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(415029) 2011 UL21
(415029) 2011 UL21 (former is 2011 UL21) is an Apollo class potentially hazardous asteroid (PHA) discovered on October 17, 2011 by the Catalina Sky Survey project.〔 The asteroid is estimated to have a diameter of . It was rated at Torino Scale 1 on October 27, 2011 with an observation arc of 9.6 days.〔 (415029) 2011 UL21 briefly had about a 1 in a million chance of impacting in 2029.〔 Its cumulative impact probability was dropped to 1 in 71 million by 2 November 2011 with an observation arc of 15 days. The 2029 impact date has been ruled out. The asteroid will be farther than 0.9. AU in 2029. It was removed from the Sentry Risk Table on 4 November 2011.〔 Palomar Observatory precovery images from 1989 and 1990 have extended the observation arc to 22 years.〔 Its next notable close approach to the Earth will be on June 27, 2024 at a distance of .〔
With an absolute magnitude (H) of 15.8,〔 it is potentially the largest/brightest potentially hazardous asteroid detected since .〔 The next largest PHA (based on absolute magnitude) discovered in 2011 is with an absolute magnitude of 16.8.〔
==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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