2015 SO2

is an Aten asteroid that is a temporary horseshoe companion to the Earth, the ninth known Earth horseshoe librator. Prior to its most recent close encounter with our planet it was an Apollo asteroid.

Discovery

was discovered on 21 September 2015 by B. Mikuž observing with the 0.6-m f/3.3 Cichocki telescope at the Črni Vrh Observatory in Slovenia. As of 30 November 2015, it has been observed 84 times with an observation arc of 9 days.

is currently an Aten asteroid. Its semi-major axis is similar to that of Earth, but it has a relatively low eccentricity and moderate orbital inclination. Gravitational interaction with Earth causes its orbit to change so that its average period is one year. It alternates between being an Aten asteroid and being an Apollo asteroid, changing dynamical status every 113 years approximately. As of 30 November 2015, this object is the 14th known Earth co-orbital and the 9th known object following a horseshoe path with respect to our planet. Its orbital evolution is characterized by alternating horseshoe and quasi-satellite episodes.

With an absolute magnitude of 23.9, it has a diameter in the range of 50–111 meters.