The case for 1991 VG: A long sought recovery

27/06/2017

On the morning of 30 May 2017 our team, in collaboration with ESO and using the Very Large Telescope (VLT) in Chile, obtained recovery observations of 1991 VG, a famous near-Earth object with an interesting historical background.

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1991 VG, discovered in November 2001 by Jim Scotti during the early phases of the Spacewatch survey, is a roughly 10-metre object that has the peculiarity of being in a very Earth-like orbit, with a semimajor axis of just 1.03 au, an eccentricity of less than 0.05 and an inclination of only 1.5°. It was the first object in this kind of orbit ever discovered, and its identification led to an interesting debate on its possible origin.

At the time of discovery two distinct possibilities for its origin were proposed. The most obvious was that 1991 VG is just a natural small asteroid that happened to evolve into this type of orbit, maybe coming from a population of Earth Trojan asteroids hypothesised but still undiscovered at the time. It could also have been a piece of the Moon, ejected from a lunar impact with low relative velocity with respect to the Earth, and therefore injected in an Earth-like orbit.

 

 

An animation of the recovery images of 1991 VG (left panel) taken on 30 May 2017 and its peculiar Earth-like orbit at its close approach with our planet.

 

However, an alternative option was that 1991 VG could be a man-made object, a piece of rocket hardware or an old spacecraft that was launched many decades before, and remained in an Earth-like orbit for all the time, until coming back for a close encounter with Earth in 1991.

An object in such an extreme Earth-like orbit has a very low relative velocity with respect to our planet. On one side this means that the 1991 close approach lasted unusually long and astronomers were able to keep observing 1991 VG for quite a while (almost a month in the fall of 1991 and another month in April 1992). This allowed computing an extremely accurate orbit for the asteroid, sufficient to attempt a recovery at its next available apparition.

But when 1991 VG would show up again? Unfortunately a low relative velocity translates into a long synodic period and as a consequence 1991 VG spent decades hiding behind the Sun or too close to it, thus being unobservable from Earth. Nevertheless thanks to the good quality of its orbit it could be estimated that the next favourable opportunity to recover the asteroid would occur about 25 years after discovery (e.g. as reported on the NEODyS site). That's why we took our chance and scheduled an observation attempt with VLT. On 30 May 2017 the object was detected while still at a challenging magnitude of 25. The recovery was subsequently confirmed in another set of observations taken on 1 June, and resulted in a recovery announcement circular issued by the Minor Planet Center on the same day (http://www.minorplanetcenter.net/mpec/K17/K17L02.html).

Our additional observations seem to support the idea that the object is indeed of natural origin. If it had been an artificial object, its low bulk density would have resulted in a noticeable drift from a purely gravitational orbit during these 25 years, due to the effects of solar radiation pressure. Our updated orbit does not show apparent evidence of such drift at the level expected from a hollow rocked body or piece of hardware, while it is compatible with the typical densities of a rocky or metallic asteroidal object. Further detailed studies shall confirm this guess.