OTHER MOONS OF THE EARTH

The Moon seems to be the only satellite of the Earth. The Earth-Moon system is elsewhere fairly close to a binary system, the center of mass of the two bodies being close to the surface of the Earth. A question quickly arose whether the Earth had no other natural satellites near or far, small enough not to have been observed yet. Celestial mechanics does not allow other satellites to have a stable orbit around the Earth due to strong perturbations due to the Earth and the Moon, which opened explaining the absence of such satellites. 

Among the asteroids approaching Earth, some undergo a strong gravitational effect that they seem to orbit around the Earth while also revolving around the Sun. This is particularly the case with two asteroids, (3753 Cruithne) and 2002 AA29 of which the trajectories relative to the Earth look like the orbits of satellites. These configurations are not stable in the very long term but these two objects can to be considered as quasi-satellites of the Earth. Other asteroids such as 54509 YORP, (85770) 1998 UP1 et 2003 YN107 have also similar orbits named "horse shoe orbits".