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.