762 Pulcova
These are the first images of a small moon orbiting the large asteroid Pulcova.

It is only the third asteroid discovered to have a small moon. The team named the companion Petit-Prince, officially accepted by the International Astronomical Union.
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Image Credit: Canada-France-Hawaii Telescope & Southwest Research Institute.  

An international team of astronomers led by Dr. William Merline of the Boulder office of Southwest Research Institute (SwRI) have released the first images of a small moon orbiting the large asteroid Pulcova. It is only the third asteroid discovered to have a small moon. The team named the companion Petit-Prince, officially accepted by the International Astronomical Union.

Pulcova is an asteroid about 145 km in diameter. Its small satellite, roughly a 10th its size, orbits Pulcova every four days at a distance of about 805 km.
Asteroidal companions provide vital information about asteroids that has been difficult to obtain. Until now, the best measurements of asteroid masses - their bulk densities, such as whether they are "light" like ice, "dense" like metal, or inbetween like rocks - came from deflections of spacecraft flying past an asteroid. Such spacecraft encounters are rare, and deflections of other distant objects by an asteroid's gravity are weak and difficult to measure. But an asteroidal satellite, or twin, is a body whose trajectory is so strongly deflected by the asteroid's gravity that it is actually forced to orbit around it. The revolution time provides a measure of the body's mass, hence density.

Using such techniques, Merline's team find that Eugenia, Pulcova, and Antiope are all rather light bodies. They are much less dense than familiar rocks, more like ice, but their surfaces appear very dark, like rock. Interesting differences in the densities motivate further research on asteroids with satellites.
 
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