It was a theory that earth had two moons. After the collision of these two moons the moon we see now is formed, that's why our moon has one thicker surface side and other is thinner. You can find out that here in this article.
Recently NASA has found out that a new asteroid named "2016 HO3". This new asteroid orbits around the Sun and Earth as well. It is at quite far distance from the earth but is earth's companion for a long time and can be termed as "quasi-satellite".
There was one more asteroid called 2003YN107 had similar orbit like this asteroid but it got lost in the vicinity of the earth. This asteroid was discovered in 2003 and came into Earth's vicinity in 1999. In 2006 it regained another orbit going out of earth's vicinity. It will come back again in 60 years and in around 600 years it will be earth's quasi satellite.
The new asteroid "2016 HO3" has been around earth since a century and it is locked up on earth and it will stay in the same orbit for a century. "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon," said Chodas (NASA JPL).
It was found on 27 April 2016, by the Pan-STARRS 1 asteroid survey telescope Haleakala, Hawaii,operated by the University of Hawaii's Institute for Astronomy .