At first sight, it looked like an ordinary asteroid. It is anything but. In tomorrow's issue of Nature, three astronomers report that a 5-kilometer-wide rock follows Earth around the sun in a complicated, horseshoe-shaped orbit, making it Earth's only companion object besides the moon.
Theorists had long recognized the possibility that our planet might have a small partner conducting a complex orbital dance. In 1772, French mathematician Joseph-Louis Lagrange showed that a planet orbiting the sun is flanked by two points in space where a third object, like an asteroid, can sit comfortably in a stable orbit. These Lagrangian points lie on the planet's orbital path, a sixth of an orbit ahead of it and a sixth of an orbit behind. Since then, astronomers have found that Jupiter's Lagrangian points swarm with asteroids, known as the Trojans. But no one had detected any such companions of Earth until Paul Wiegert and Kimmo Innanen of York University in Ontario and Seppo Mikkola at the University of Turku in Finland analyzed the path of a small asteroid first spotted 11 years ago.
They found that it follows a circuit taking it around both Lagrangian points. It travels first around one Lagrangian point, then takes the long way around the Earth's orbit to the other Lagrangian point, circles it, and goes back again, completing the cycle in 385 years. To complicate matters further, this horseshoe-shaped track is only the asteroid's average path; like a moth circling a moving candle, it also describes large loops around this path, crossing the path of Venus and occasionally coming within a few million kilometers of Earth.
Astronomers are looking forward to studying Earth's new companion to find out how it got caught up in this orbital dance. They are also excited about seeing theoretical constructs reflected in reality. "The [horseshoe] orbit is a nice mathematical abstraction," says Carl Murray, an astronomer at Queen Mary and Westfield College in London. "Now nature provides an example. It's beautiful."