary Science. 37 (10): 1435–1441. Bibcode:2002M&PS...37.1435C. doi:10.1111/j.1945-5100.2002.tb01039.x.
  2. ^ Mikkola, S.; Brasser, R.; Wiegert, P.; Innanen, K. (2004). "Asteroid 2002 VE68, a quasi-satellite of Venus". Monthly Notices of the Royal Astronomical Society. 351 (3): L63–L65. Bibcode:2004MNRAS.351L..63M. doi:10.1111/j.1365-2966.2004.07994.x.
  3. ^ Brasser, R.; et al. (September 2004). "Transient co-orbital asteroids". Icarus. 171 (1): 102–109. Bibcode:2004Icar..171..102B. doi:10.1016/j.icarus.2004.04.019.
  4. ^ Wajer, Paweł (October 2010). "Dynamical evolution of Earth's quasi-satellites: 2004 GU9 and 2006 FV35" (PDF). Icarus. 209 (2): 488–493. Bibcode:2010Icar..209..488W. doi:10.1016/j.icarus.2010.05.012.
  5. ^ a b de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "From horseshoe to quasi-satellite and back again: The curious dynamics of Earth co-orbital asteroid 2015 SO2". Astrophysics and Space Science. 361: 16. arXiv:1511.08360. Bibcode:2016Ap&SS.361...16D. doi:10.1007/s10509-015-2597-8. S2CID 189842725.
  6. ^ de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2014). "Asteroid 2014 OL339: Yet another Earth quasi-satellite". Monthly Notices of the Royal Astronomical Society. 445 (3): 2985–2994. arXiv:1409.5588. Bibcode:2014MNRAS.445.2961D. doi:10.1093/mnras/stu1978.
  7. ^ Agle, D.C.; Brown, Dwayne; Cantillo, Laurie (15 June 2016). "Small asteroid is Earth's constant companion". NASA. Retrieved 15 June 2016.
  8. ^ a b c de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "Asteroid (469219) 2016 HO3, the smallest and closest Earth quasi-satellite". Monthly Notices of the Royal Astronomical Society. 462 (4): 3441–3456. arXiv:1608.01518. Bibcode:2016MNRAS.462.3441D. doi:10.1093/mnras/stw1972.
  9. ^ Christou, Apostolos A.; Asher, David J. (2011). "A long-lived horseshoe companion to the Earth". Monthly Notices of the Royal Astronomical Society. 414 (4): 2965–2969. arXiv:1104.0036. Bibcode:2011MNRAS.414.2965C. doi:10.1111/j.1365-2966.2011.18595.x. S2CID 13832179.
  10. ^ a b "Small Asteroid is Earth's Constant Companion". Jet Propulsion Laboratory.
  11. ^ a b de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (September 2012). "(309239) 2007 RW10: a large temporary quasi-satellite of Neptune". Astronomy and Astrophysics Letters. 545: L9. arXiv:1209.1577. Bibcode:2012A&A...545L...9D. doi:10.1051/0004-6361/201219931. S2CID 118374080.
  12. ^ Wiegert, P.; Innanen, K. (2000). "The stability of quasi satellites in the outer solar system". The Astronomical Journal. 119 (4): 1978–1984. Bibcode:2000AJ....119.1978W. doi:10.1086/301291.
  13. ^ Green, LM; Zakharov, AV; Pichkhadze, KM. Что мы ищем на Фобосе [What we are looking for [on] Phobos] (in Russian). Archived from the original on 2009-07-20.
  14. ^ a b c d de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2012). "Plutino 15810 (1994 JR1), an accidental quasi-satellite of Pluto". Monthly Notices of the Royal Astronomical Society: Letters. 427 (1): L85. arXiv:1209.3116. Bibcode:2012MNRAS.427L..85D. doi:10.1111/j.1745-3933.2012.01350.x. S2CID 118570875.
  15. ^ "Pluto's fake moon". Archived from the original on 2013-01-05. Retrieved 2012-09-24.
  16. ^ de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "The analemma criterion: accidental quasi-satellites are indeed true quasi-satellites". Monthly Notices of the Royal Astronomical Society. 462 (3): 3344–3349. arXiv:1607.06686. Bibcode:2016MNRAS.462.3344D. doi:10.1093/mnras/stw1833.
  • Quasi-satellite Information Page
  • Astronomy.com: A new "moon" for Earth
  • Discovery of the first quasi-satellite of Venus – University of Turku news release (August 17, 2004)