2003 YN107

 is a tiny asteroid, classified as a near-Earth object of the Aten group moving in a 1:1 mean-motion resonance with Earth. Because of that, it is in a co-orbital configuration relative to Earth.

Discovery, orbit and physical properties
was discovered by the Lincoln Near Earth Asteroid Research (LINEAR) system in orbit around the Sun on 20 December 2003. Its diameter is approximately 10 to 30 metres. The object is on NASA's Earth Close Approach list, and is estimated to miss Earth by 0.01 AU. It revolves around the Sun on an Earth-like, almost circular, orbit. Its orbital period of 363.846 days also is very close to the sidereal year.

Co-orbital with Earth and orbital evolution
From approximately 1997 to 2006, the asteroid remained within 0.1 AU of Earth and it appeared to slowly orbit Earth. However, is no second moon, as it is not bound to Earth. It is the first discovered member of a postulated group of coorbital objects, or quasi-satellites, which show these path characteristics. Other members of this group include 10563 Izhdubar, 54509 YORP,, , and. Before 1996, the asteroid had been on a so-called horseshoe orbit around the Sun, along the Earth's orbit. After 2006, it had regained such an orbit. This makes it very similar to, which will become a quasi-satellite of Earth in approximately 600 years.