Two apparently and the 2nd one is called Cruithne. Discovered sometime in 1997. wow.
http://www.space.com/scienceastronom...on_991029.html
Earth has a second moon, of sorts, and could have many others, according to three astronomers who did calculations to describe orbital motions at gravitational balance points in space that temporarily pull asteroids into bizarre orbits near our planet.
The 3-mile-wide (5-km) satellite, which takes 770 years to complete a horseshoe-shaped orbit around Earth, is called Cruithne and will remain in a suspended state around Earth for at least 5,000 years.
Cruithne, discovered in 1986, and then found in 1997 to have a highly eccentric orbit, cannot be seen by the naked eye, but scientists working at Queen Mary and Westfield College in London were intrigued enough with its peregrinations to come up with mathematical models to describe its path.
That led them to theorize that the model could explain the movement of other objects captured at the gravitational balance points that exist between all planets and the sun.
"We found new dynamical channels through which free asteroids become temporarily moons of Earth and stay there from a few thousand years to several tens of thousands of years," said Fathi Namouni, one of the researchers, now at Princeton University.
"Eventually these same channels provide the moons with escape routes. So the main difference between the moon (weve always known) and the new moons is that the latter are temporary -- they come and go, but they stay for a very long time before they leave."
Astronomers have long known that the solar system is full, relatively speaking, of asteroids.
Most orbit the sun in a belt between Mars and Jupiter, but a handful cross Earth's orbital path -- an imaginary curve through space along which our planet travels around the sun.
Namouni and his colleagues discovered several new types of orbital motion, which showed that some asteroids that cross Earths path may be trapped in orbits caused by the gravitational dance between Earth and the sun.
The work was published in a recent issue of Physical Review Letters.