NASA's Wide-field Infrared Survey Explorer (WISE) has uncovered Earth's first Trojan asteroid – a 300-metre wide rocky inhabitant orbiting the Sun in loops around the plane of Earth's orbit.

The term 'Trojan asteroid' is typically associated with the thousands of asteroids that occupy gravitationally stable locations, known as Lagrangian points, 60 degrees ahead of and behind Jupiter's orbit, but Trojans have also recently been discovered at Neptune and Mars, too. A recent discovery of an asteroid "stalking" Earth in its orbit (see our news story here) was interpreted as potential leakage from a then undiscovered Trojan population.

WISE stepped up to the game to identify the new object, 2010 TK7, which was confirmed by follow up observations with the Canada-France-Hawaii Telescope in Mauna Kea, Hawaii as the first official Earth Trojan.

"These asteroids dwell mostly in the daylight, making them very hard to see," says Martin Connors of Athabasca University in Canada, lead author of the paper describing the discovery in the 28 July issue of Nature. "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."

2010 TK7 is 300 metres wide and resides 50 million miles from Earth. It has a complex orbit that sees it frequently cross the plane of Earth's orbit. Simulations suggest that for the next 100 years, the asteroid will not come closer to the Earth than about 15 million miles.

"It's as though Earth is playing follow the leader," adds Amy Mainzer, the principal investigator of NEOWISE at NASA's Jet Propulsion Laboratory. NEOWISE is the Near Earth Object (NEO) subset of the primary WISE mission. "Earth always is chasing this asteroid around."

Earlier this year WISE identified a similarly sized asteroid stalking Earth in a horseshoe orbit – one that closely mimics the orbital motion of Earth around the Sun, but from the point of view of our planet, appears to slowly trace out a horseshoe shape in space.

"2010 SO16 is further up in the gravity well than 2010 TK7, so TK7 presumably has stronger links to a possible larger population," Connors tells Astronomy Now. "However, asteroids can move around in unexpected ways, particularly if non-gravitational forces [like the Yarkovsky effect which is caused by heat emitted from a small rotating body, carrying momentum with it] are included. I suspect there is a reservoir but think it is premature to say much about it. We need more detections of related objects, and of course preferably of Trojans that stay very near the Lagrange points."

Connors adds that the possible eventual detection of several Trojans would be needed to tell us anything about the history and evolution of our planetary neighbourhood. "Statistics of properties easily determined (colour, size, etc.) would be needed, and things hard to determine (composition from samples, etc.) would also be desired," he says.

The NEOWISE project provided observations of 155,000 main belt asteroids and more than 500 NEOs, discovering 132 that were previously unknown.

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