1991 VG is a Near-Earth object discovered by American astronomer James Scotti on 6 November 1991. Due to its unusual orbit and rapid variation in brightness, there was early speculation that it could be a man-made or even extraterrestrial object. In more recent years, however, additional discoveries have accounted for the unusual characteristics of 1991 VG, and it is no longer regarded as anomalous.
On November 6, 1991, Scotti discovered a faint object which was designated 1991 VG soon after discovery. The object's heliocentric orbit was found to be very similar to Earth's orbit and it was found that it would make a close approach to Earth just a month after discovery. 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. More recently, the first Earth Trojan asteroid - 2010 TK7 has been identified and such objects could well be a source for objects like 1991 VG.
Possible monolithic structure
Since the discovery of 1991 VG, about 80% of small asteroids with absolute magnitudes (H) fainter than 22.0 (corresponding to sizes smaller than about 200 meters) which have had their lightcurve measured have rotation periods under 2 hours. Such rapid rotation would cause objects that are rubble piles - i.e. conglomerates of debris held together only by their gravitational attraction - to disrupt due to centripetal acceleration. Most such objects, therefore, are thought to be monolithic bodies or conglomerates which are held together by forces other than gravity (monolithic rocks, groups of debris held together by impact melt or welded by some other natural process). The unusual rapid lightcurve variation seen on the long trailed images of 1991 VG during its 1991 December close flyby of Earth is thus no longer considered unique or unexpected.
Possible artificial origin
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 artificial in origin. There was much speculation that it could be a rocket body from a satellite launched in the early 1970s when 1991 VG made its previous close approach with Earth. Earlier close approaches to Earth were before the start of the space age. Some even went so far as to suggest the possibility that it might be an extraterrestrial object such as a Bracewell probe, because its orbital path did not closely match any known man-made spacecraft or rocket bodies. Dr. Duncan Steel suggested extraterrestrial origin in an article in The Observatory.
1991 VG has absolute magnitude H=28.5 which would correspond to an asteroid between about 6 and 12 meters diameter and it is therefore far too faint for further study, apart from during close approaches to Earth. It will make its next close approach to earth in 2017.
- satobs.org: James Scotti, Re: What is 1991 VG?
- jpl.nasa.gov, 1991 VG Earth Impact Risk Summary
- Steele, D. (1995). "SETA and 1991 VG". The Observatory 115: 78–83. Bibcode:1995Obs...115...78S.
- Hergenrother, C. W.; Whiteley, R. J. (2011). "A survey of small fast rotating asteroids among the near-Earth asteroid population". Icarus 214: 194. Bibcode:2011Icar..214..194H. doi:10.1016/j.icarus.2011.03.023.
- JPL Small-Body Database Browser: 1991 VG