Strange Asteroid Shapes Explained

Gyula Szabó from the University of Szeged [Hungary] is the lead author of the study, which was published in the July edition of Icarus. He explains, "There are several hundred thousand asteroids in our solar system. They orbit the sun, but because they are small their surface gravity is low. This means that many have strange, irregular shapes."

The most accurate data about asteroids comes from spacecraft fly-bys, but only a few asteroids have been examined that way. Radar observations can only be made of objects that get close to the Earth. Telescopes produce detailed images, but only for the largest asteroids.

Another option for obtaining information about asteroids is called "time-resolved photometry." The technique is surprisingly simple: By observing asteroids as they spin in space and then studying the amount of light reflected, scientists can get an idea of their shape. Getting accurate results from this method can take a long time, but the researchers realised that digital sky surveys could speed up the process. Such projects study thousands of objects every night. The Sloan Digital Sky Survey, for instance, mainly looks at stars and galaxies, but it also has gathered data on asteroids.

"This procedure was very economical," says Gyula. "Using photometry, astronomers have determined shapes for about 1,200 asteroids in the past 30 to 40 years. We derived the shapes for ten times more asteroids, but in half an hour!"

"The results were really surprising," says Gyula. "We saw there were families that included many elongated asteroids, and there were other ones which consisted of mostly spheroidal bodies."

But what changes the asteroids' shape? Gyula and his team have shown that asteroids change shape from elongated to roughly spherical due to being impacted during their lifetimes. They are like pebbles on the beach that become worn smooth over many years -- only in space, erosion is caused by small impacts as rocks knock into each other and chip pieces off.