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Curiosity rover watches Mars’ moons pass in the night

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Although most of the images we get from the Mars Curiosity rover are of the ground, every once in awhile the rover looks upwards to catch something particularly cool going on in the sky.

The latest event it captured was one of Mars' moons, Phobos, passing in front of the other, Demos:

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Phobos and Demos are thought to be large asteroids that were caught in Mars' orbit long ago.

Phobos is the bigger of the two, and the closer of the two to Mars. It orbits so quickly around the planet (in around 7 hours and 40 minutes) that it actually rises in the west and sets in the east! We'd actually see the same thing here on Earth if the Moon went around us as quickly! Phobos is destined for destruction someday, as its orbit gets smaller by a small bit every year. In roughly 30-50 million years, it will be torn apart by Mars' gravity, becoming a debris ring around the planet. Although it hasn't been confirmed yet, Demos might be headed in the opposite direction, getting further away from Mars every year.

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