Phobos is a bit of an enigma. It looks like a lumpy potato, barely 17 miles across. Its small size and low orbit around Mars once made people wonder if it wasn't a moon at all, but a space station put in orbit by an advanced Martian civilization.
Now scientists are reasonably sure it is a moon, but they'd like to know more about it.
Since the end of February, the Mars Express spacecraft has been orbiting ever closer to the tiny moon.
The closest approach will have the spacecraft a mere 42 miles from the surface, but instead of taking pictures, scientists will monitor radio signals from the spacecraft as it flies by. Tiny shifts in frequency will be caused by changes in the gravitational pull of Phobos ... and those changes can be used to measure the distribution of mass inside the moon.
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