Earth's first trojan asteroid was recently detected by Martin Conners (Athabasca University) and colleagues using NASA's WISE satellite, which is surveying the sky at infrared wavelengths. Trojans are asteroids that reside in or near two sites that lead or trail a planet's orbit by 60 degrees, and these orbits are stablized by the combined gravities of the Sun, the planet, plus the centrifugal force that is due to the asteroid's orbital motion. This particular asteroid is known at 2010 TK7, and it is small, maybe a half mile across. This Trojan is indicated by the small white dot in the above graphic. The blue dots show the Earth's motion about the Sun, while the green dots show that 2010 TK7's motion can carry it quite far from the stable equilibrium site at 60 degrees, which is also known at the L4 Lagrange point. Indeed, orbit calculations by Paul Wiegert (University of Western Ontario) show that this is only a temporary Trojan, since Earth captured that object at its L4 point about 1500 years ago, and it will probably escape back into interplanetary space in a comparable amount of time. See Wiegert's very nice website on 2010 TK7 for more details.