2009 HC82: A Burnt-Out, Eccentric and Backward Near-Earth Asteroid

The Solar System often throws up surprises for astronomers, but the recent discovery of a 2- to 3-km wide asteroid called 2009 HC82 has sent observers in a spin. A retrograde spin to be precise.

This particular near-Earth asteroid (NEO) should have already been spotted as it has such a strange orbit. It is highly inclined, making it orbit the Sun backwards (when compared with the rest of the Solar System’s planetary bodies) every 3.39 years. What’s more, it ventures uncomfortably close (3.5 million km) to the Earth, making this NEO a potentially deadly lump of rock…

2009 HC82 was discovered on April 29th by the highly successful Catalina Sky Survey, and after independent observations by five different groups, it was determined that the asteroid has an orbit of 3.39 years and that its orbit is very inclined. So inclined in fact that the asteroid’s orbit takes it well out of the Solar System ecliptic at an angle of 155°. Inclined orbits aren’t rare in themselves, but if you find an asteroid with an inclination of more than 90°, you are seeing a very rare type of object: a retrograde asteroid.

The last time I wrote about a retrograde asteroid was back in September 2008 (Kuiper Belt Object Travelling the Wrong-Way in a One-Way Solar System), when a University of British Columbia researcher spotted a rather unique retrograde Kuiper belt object (called 2008 KV42) that had a large looping orbit with an inclination larger than 90°. It was nicknamed “Drac” after Dracula’s ability to walk on walls.

2009 HC82 is therefore not only rare, it is also very strange. It orbits the Sun the wrong way (therefore making it very inclined), it is a potentially hazardous NEO (it is smaller than the 10 km asteroid that is attributed to wiping out the dinosaurs, but it would cause significant devastation on a global scale if it did hit us) and it is very eccentric.

All these orbital components have led to speculation that 2009 HC82 is in fact a “burnt out” comet. Comets originate from the Oort Cloud, a theoretical region cometary nuclei that occasionally gets nudged by gravitational disturbances when stars pass by. The Oort Cloud is not restricted to a belt along the ecliptic (like the asteroid belt or the Kuiper belt), it encapsulates our Solar System. Therefore, this may explain 2009 HC82’s bizarre trajectory; it was a comet, but all the ice has vaporized, leaving a rocky core to fling around the Sun on a death-defying orbit, buzzing the inner Solar System.

Brian Marsden of the Minor Planet Center agrees that some retrograde asteroids could be burnt-out comets. The size and shape of the new asteroid’s orbit “is very like Encke’s comet except for inclination,” he said, but the only difference is the fact that 2009 HC82 has no cometary tail.

More observations are needed before a definitive conclusion can be made, but Marsden is confused as to why this object has not been discovered before now. “It should have been easily observable in 2000,” says Marsden. “Why wasn’t it seen then?”

It is hoped further investigation may answer this question…

Source: New Scientist