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Apollo class asteroid
(415029) 2011 UL21, provisional designation 2011 UL21, is an Apollo class potentially hazardous asteroid discovered on October 17, 2011, by the Catalina Sky Survey project.[1] The asteroid is estimated to have a diameter of 2.5 kilometres (1.6 mi). It was rated at Torino Scale 1 on October 27, 2011, with an observation arc of 9.6 days.[4] 2011 UL21 is the largest asteroid ever to be rated above a 0 on the Torino Scale.
2011 UL21 briefly had about a 1 in a million chance of impacting in 2029.[5] Its cumulative impact probability dropped to 1 in 71 million by 2 November 2011 when the observation arc reached 15 days. It was removed from the Sentry Risk Table on 4 November 2011 when all impact scenarios for the next 100 years or more were ruled out.[6] During 2029, the closest approach to Earth is 1.6 AU. Palomar Observatory precovery images from 1989 and 1990 have extended the observation arc to 22 years.[7]
With an absolute magnitude of 15.8,[2] it is one of the brightest and therefore largest potentially hazardous asteroids (PHA) detected since (242450) 2004 QY2.[8] The next largest PHA (based on absolute magnitude) discovered in 2011 is 2011 WO41 with an absolute magnitude of 16.8.[8]
On 27 June 2024 it had a relatively close fly-by with the Earth, reaching a minimum distance of 0.044 AU (6,600,000 km; 4,100,000 mi) from it.[9][10] It was revealed to be orbited by a moonlet.[11]
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