Tags: Planet.
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 2.5 km. It was rated at Torino Scale 1 on October 27 2011 with an observation arc of 9.6 days. 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.