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In research published today in Nature, scientists, led by Stephen Silvan from MIT, have discovered that asteroids in the Koronis family mysteriously seem to spin in only two different orientations. The Koronis family, like the other families of main belt asteroid from between Mars and Jupiter, are a group of 200 asteroids that formed as the debris from the break-up of a pre-existing larger asteroid due to a giant collision. When the orbits of the asteroids in the family are traced back in time they all meet up at a single point. From this the Koronis family is thought to have formed in a collision billions of years ago. The great age of the Koronis asteroids is also suggested by the surface of 243 Ida, that was visited by the NASA Galileo spacecraft, since it is covered in a large number of craters. In contrast Dactyl, the small moon of Ida, seems to be much younger at 100,000 years.
Silvan and colleages studied how fast and in what direction Koronis asteroids were spinning by examining how their brightness changes. Because asteroids are irregular in shape the amount of light they reflect changes as they spin. This light curve can be used to determine the rotation rates.
The researchers expected to find that the spin rates and directions of the asteroids were random, however, the measurements instead indicate that they spin in only one of two directions. This completely unexpected result is a profound mystery but may be an important clue to how asteroids behave when they are completely broken up in collisions with other asteroids.
More info: Nature
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