Barbara's correct. Phobos orbits Mars faster than Mars rotates, leading some to speculate that it's artificial. It's orbital decay was also said to be too rapid for a natural object.
It has been found, though, that Phobos' orbital peculiarities are completely natural, after the Mariner 9 probe went to Mars and got its tape measure out (so to speak).
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Phobos the Space Station?
Martian intelligent-design speculation has not been limited to Mars features. In 1945, Bevan Sharpless discovered that Mars's moon Phobos is slowly spiraling into Mars. And in 1959, Iosif Shklovskii analyzed possible causes of that inspiral and concluded that Phobos has a density much lower than that of most solid objects, implying that it must be hollow. So could Phobos be a space station? Spacecraft pictures, however, reveal that Phobos looks like a lumpy, cratered potato, and its gravitational attraction of spacecraft reveals that its density is about 2 g/cm^3, not much less than that of typical rocks (3 g/cm^3). Mars's other moon Deimos has a similar appearance and a slightly-smaller density, and the four asteroids flown by to date (243 Ida, 951 Gaspra, 253 Mathilde, and 433 Eros) are likewise similar. So if Phobos is a space station, it is a well-camouflaged one that is largely rock.
from: [
wiki.cotch.net]
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[
www.mercuryrapids.co.uk]
[
www.ufodata.co.uk]
