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(99907) 1989 VA is an Aten asteroid located in Venus' zone of influence that has frequent close encounters with the Earth. It was discovered on 2 November 1989 by C. S., E. M. Shoemaker and David H. Levy at Mount Palomar and was the eighth Aten asteroid discovered. 1989 VA was the first asteroid discovered with such a small semi-major axis (0.728 au, about the same as Venus), breaking 2100 Ra-Shalom's distance record (0.832 au), which had held for over a decade. It remained the asteroid with the smallest known semi-major axis for five years until the discovery of 1994 GL (0.684 au), which was the first asteroid discovered closer to the Sun than Venus. Being so close to Venus, it is also the first asteroid discovered within Venus' zone of influence. This means that it is close enough to Venus for the planet to capture 1989 VA into a co-orbital relationship. Though it is not a Venus co-orbital at the moment, it may become one in the future and may have been one in the past. Currently, the only known Venus co-orbitals are , and (as of 18 March 2013). Of the seven known objects in Venus' zone of influence, 1989 VA is the largest at about 1400 metres. All of these objects, like 1989 VA, have eccentric orbits that cross Mercury's and Earth's orbits as well. 1989 VA briefly held the title to Aten asteroid with the highest eccentricity (0.595), which was higher than the record the Earth co-orbital 3753 Cruithne (0.516) had set a few months earlier. 1991 VE (0.664) claimed that record two years later. The combination of a small semi-major axis and high eccentricity made 1989 VA the first Aten asteroid discovered to get closer to the Sun (0.295 au) than Mercury ever does. 2340 Hathor (the second Aten discovered, in 1976) had the smallest perihelion (0.464 au) earlier, which was about the same distance as Mercury's aphelion (0.467 au). It was not until (0.277 au) was discovered that an Aten asteroid with a lower perihelion was found. 1989 VA's eccentric orbit takes it out past the Earth, where it has encounters of about 0.15 to 0.20 au about every 3 to 5 years around October–November. It was discovered during its 1989 encounter and was about 0.17 au away at the time. Further observations were made in October 2002 and during the most recent close encounter in November 2007. 1989 VA is an 1400-metre diameter type Sq asteroid with a rotation period of 2.5 hours. == References == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「(99907) 1989 VA」の詳細全文を読む スポンサード リンク
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