The discovery of a 2- to 3-kilometre-wide asteroid in an orbit that goes backwards has set astronomers scratching their heads. It comes closer to Earth than any other object in a 'retrograde' orbit, and astronomers think they should have spotted it before.

The object, called 2009 HC82, was discovered by the Catalina Sky Survey in Arizona on the morning of 29 April.

From observations of its position by five different groups, Sonia Keys of the International Astronomical Union's Minor Planet Center calculated it orbits the sun every 3.39 years on a path that ventures within 3.5 million km of the Earth's orbit. Combined with its size, that makes 2009 HC82 a potentially hazardous asteroid.

What's really unusual is that the calculated orbit is inclined 155° to the plane of the Earth's orbit. That means that as it orbits the Sun, it actually travels backwards compared to the planets. It is only the 20th asteroid known in a retrograde orbit, a very rare group. None of the others comes as close to the Earth.

More observations needed

Comets, which originate on the outer fringes of the solar system, are much more likely to have retrograde orbits than asteroids. In part, this is because passing stars or planets can kick them out of their original orbits and onto unusual paths, bringing them into the inner solar system, where we tend to see them.

Some retrograde asteroids may in fact be burnt-out comets, says Brian Marsden of the Minor Planet Center. The size and shape of the new asteroid's orbit "is very like Encke's comet except for inclination," he told New Scientist, although it shows no sign of a cometary tail.

The calculated orbit is the best fit to the available observations, but small observational errors could make a big difference in that calculation. "I'd feel happier about it if we get some more observations," says Marsden.

The asteroid is now far beyond Mars, but its orbit periodically brings it fairly close to Earth. "It should have been easily observable in 2000," says Marsden. "Why wasn't it seen then?" He hopes new observations will answer that question.