Scientists have discovered first Trojan asteroid companion to Earth that orbits around the sun along with Earth for thousands of years.

Asteroids are giant space rocks that orbit the sun and the one which shares an orbit with a planet are called Trojans.

This artist's concept illustrates the -first known Earth Trojan asteroid, discovered by WISE. The asteroid is gray and its extreme orbit is shown in green.

Trojans are asteroids that share the same orbit as Earth shares they constantly lead or follow the planet but they never collide with it.

Apart from Earth Trojan asteroid also share orbits with Saturn’s two moons, Neptune, Mars and Jupiter.

"These asteroids dwell mostly in the daylight, making them very hard to see. But we finally found one, because the object has an unusual orbit that takes it farther away from the sun than what is typical for Trojans. WISE was a game-changer, giving us a point of view difficult to have at Earth's surface," said Martin Connors of Athabasca University in Canada, lead author of a new paper on the discovery in the July 28 issue of the journal Nature.

Connors said the launch of NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft in 2009 changed all that.

"WISE looked at the infrared light coming from all over the sky and was particularly good at detecting asteroids, finding about 150,000 of them, including 500 which come near the Earth."

The asteroid, called 2010 TK7 was confirmed as an Earth Trojan and is nearly 1,000 feet (300 meters) in diameter. The asteroid is currently leading the Earth by about 50 million miles (80 million kilometers).

At least for the next 100 years, the asteroids will not come closer to Earth than 15 million miles (24 million kilometers).

The researchers by using ground-based telescopes have confirmed that the asteroid has been in a stable orbit with Earth for more than 10,000 years.

"It

Asteroid 2010 TK7 is circled in green, in this single frame taken by NASA's Wide-field Infrared Survey Explorer, or WISE.

's as though Earth is playing follow the leader," said Amy Mainzer, the principal investigator of NEOWISE at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "Earth always is chasing this asteroid around."

The asteroids that have orbits similar to Earth could make excellent candidates for future robotic or human exploration.

Asteroid 2010 TK7 travels too far above and below the plane of Earth's orbit, and is not a good target for future robotic or human exploration because it would require large amounts of fuel to reach it.

"This observation illustrates why NASA's NEO Observation program funded the mission enhancement to process data collected by WISE," said Lindley Johnson, NEOWISE program executive at NASA Headquarters in Washington.

"We believed there was great potential to find objects in near-Earth space that had not been seen before," Johnson added.

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