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WT1190F (9U01FF6, UDA34A3, or UW8551D) was a small temporary satellite of the Earth, thought to be space junk, that impacted the Earth on 13 November 2015 at 06:18:34.3 (±1.3 seconds) UTC.〔 It was first discovered on 18 February 2013 by the Catalina Sky Survey.〔 It was then lost, and reacquired on 29 November 2013. It was again discovered on 3 October 2015, and the object was soon identified to be the same as the two objects previously sighted by the team, who have been sharing their data through the International Astronomical Union's Minor Planet Center (MPC). An early orbit calculation showed that it was orbiting the Earth in an extremely elliptical orbit, taking it from within the geosynchronous satellite ring to nearly twice the distance of the Moon.〔 It was also probably the same object as 9U01FF6, another object on a similar orbit discovered on 26 October 2009. WT1190F had been orbiting the Earth as a temporary satellite since mid 2009 (named as UWAIS), if not longer. While it has not been positively identified with any known artificial satellites, its estimated density of 0.1 g/cm³ was much lower than would be expected of a natural object as even water has a density of 1 g/cm³. Hence, European Space Agency astronomers have concluded that the object was likely a fuel tank of some sort.〔〔 After more observations, astronomers determined that the object would impact the Earth on 13 November 2015 at 06:18 UTC (11:48 local time),〔 south of Sri Lanka.〔〔 Due to its small size, it was expected that most or all of the object would burn up in the atmosphere before impacting, but would be visible as a bright daytime fireball if the sky was not badly overcast.〔〔 ==Observations== WT1190F was first discovered by the Mount Lemmon Survey, a participant in the Catalina Sky Survey Near-Earth Object surveying program. The object was identified with an apparent magnitude 19.5 on 18 February 2013, and given the temporary designation UDA34A3, but was lost soon after, with an observation arc of only 5 hours.〔(【引用サイトリンク】url=http://www.projectpluto.com/pluto/mpecs/uda.htm )〕 However, it was again seen by the same survey on 29 November 2013 and given the designation UW8551D and lost again, only being observed for 1 hour 35 minutes.〔 Most recently, it was recovered on 3 October 2015 and given the designation WT1190F. Its orbit was soon calculated and found to be orbiting the Earth, but not with the orbit of any known artificial satellite. The object's orbit was soon connected, allowing more observations to be made, and several precovery observations have been found of the object, dating back to June 2009.〔 The type of orbit that WT1190F had was not stable long-term. An object in this type of orbit was likely to impact into Earth or the Moon, or acquire enough orbital speed to be ejected into orbit around the Sun.〔 It was not likely that it had been orbiting Earth for decades.〔 In 2011 the orbit had an eccentricity of 0.33 and perigee (closest approach to Earth) of .〔 It passed about from the Moon on 2012 May 24.〔 By 2013 the eccentricity had increased to 0.70 and the perigee decreased to .〔 During WT1190F's orbit, it changed significantly in brightness, from an apparent magnitude 16 at perigee, to magnitude 23 at apogee. It spent most of its time dimmer than magnitude 20.〔 This, combined with solar pressure acceleration, the Yarkovsky effect, and frequent orbital perturbations by the Moon, made it difficult to precisely predict its orbit and location. About one hour before atmospheric entry, the object had a R magnitude of 13.6,〔 roughly the brightness of Pluto. 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「WT1190F」の詳細全文を読む スポンサード リンク
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