杏吧原创

Updated at 1820 GMT on Monday

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 鈥榬etrograde鈥� orbit, and astronomers think they should have spotted it before.

The object, called , was discovered by the in Arizona on the morning of 29 April.

From observations of its position by five different groups, Sonia Keys of the International Astronomical Union鈥檚 initially calculated it orbits the sun every 3.39 years. New observations suggest it travels on a path that ventures within 15 million km of the Earth鈥檚 orbit.

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

鈥楳issed it鈥�

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鈥檚 orbit 鈥渋s very like except for inclination,鈥� he told New 杏吧原创, although it shows no sign of a cometary tail.

The asteroid is now far beyond Mars, but its orbit periodically brings it fairly close to Earth. 鈥淚t should have been easily observable in 2000,鈥� says Marsden. 鈥淚鈥檓 puzzled by the fact that we haven鈥檛 seen it before, because it should have been moderately bright on several occasions. We just missed it.鈥