NEO: 3Km wide Asteroid in contra orbit - potential impact threat

The newly discovered asteroid 2009 HC82 travels on an orbit that is tilted by 155° with respect to the orbital plane of the planets, which means it travels in the opposite direction (Illustration: NASA/JPL)

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.

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 20^th asteroid known in a retrograde orbit, a very rare group. None of the others comes as close to the Earth.

The retrograde orbit of this asteroid will greatly increase the 'speed at impact' of this object, colliding with the Earth

The retrograde orbit of this asteroid will greatly increase the 'speed at impact' of this object and its potential for major damage. This is because both the Earth and HC82 will be travelling towards each other at high speed, like two trains heading towards each other on the same track.