(308242) 2005 GO21

 is a large Aten near-Earth asteroid and potentially hazardous object. It has a well determined orbit with an observation arc of 7 years and an uncertainty parameter of 0. It was discovered on 1 April 2005 by the Siding Spring Survey at an apparent magnitude of 18.1 using the 0.5 m Uppsala Southern Schmidt Telescope.

Based on an absolute magnitude of 16.4, the asteroid has an estimated diameter of 1.6 km (within a factor of two). is the largest potentially hazardous asteroid (PHA) discovered in 2005. On 21 June 2012 it passed Earth at a distance of 0.043963 AU. The 2012 passage was studied with radar using Goldstone and Arecibo.