An asteroid (circled in green) is tracking Earth from 50 million miles away, according to astronomers.

NASA/JPL-Caltech/UCLA

he image: Scientists have known for many years that asteroids often follow planets, moving in the same orbit around the sun. These so-called "Trojan asteroids" have been spotted tagging alongside Jupiter, Mars, and Neptune. Earth, however, was believed to be free of these celestial stalkers — until now. Astronomers from Athabasca University in Canada, using data from an infrared NASA telescope that's orbiting the Earth, discovered an asteroid tracking us from just 50 million miles away. (See an image at right, and an artist's rendering below.) Earth's Trojan asteroid is usually obscured by the light from the Sun, which is why it's only recently been discovered. Besides its orbit, researchers know very little about this asteroid, named 2010TK7. It's estimated to be roughly 1,000 feet across, "about the size of a small neighborhood," says astronomer Martin Connors, as quoted by the Australian Broadcasting Corp. 

The reaction: "Fascinating," says Jonathan Amos at BBC News. Trojans like 2010TK7 "would make possible targets for astronaut missions." And finding this Trojan asteroid means there are probably other asteroids following Earth, just waiting to be discovered. In the meantime, there's no need to worry that our little asteroid will collide with Earth, says Charles Q. Choi at Space.com. Because 2010TK7 is expected to maintain a stable orbit for at least 10,000 more years, it won't get "any closer than 12.4 million miles, which is more than 50 times the distance from Earth to the moon." See for yourself:

Image credit: Paul Wiegert, University of Western Ontario, Canada