(308242) 2005 GO21 is a large Aten near-Earth asteroid and potentially hazardous object.

[2] It has a well determined orbit with an observation arc of 7 years and an uncertainty parameter of 0.

[2] It was discovered on 1 April 2005 by the Siding Spring Survey at an apparent magnitude of 18.1 using the 0.5-metre (20 in) Uppsala Southern Schmidt Telescope.

[1] Based on an absolute magnitude of 16.4,[2] the asteroid has an estimated diameter of 1.6 km (within a factor of two).

[4] On 21 June 2012 it passed Earth at a distance of 0.043963 AU (6,576,800 km; 4,086,600 mi).