1991 VG

1991 VG is a very small near-Earth object of the Apollo group, approximately 5–12 meters (16–39 feet) in diameter. It was first observed by American astronomer James Scotti on 6 November 1991, using the Spacewatch telescope on Kitt Peak National Observatory near Tucson, Arizona, in the United States.[1][4]

1991 VG
Recovery of 1991 VG with the VLT, on 30 May 2017. The asteroid is marked with green lines, and appears several times on this time-lapse composite.
Discovery [1]
Discovered bySpacewatch
Discovery siteKitt Peak Obs.
Discovery date6 November 1991
(first observed only)
Designations
1991 VG
NEO · Apollo [1][2]
Orbital characteristics [2]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc25.57 yr (9,339 d)
Aphelion1.0858 AU
Perihelion0.9765 AU
1.0312 AU
Eccentricity0.0530
1.05 yr (382 d)
75.828°
0° 56m 28.68s / day
Inclination1.4270°
71.481°
25.947°
Earth MOID0.0043 AU (1.67 LD)
Jupiter MOID3.92 AU
Physical characteristics
Mean diameter
5–12 m[3]
28.3[1][2]

    Earth-like orbit

    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 and 0.0472 au on 11 February, 2018.[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 (for example between AD 1382 and 1667) and it will return to such in the future. In 1991-1992 it looped around the earth in a retrograde manner (east to west) with closest approach at 0.00306 au on December 5, 1991.[6][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] This temporary capture has taken place multiple times in the past and it is expected to repeat again in the future.[7]

    It is now falling behind the earth as it and the earth orbit the sun. The two will again be on the same side of the sun in November, 2038.[2]

    Simulation has been performed for 5000 years into the future without detecting an impact on Earth. Around 400 BC there was an approach to a distance of around 0.001 au, closer than the moon (see Figure 1 in the paper by Carlos and Raúl de la Fuente Marcos[7]).

    Possible monolithic structure

    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 or rubble piles.[8]

    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 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 the 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.

    Recovery

    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.

    Exploration

    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]

    Notes
    1. Computed with JPL Horizons using a geocentric solution. Ephemeris Type: Orbital Elements / Center: 500
    2. On 28 February 1992, 1991 VG was 0.043 AU (6,400,000 km; 4,000,000 mi) from Earth moving at a relative speed of 0.22 km/s (490 mph) and had an eccentricity of 0.67 with respect to Earth. It was not orbiting Earth because it was outside of the Earth's Hill sphere which has a radius of roughly 0.01 AU (1.5 million km).

    References
    1. "1991 VG". Minor Planet Center. Retrieved 27 June 2018.
    2. "JPL Small-Body Database Browser: (1991 VG)" (2017-06-01 last obs.). Jet Propulsion Laboratory. Retrieved 27 June 2018.
    3. "Asteroid Size Estimator". CNEOS NASA/JPL. Retrieved 12 November 2017.
    4. Steel, Duncan (April 1995). "SETA and 1991 VG". The Observatory. 115: 78–83. Bibcode:1995Obs...115...78S. Retrieved 27 June 2018.
    5. James Scotti (17 July 1996). "Re: What is 1991 VG?". www.satobs.org. Retrieved 27 June 2018.
    6. Tancredi, G. (September 1997). "An Asteroid in a Earth-like Orbit". Celestial Mechanics and Dynamical Astronomy. 69 (1/2): 119–132. Bibcode:1997CeMDA..69..119T. doi:10.1023/A:1008378316299.
    7. de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (January 2018). "Dynamical evolution of near-Earth asteroid 1991 VG". Monthly Notices of the Royal Astronomical Society. 473 (3): 2939–2948. arXiv:1709.09533. Bibcode:2018MNRAS.473.2939D. doi:10.1093/mnras/stx2545.
    8. Hergenrother, Carl W.; Whiteley, Robert J. (July 2011). "A survey of small fast rotating asteroids among the near-Earth asteroid population". Icarus. 214 (1): 194–209. Bibcode:2011Icar..214..194H. doi:10.1016/j.icarus.2011.03.023. Retrieved 27 June 2018.
    9. "ESA/ESO Collaboration Successfully Tracks Its First Potentially Threatening Near-Earth Object". European Southern Observatory. 21 January 2014. Retrieved 27 June 2018.
    10. "A Very Close Encounter". European Southern Observatory. 10 August 2017. Retrieved 27 June 2018.
    11. "MPEC 2017-L02 : 1991 VG". Minor Planet Center. 1 June 2017. Retrieved 27 June 2018.
    12. Mahoney, Erin (21 May 2016). "NEA Scout". NASA. Retrieved 27 June 2018.
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