The Earth has a very skittish dance partner. Locked ahead of the Earth’s orbit lies a 300-metre lump of rock — Earth’s first Trojan asteroid, astronomers report today in Nature.
The points 60 degrees ahead of and behind a planet are one type of Lagrange point — stable gravitational wells where objects can lurk, caught between the Sun’s gravity and the planet’s. Objects residing in these spots are known as Trojans. With the discovery, the Earth joins Mars, Neptune and Jupiter as planets with known Trojans.
Trojans around Earth have been difficult to find because the objects dwell mostly in the daytime sky. But Martin Connors of Athabasca University in Canada and his colleagues used data from the Wide-field Infrared Survey Explorer (WISE), which launched in 2009 and has found more than 500 near-Earth objects. The researchers confirmed that the object, called 2010 TK7, was a Trojan through follow-up observations with telescopes in Hawaii.
The authors say the object’s behavior is fairly chaotic — it dances around the orbital plane with a tadpole-like motion. They say the asteroid might jump to other Lagrange points or orbital modes, and its stability can’t be guaranteed beyond about 7,000 years. The Earth better enjoy its pas de deux while it can.