View source for 1991 VG

{{Short description|Near-Earth object}}
{{Use dmy dates|date=February 2018}}
{{Infobox planet
| minorplanet = yes
| name = 1991 VG
| background = #FFC2E0
| image = 1991VG-2017.png
| image_scale =
| caption = Recovery of ''1991 VG'' with the [[Very Large Telescope|VLT]], on 30 May 2017. The asteroid is marked with green lines, and appears several times on this time-lapse composite.
| discovery_ref = <ref name="MPC-object" />
| discoverer = [[Spacewatch]]
| discovery_site = [[Kitt Peak National Observatory|Kitt Peak Obs.]]
| discovered = 6 November 1991<br />{{small|(first observed only)}}
| mpc_name = 1991 VG
| alt_names =
| pronounced =
| named_after =
| mp_category = [[Near-Earth object|NEO]]{{·}}[[Apollo asteroid|Apollo]]<ref name="MPC-object" /><ref name="jpldata" />
| orbit_ref = <ref name="jpldata" />
| epoch = 2022-Aug-09 ([[Julian day|JD]] 2459800.5)
| uncertainty = 0
| observation_arc = 25.57 [[Julian year (astronomy)|yr]] (9,339 d)
| aphelion = 1.0866 [[Astronomical unit|AU]]
| perihelion = 0.97823 AU
| semimajor = 1.0324 AU
| eccentricity = 0.05247
| period = 1.05 yr (383 d)
| mean_anomaly = 138.576[[Degree (angle)|°]]
| mean_motion = {{Deg2DMS|0.9413|sup=ms}} / day
| inclination = 1.4298°
| asc_node = 71.267°
| arg_peri = 26.921°
| moid = {{convert|0.00083|AU|LD|abbr=on|lk=out|sigfig=3}}
| jupiter_moid = 3.92 AU
| mean_diameter = {{val|5|–|12|ul=m}}<ref name="h" />
| abs_magnitude = 28.3<ref name="MPC-object" /><ref name="jpldata" />
}}
'''1991 VG''' is a very small [[near-Earth object]] of the [[Apollo asteroid|Apollo group]], approximately {{convert|5|-|12|m|ft|abbr=off|sigfig=2|sp=us}} in diameter. It was first observed by American astronomer [[James V. Scotti|James Scotti]] on 6 November 1991, using the [[Spacewatch]] telescope on [[Kitt Peak National Observatory]] near Tucson, Arizona, in the United States.<ref name="MPC-object" /><ref name="Steel-1995" /> On 6 December 1991 it passed about {{val|369635|u=km}} from the Moon.<ref name=jpldata/> The asteroid then went unobserved from April 1992 until it was recovered by [[Paranal Observatory]] in May 2017.<ref name="MPC-object" /> It was removed from the [[Sentry (monitoring system)|Sentry Risk Table]] on 1 June 2017.<ref name="removed"/>
== Earth-like orbit ==
On 6 November 1991, Scotti discovered a faint object which was designated ''1991 VG'' soon after discovery.<ref name="Scotti-satobs" /> 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 distance (astronomy)|lunar distances]] or {{convert|0.003|AU|km mi|abbr=on|lk=off}} on 5 December 1991.<ref name="jpldata" /> ''1991 VG'' also passed 0.0568&nbsp;[[Astronomical unit|AU]] from Earth on 7 August 2017 and 0.0472&nbsp;au on 11 February 2018.<ref name="jpldata" /> 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.<ref name="Steel-1995" /><ref name="Tancredi-1997" /> The first [[Earth Trojan asteroid]], {{mp|2010 TK|7}}, 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 orbit|horseshoe type]] in the past (for example between AD&nbsp;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&nbsp;au on 5 December 1991.<ref name="Tancredi-1997" /><ref name="Fuente-Marcos-2018" /> It had an eccentricity of less than 1 with respect to the Earth from 23 February to 21 March 1992, but was outside of Earth's [[Hill sphere]].<ref name="Geocentric1992"/>{{efn|name=ecc}} This temporary capture has taken place multiple times in the past and it is expected to repeat again in the future.<ref name="Fuente-Marcos-2018" />
Currently, it is falling behind Earth as it orbits the Sun along with Earth. The two will again be on the same side of the Sun in November 2038.<ref name="jpldata" />
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&nbsp;au, closer than the Moon (see Figure 1 in the paper by Carlos and Raúl de la Fuente Marcos<ref name="Fuente-Marcos-2018" />).
