Abridged from a New Scientist article by Rebecca Boyle, September 30, 2014:
A newly discovered asteroid called 2014 OL339 is the latest quasi-satellite of Earth – a space rock that orbits the Sun but is close enough to Earth to look like a companion. The asteroid has been hanging out near Earth for about 775 years, but its orbit is unstable – it will probably move on about 165 years from now.
Quasi-satellites orbit in resonance with Earth, allowing our planet’s gravity to shift the rock’s position. The asteroid orbits the Sun every 365 days, as Earth does, but Earth’s gravity guides it into an eccentric wobble, which causes the rock to appear to circle backward around the planet.
The asteroid, which is between 90 and 200 metres in diameter, is among several different categories of space rock in Earth’s retinue besides our one satellite, the Moon. Rocks that hang out at a gravitational middle ground known as a Lagrange point, where they follow or lead Earth in its orbit, are called Trojans.