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Why do asteroids change shape over time?
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16:43, July 11, 2008

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Researchers using a vast database to study 11,735 asteroids have discovered that asteroids change shape over time, and they think they know the reason why.

Gyula Szabó from the University of Szeged [Hungary] is the lead author of the study, which was published in the July edition of Icarus.

"There are several hundred thousand asteroids in our solar system," said Szabo. "They orbit the sun, but because they are small their surface gravity is low. This means that many have strange, irregular shapes."

Determining the shapes of these asteroids presented difficulties for Szabo and his colleague Laszlo Kiss from the University of Sydney. 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 realized 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 Szabo. "Using photometry, astronomers have determined shapes for about 1,200 asteroids in the past 30 to 40 years. We derived the shapes for 10 times more asteroids, but in half an hour! The results were really surprising. We saw there were families that included many elongated asteroids, and there were other ones which consisted of mostly spheroidal bodies."

In young groups of asteroids there are a great variety of shapes, hinting that they formed relatively recently from fragments of rock that later bound together. Asteroids in older families tend to be rounder. It seems to take one billion to two billion years for irregular asteroids to be transformed into smooth balls.

But what changes the asteroids' shape? Szabo 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.


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