== Possible monolithic structure ==
Since the discovery of ''1991 VG'', about 80% of small asteroids with [[Absolute magnitude#Solar System bodies (H)|absolute magnitudes]] fainter than 22.0 (corresponding to sizes smaller than about 200 meters) which have had their [[lightcurve]] measured have [[rotation period]]s under 2 hours. The so-called [[List of fast rotators (minor planets)|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 pile]]s.<ref name="Hergenrother-2011" />
== 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.<ref name="Steel-1995" /><ref name="Scotti-satobs" /> There was speculation that it could be a rocket body from a satellite launched in the early 1970s,<ref name="Scotti-satobs" /> or from the [[Apollo 12]] mission.{{citation needed|date=May 2020}} A detailed analysis of the available evidence confirms that there is no compelling reason to believe that ''1991 VG'' is not natural.<ref name="Fuente-Marcos-2018" /> The natural versus artificial issue also emerged in the case of the temporary capture episode experienced by near-Earth asteroid {{mpl|2022 NX|1}}.<ref name="spectrum">{{cite journal |last1=de la Fuente Marcos|first1=Raúl |first2=Julia |last2=de León |last3=de la Fuente Marcos|first3=Carlos |first4=Javier |last4=Licandro  |first5=Miquel |last5=Serra-Ricart |first6=Antonio |last6=Cabrera-Lavers |date=2 February 2023 |title=Mini-moons from horseshoes: A physical characterization of 2022 NX<sub>1</sub> with OSIRIS at the 10.4 m Gran Telescopio Canarias  |journal=Astronomy & Astrophysics Letters |volume=670 |issue=1 |pages=L10 (8 pages) |arxiv=2301.10797|bibcode=2023A&A...670L..10D |doi=10.1051/0004-6361/202245514 |url=https://www.aanda.org/articles/aa/full_html/2023/02/aa45514-22/aa45514-22.html | doi-access=free}}</ref>
== Recovery ==
''1991 VG'' was not observed between 1992 and 2017.<ref name="MPC-object" /> But after 26 years, ''1991 VG'' had returned to the vicinity of Earth. As part of a program<ref name="ESO-14004" /><ref name="ESO-17052" /> by [[European Space Agency|ESA]] and [[European Southern Observatory|ESO]] to secure the orbit of faint but potentially threatening Near-Earth Objects, ''1991 VG'' was recovered<ref name="MPEC" /> by the [[Very Large Telescope|ESO VLT]] on 30 May 2017, at magnitude 25. With this recovery, the orbit of ''1991 VG'' is now determined with a high precision.<ref name="jpldata" /> It was removed from the [[Sentry Risk Table]] on 1 June 2017.<ref name="removed"/>
==Exploration==
The [[Near-Earth Asteroid Scout]] is a low-cost [[CubeSat]] [[solar sail]] spacecraft by [[NASA]] capable of encountering [[near-Earth asteroid]]s and taking high resolution images and ''1991 VG'' was the planned target before 2020 GE became the new target.<ref name="NEA-Scout" /> The spacecraft was successfully launched into heliocentric orbit on 16 November 2022, however contact with the spacecraft was not successfully established and it is considered lost.
== Notes ==
{{notelist|refs=
{{efn|name=ecc|1=On 18 March 1992, ''1991 VG'' was {{convert|0.0426|AU|km mi|abbr=on|lk=off}} from Earth moving at a relative speed of {{convert|0.26|km/s|mph|abbr=on}} and had an eccentricity of 0.55 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 {{convert|0.01|AU|e6km|abbr=unit}}.}}
}} <!-- end of notelist -->
== See also ==
* {{mpl|2024 PT|5}}{{Snd}} another temporary Earth satellite discovered in 2024
* {{mpl|2023 FY|3}}{{Snd}} another temporary Earth satellite discovered in 2023
* {{mpl|2022 NX|1}}{{Snd}} near-Earth asteroid temporarily captured by Earth in 2022
* {{mpl|2020 CD|3}}{{Snd}} another temporary Earth satellite discovered in 2020
* {{mpl|2006 RH|120}}{{Snd}} the first temporary Earth satellite discovered ''in situ'' 2006
== References ==
{{Reflist|refs=
<ref name="jpldata">{{cite web
|type       = 2017-06-01 last obs.
|title      = JPL Small-Body Database Browser: (1991 VG)
|url        = https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=3005816&view=OPC
|publisher  = [[Jet Propulsion Laboratory]]
|accessdate = 13 August 2022}}</ref>
<ref name="MPC-object">{{cite web
|title      = 1991 VG
|work       = Minor Planet Center
|url        = https://www.minorplanetcenter.net/db_search/show_object?object_id=1991+VG
|accessdate = 27 June 2018}}</ref>
<ref name="MPEC">{{cite web
|title      = MPEC 2017-L02 : 1991 VG
|work       = Minor Planet Center
|date       = 1 June 2017
|url        = http://www.minorplanetcenter.net/mpec/K17/K17L02.html
|accessdate = 27 June 2018}}</ref>
<ref name="Steel-1995">{{Cite journal
|last1      = Steel |first1 = Duncan
|date       = April 1995
|title      = SETA and 1991 VG
|url        = http://adsabs.harvard.edu/full/1995Obs...115...78S
|journal    = The Observatory
|volume = 115
|pages      = 78–83
|bibcode    = 1995Obs...115...78S
|access-date= 27 June 2018}}</ref>
<ref name="Hergenrother-2011">{{Cite journal
|first1     = Carl W. |last1 = Hergenrother
|first2     = Robert J. |last2 = Whiteley
|date       = July 2011
|title      = A survey of small fast rotating asteroids among the near-Earth asteroid population
|url        = http://adsabs.harvard.edu/cgi-bin/bib_query?bibcode=2011Icar..214..194H
|journal    = Icarus
|volume     = 214
|issue      = 1
|pages      = 194–209
|bibcode    = 2011Icar..214..194H
|doi        = 10.1016/j.icarus.2011.03.023
|access-date= 27 June 2018}}</ref>
<ref name="Tancredi-1997">{{Cite journal
|last1      = Tancredi |first1 = G.
