12:26 30 January 2009 by Jessica Griggs

The Earth had company this week when an asteroid practically grazed the planet - passing by just 400,000 miles away. That's less than twice the distance of the Earth to the Moon.

The 8-metre-wide rock, known as 2009 BD, is a co-orbital asteroid, which means it orbits the Sun on almost the same plane as Earth does. The coupling offers rare opportunities to discover more about asteroids.

Rather than speeding past fleetingly like most asteroids, co-orbital ones synch up with Earth and perform a spiralling dance around it, sometimes lasting for many months or even years. They are sometimes referred to as second moons, despite their diminutive size.

On average, eight co-orbital asteroids are discovered a year, but 2009 BD is special because, with a tilt of just 1.5 degrees from the Earth-Sun plane, it has one of the most Earth-like orbits of any yet observed, says Paul Chodas of the Near Earth Object project at NASA's Jet Propulsion Laboratory in Pasadena, California.

This should make it easier to reach the asteroid in a spacecraft where a lander could scoop up samples and send them back to Earth, he says.

Mission window

The asteroid last passed close to us in 1955, but because its orbit is not quite a perfect circle, it will pass by again fairly soon on 2 June 2011.

"If we were to send a probe to this object, we should do it in 2011, because it won't be near the Earth again until 2034," says Chodas.

"Sending a spacecraft out to an asteroid to retrieve a sample is certainly a worthwhile mission for NASA to consider - we could learn much about the composition of asteroids," he says.

The Japanese have attempted such a feat with their Hayabusa spacecraft, which is on its way back from the asteroid Itokawa. It is expected to deliver its cargo in June 2010, but due to several problems during the mission it is not known whether it has been successful in obtaining a sample.