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 twelve months from July 2006 to July 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]
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[update], 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[update], 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] Another temporary capture episode was experienced by 2022 NX1 that may return as a mini-moon in December 2051.[13][14]
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.[15][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]