(415029) 2011 UL21

, provisional designation, is an Apollo class potentially hazardous asteroid 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.

Description
briefly had about a 1 in a million chance of impacting in 2029. 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. 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.

With an absolute magnitude of 15.8, it is one of the brightest and therefore largest potentially hazardous asteroids (PHA) detected since. The next largest PHA (based on absolute magnitude) discovered in 2011 is with an absolute magnitude of 16.8.

On 27 June 2024 it had a relatively close fly-by with the Earth, reaching a minimum distance of 0.044 AU from it. It was revealed to be orbited by a moonlet.