2015 XX₁₆₉ (also written 2015 XX169) is an Apollo asteroid that is a temporary horseshoe companion to the Earth, the tenth known Earth horseshoe librator.^[5] A close encounter with the Earth on 14 December 2015 caused the value of the semi-major axis of 2015 XX₁₆₉ to drift slowly upwards, and the object evolved from an Aten asteroid to an Apollo asteroid about a year after this close approach.

Discovery

2015 XX₁₆₉ was discovered on 9 December 2015 by R. G. Matheny observing with the 1.5-m reflector telescope at the Mount Lemmon Survey.^[6] As of 6 December 2016, it has been observed 47 times with an observation arc of 363 days.^[2]

Orbit and orbital evolution

2015 XX₁₆₉ is currently an Apollo asteroid (Earth-crossing but with a period greater than a year). Its semi-major axis (currently 1.00096 AU) is similar to that of Earth (1.00074 AU), but it has a relatively low eccentricity (0.18431) and moderate orbital inclination (7.640°). It alternates between being an Apollo asteroid and being an Aten asteroid, changing dynamical status approximately every 130 years. As of 9 March 2016, this object is the 15th known Earth co-orbital and the 10th known object following a horseshoe path with respect to our planet. Asteroid 2015 XX₁₆₉ follows an asymmetrical horseshoe path with respect to our planet; the value of its relative mean longitude oscillates about 180°, but enclosing 0°.^[5]

Physical properties

With an absolute magnitude of 27.4, it has a diameter in the range of 9–22 meters (for an assumed albedo range of 0.20–0.04, respectively).