Trojan asteroid: Earth stalker found in eternal twilight

AN ASTEROID 300 metres in diameter is stalking the Earth. 

Hiding in the pre-dawn twilight, it has marched in lockstep with our planet for years, all but invisible to our telescopes.

The rock is Earth's first confirmed Trojan, which can orbit the sun in either of two gravitational wells along the same orbital path as our planet. 

From the sun's point of view, these wells lie 60 degrees ahead of and behind the Earth, at Lagrange points where gravitational forces between the sun and the Earth balance out.

Trojans are common - Jupiter alone boasts about 5000, and Neptune and Mars each have their own smaller collections. But finding Earth's has proven difficult, because the Lagrange points lie towards the sun in the sky. 

Astronomers must look for the objects just before the sun rises or after it sets, and until now the glare of this sunlight has obscured the feeble light reflected from any rocks that might be hiding there.

Now Martin Connors of Athabasca University in Alberta, Canada, and colleagues have used a heat sensor to see past the gloaming. Using data from NASA's Wide-field Infrared Survey Explorer (WISE) satellite, they identified a 300-metre-wide Trojan now dubbed 2010 TK₇.

The rock is leading the Earth, and based on the team's calculations it is expected to be stable in an elliptical orbit around its Lagrange point for at least the next 10,000 years, drifting at between 20 million and 300 million kilometres from us (Nature, DOI: 10.138/nature/10233).

Like most Trojans, says Connors, the story of where 2010 TK₇ came from, and what it is made of, is an utter mystery.

It could be an errant, captured asteroid, or perhaps a "genesis rock" - a long-sought relic from the birth of the solar system about 4.5 billion years ago. 

If so, it may be identical to the rocks that came together to form the Earth, which means that studying its composition would tell us what the chemistry of our planet was like in the earliest stages of its existence.