1991 VG

On 6 November 1991, Scotti discovered a faint object which was designated 1991 VG soon after discovery.[5] The object's heliocentric orbit was found to be very similar to Earth's orbit and it was calculated that it would make a close approach to Earth in the month after discovery at 1.2 lunar distances or 0.003 AU (450,000 km; 280,000 mi) on 5 December 1991.[2] 1991 VG also passed 0.0568 AU from Earth on 7 August 2017.[2] Given such an Earth-like orbit, the dynamical lifetime of such an object is relatively short with the object quickly either impacting Earth or being perturbed by Earth onto a different orbit. The similarity of its orbit with Earth was also very difficult to explain from natural sources, with ejecta from a recent Lunar impact or non-gravitational perturbations such as the Yarkovsky effect having been suggested.[4][6] The first Earth Trojan asteroid, 2010 TK₇, was later identified and such objects could well be a source for objects like 1991 VG.

1991 VG has been a transient co-orbital of the horseshoe type in the past and it will return as such in the future;[7] it was a natural satellite of Earth for about a month since 1992.[6][7] This temporary capture may have taken place multiple times in the past and it is expected to repeat again in the future.[7] It had an eccentricity of less than 1 with respect to the Earth from 23 February to 21 March 1992.[lower-alpha 1][lower-alpha 2]

Since the discovery of 1991 VG, about 80% of small asteroids with absolute magnitudes fainter than 22.0 (corresponding to sizes smaller than about 200 meters) which have had their lightcurve measured have rotation periods under 2 hours. The so-called fast rotators are typically monolithic bodies or, alternatively, welded conglomerates with a sufficient intrinsic strength to counteract centrifugal forces. More slowly rotating asteroids are sometimes gravitationally bound aggregates.[8]

The uncertainty of the object's origin, combined with rapid variation in the object's brightness in images obtained during its close passage with Earth in early December 1991, led to some speculation that 1991 VG might be a spent rocket fuel tank.[4][5] There was speculation that it could be a rocket body from a satellite launched in the early 1970s,[5] or from Apollo 12 mission. A detailed analysis of the available evidence confirms that there is no compelling reason to believe that 1991 VG is not natural.[7]

As of 2018, with a highly accurate orbit, it is highly unlikely that 1991 VG is artificial, as it did not approach anywhere near Earth at any point since 1900.

1991 VG was not observed between 1992 and 2017.[1] But after 26 years, 1991 VG had returned to the vicinity of Earth. As part of a program[9][10] by ESA and ESO to secure the orbit of faint but potentially threatening Near-Earth Objects, 1991 VG was recovered[11] by the ESO VLT on 30 May 2017, at magnitude 25. With this recovery, the orbit of 1991 VG is now determined with a high precision.[2] It was removed from the Sentry Risk Table on 1 June 2017. The fact that it has stayed on a stable orbit for many years indicates not only that it is an inert object (i.e. not an active spacecraft), but also that its density is fairly high: an empty fuel tank, for instance, would have its orbit slightly changing in time due to the radiation pressure from the Sun. This strengthens the probability that 1991 VG is a near Earth asteroid. It was last observed 1 June 2017.[1]

The Near-Earth Asteroid Scout is a planned mission by NASA to develop a low-cost CubeSat solar sail spacecraft capable of encountering near-Earth asteroids and taking high resolution images. It is expected to launch in 2021, and 1991 VG is the planned target.[12]

• 1991 VG at NeoDyS-2, Near Earth Objects—Dynamic Site
  • Ephemeris · Obs prediction · Orbital info · MOID · Proper elements · Obs info · Close · Physical info · NEOCC
• 1991 VG at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters