Phobos & Deimos

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Mars has two very small moons - Phobos and Deimos. They were discovered by Asaph Hall in 1877. He named them after Mars' mythological attendants: fear (Phobos) and terror (Deimos). It is likely that they did not form with the planet, but are captured asteroids. This is believed because they are both small, and Mars is reasonably close to the asteroid belt.
(Mariner 9, 1971)
Click images to enlarge (not to scale)
(MGS, 1998)

Both Phobos and Deimos are very close to the planet compared to Earth's Moon. Their orbital radii are 9,378 km and 23,459 km respectively. Phobos is the larger of the two, but would appear from the surface to be about 1/3 of the size of our Moon.

Odd Shapes

Mars's moons are so small that their gravity is not enough to have pulled them into a spherical shape. The first photograph of Phobos was taken by Mariner 9. Its mission was to map the surface of the parent planet. However, on arrival (becoming the first artifical satellite of another world), Mars was enveloped in a dust storm. It was ordered to turn its attention to the moons, delivering detailed photographs.

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Frequent Eclipses!

eclipse of the sun by PhobosPhobos orbits Mars roughly once every 7 hours - much faster than Mars's rotation. From Mars, Phobos is seen to rise then set in only 4 hours. Since Mars is so much larger, Phobos would normally be eclipsed as it passes into Mars's shadow. The first ever images of a martian eclipse (right) was taken by NASA rover Opportunity.
Image credit: NASA/JPL/Cornell

eclipse of the sun by DeimosDue to its great speed, Phobos regularly eclipses the Sun. However, unlike Earth's Moon, Phobos is too small to completely cover the solar disc. Instead it cuts off about a quarter of its light. This happens about 1,300 times a year, each lasting a maximum of 19 seconds!

Slow Mover

Deimos, on the other hand, is nearly three times further away than Phobos, and orbits Mars in about 30 hours. This is just a little longer than the martian day, so Deimos would hardly move in the night sky - although again it would probably be eclipsed, and would be very small and feint anyway. Remember that Mars is considerably further away from the Sun than Earth, so recives less sunlight.

European Study!

ESA's Mars Express probe entered orbit around Mars early in 2004. It will pass Phobos over 1000 times. It carried the unsuccessful British Beagle 2 probe that landed on the martian surface on Christmas Day 2003. Click for more.


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