More Moon and Mars

StarDate: August 15, 2009

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The Moon has a well-known companion at first light tomorrow: the planet Mars. It looks like a bright orange star a little to the Moon's upper right.

Mars itself has two moons -- Phobos and Deimos. They're nowhere near as impressive as our moon, though. They're small, oddly shaped chunks of rock. In fact, there's a good chance that they started out as asteroids, and were captured by Mars long ago.

Phobos is the larger of the two. It's shaped like a potato -- about 15 miles long by 10 miles wide. One end is scarred by a giant crater. The impact that created it may have fractured the entire moon, leaving big cracks that travel from one end to the other.

Phobos is in a low orbit, so it goes around Mars in a hurry. In fact, it's so fast that it completes about three orbits every day. It rises in the west, then sets in the east just a few hours later. As seen from the equator, it looks less than half as big as our Moon looks from Earth. And it's so low in the sky that you can't see it from the poles at all.

Deimos is both smaller and farther out. It takes almost three days to complete one orbit. From the Martian surface, it looks about like Venus looks from Earth -- a brilliant star moving leisurely across an alien sky.

And speaking of Venus, it's the dazzling "morning star" just below Mars and the Moon tomorrow. But the Moon will move much closer to it by Monday morning, and we'll have more about that tomorrow.

Script by Damond Benningfield, Copyright 2009

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