|date       = September 1997
|title      = An Asteroid in a Earth-like Orbit
|journal    = [[Celestial Mechanics and Dynamical Astronomy]]
|volume = 69
|issue      = 1/2
|pages      = 119–132
|bibcode    = 1997CeMDA..69..119T
|doi        = 10.1023/A:1008378316299
|s2cid = 189823446
}}</ref>
<ref name="h">{{cite web
|title      = Asteroid Size Estimator
|publisher  = CNEOS NASA/JPL
|url        = https://cneos.jpl.nasa.gov/tools/ast_size_est.html
|accessdate = 12 November 2017}}</ref>
<ref name="Fuente-Marcos-2018">{{Cite journal
|first1     = Carlos |last1 = de la Fuente Marcos
|first2     = Raúl |last2 = de la Fuente Marcos
|date       = January 2018
|title      = Dynamical evolution of near-Earth asteroid 1991 VG
|journal    = [[Monthly Notices of the Royal Astronomical Society]]
|volume     = 473
|issue      = 3
|pages      = 2939–2948
|bibcode    = 2018MNRAS.473.2939D
|doi        = 10.1093/mnras/stx2545
|doi-access = free
|arxiv      = 1709.09533
}}</ref>
<ref name="Scotti-satobs">{{cite web
|title      = Re: What is 1991 VG?
|author     = James Scotti
|date       = 17 Jul 1996
|publisher  = www.satobs.org
|url        = http://www.satobs.org/seesat/Jul-1996/0149.html
|accessdate = 27 June 2018}}</ref>
<ref name="ESO-14004">{{Cite web
|title      = ESA/ESO Collaboration Successfully Tracks Its First Potentially Threatening Near-Earth Object
|date       = 21 January 2014
|publisher  = European Southern Observatory
|url        = https://www.eso.org/public/unitedkingdom/announcements/ann14004/
|accessdate = 27 June 2018}}</ref>
<ref name="ESO-17052">{{Cite web
|title      = A Very Close Encounter
|date       = 10 August 2017
|publisher  = European Southern Observatory
|url        = https://www.eso.org/public/unitedkingdom/announcements/ann17052/
|accessdate = 27 June 2018}}</ref>
<ref name="NEA-Scout">{{cite web
|last1      = Mahoney |first1 = Erin
|title      = NEA Scout
|date       = 21 May 2016
|url        = https://www.nasa.gov/content/nea-scout
|website    = NASA
|accessdate = 27 June 2018}}</ref>
<ref name="Geocentric1992">{{cite web
|title=Horizons Batch for 1991 VG Geocentric orbital elements for 22 Feb to 22 March 1992
|publisher=[[JPL Horizons On-Line Ephemeris System|JPL Horizons]]
|type=Using 1992-Mar-18
|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271991+VG%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%271992-Feb-22%27&STOP_TIME=%271992-Mar-22%27&STEP_SIZE=%271%20day%27&CENTER=%27@399%27&OUT_UNITS=%27AU-D%27 }}</ref>
<ref name="removed">{{cite web
|title      = Removed Objects
|publisher  = NASA/JPL Near-Earth Object Program Office
|url        = https://cneos.jpl.nasa.gov/sentry/removed.html}}</ref>
}} <!-- end of refs -->
== External links ==
* {{NeoDys|1991VG}}
* {{JPL small body}}
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{{DEFAULTSORT:1991 VG}}
[[Category:Apollo asteroids|#]]
[[Category:Minor planet object articles (unnumbered)]]
[[Category:Earth co-orbital asteroids|#]]
[[Category:Claimed moons of Earth]]
[[Category:Discoveries by James V. Scotti|#]]
[[Category:Discoveries by the Spacewatch project|#]]
[[Category:Near-Earth objects in 1991|19911205]]
[[Category:Near-Earth objects removed from the Sentry Risk Table|#]]
[[Category:Astronomical objects discovered in 1991|19911106]]