2015 SO2 (astronomical naming convention: 2015 SO2) is an Aten asteroid that is a temporary horseshoe companion to the Earth, the ninth known Earth horseshoe librator.
[6] Prior to its most recent close encounter with our planet (2015 September 30) it was an Apollo asteroid.
2015 SO2 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.
Gravitational interaction with Earth causes its orbit to change so that its average period is one year.
Its orbital evolution is characterized by alternating horseshoe and quasi-satellite episodes.