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Phobos is the largest moon of the planet Mars. Phobos and Mars's other moon, Deimos, were discovered by Asaph Hall in 1877.

Its orbital period is 0.31910 Earth days, and its rotation is tidally locked to the same period so that one side of Phobos always faces the planet Mars. Its orbital inclination is 1.075 degrees from the martian equator. Its orbit is mildly eccentric (orbital eccentricity of 0.01) with a semi-major axis of 9378.5 km or about 2.5 Mars radii. This makes Phobos the closest orbiting moon in the solar system in terms of planetary radii. Phobos is just outside the Roche Limit of Mars, and is expected to be broken up by tidal forces during the next 50 million years.

Phobos is a relatively small moon, with oblong dimensions of 27 km x 18 km and a mass of 1.08 x <math>10^1</math><math>^3</math> tons. It's surface spectra and albedo are similar to carbonaceous chondrite asteroids, and it has a mean density of about 2000 kg/m<math>^3</math>. The largest crater on the moon, Stickney, has a radius of approximately 10 km, more than half the smaller diameter of Phobos.

Missions to Phobos

Phobos has been visited by several space probes:

    Mariner 9
    Viking 1 Orbiter
    Phobos 2
    Mars Global Surveyor
    Mars Express

The majority of space probe encounters with Phobos have been fly-by only. The Phobos 2 mission attempted a landing, but failed. High resolution photographs and surface spectra are available from all of these space probes.

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