title: 2015 YA
text: # 2015 YA
  
  2015 YA is a sub-kilometer asteroid, classified as near-Earth object of the Aten group, that is a temporary horseshoe companion to the Earth. It is the 11th known Earth horseshoe librator. Prior to a close encounter with the Earth on 15 December 2015, 2015 YA was an Apollo asteroid.
  
  ## Discovery
  
  2015 YA was discovered on 16 December 2015 by G. J. Leonard and R. G. Matheny observing for the Catalina Sky Survey. As of 9 March 2016, it has been observed 47 times with an observation arc of 5 days.
  
  ## Orbit and orbital evolution
  
  2015 YA is currently an Aten asteroid (Earth-crossing but with a period less than a year). Its semi-major axis (currently 0.99753 AU) is similar to that of Earth (1.00074 AU), but it has a moderate eccentricity (0.2791) and very low orbital inclination (1.6249°). It alternates between being an Aten asteroid and being an Apollo asteroid, although its orbital evolution is rather chaotic. As of 9 March 2016, this object is the 16th known Earth co-orbital and the 11th known object following a horseshoe path with respect to our planet. Asteroid 2015 YA follows an asymmetrical horseshoe path with respect to our planet; the value of its relative mean longitude oscillates about 180°, but enclosing 0°.
  
  ## Physical properties
  
  With an absolute magnitude of 27.4 mag, it has a diameter in the range 9–22 meters (for an assumed albedo range of 0.04–0.20, respectively).
  
  