co-orbital

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.