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(433953) 1997 XR2
(also written 1997 XR2) is an asteroid discovered on December 4, 1997.〔(【引用サイトリンク】url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1997+XR2 )〕 It has a diameter of 0.1-0.3 km and an estimated mass on the order of 1010 kilograms.
From early 2002 to February 24, 2006, was considered to have about a 1 in 10,000 chance of colliding with Earth on June 1, 2101, based on a 27-day observation arc in December 1997.〔
It was ranked at one on the Torino Scale of impact risk, and was the only near-Earth object to be ranked higher than zero (the scale is 0–10) until it was joined by at level one in November 2004, and then when 99942 Apophis (then known only by its provisional designation ) was temporarily assessed at level four in December 2004. and 99942 Apophis are now rated at level zero.
On February 24, 2006, was observed by the Mt. Lemmon Survey after being lost for more than 8 years.〔 The refinement of its orbit eliminated the possibility of impact in 2101. It is now known that on November 20, 2101, the asteroid will be from Earth.〔
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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