Earth’s Long-Sought-For Trojan Asteroid Companion Found!

The discovery of a long-sought-for Trojan companion, called 2010 TK7, which measures only about 300 yards across, makes Earth the fourth planet in the solar system that's known to share its orbit with an asteroid. The tiny object was discovered last year by NASA's WISE satellite. The asteroid sometimes ranges so far ahead that it's on the opposite side of the sun from Earth, said Martin Connors of Canada's Athabasca University in Alberta. Connors and colleagues were able to focus a ground-based telescope in Hawaii on it in April, determining its orbit with enough precision to show it was a Trojan.

Asteroids are giant space rocks that orbit the sun, and ones that share an orbit with a planet are called Trojans. Scientists had previously found a few for Mars and Neptune and nearly 5,000 for Jupiter. Spotting one in Earth's orbit is difficult from the ground because the potential locations are generally in the daytime sky.

Donald K. Yeomans, manager of NASA's Near-Earth Object Program Office, who didn't participate in the discovery, agreed that the asteroid is a Trojan. Most scientists suspected Earth had them, he said, and "I would guess there's others.

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