Phobos is the larger of the two moons of Mars. It was discovered by astronomer Asaph Hall on 17th August 1877 in Washington DC.

Diameter: 27 x 22 x 18 km
Mass: 1.07 x 1016 kg
Composition: Rock
Orbit: 9,378 km from Mars

Measuring 27 × 22 × 18 km, Phobos is the largest moon of Mars and orbits closer to the planet than the only other moon, Deimos. Phobos is over seven times larger than Deimos and is the only moon to orbit as close to its primary planet. Originally spelt Phobus, Phobos was discovered after Asaph Hall embarked on a search for Martian moons.

The names of both Phobos and Deimos were the idea of British scholar Henry Maden and adapted from Book XV of the Iliad. In Greek mythology, Phobos and Deimos are both sons of Ares and Aphrodite, who are the Greek versions of the Mars, the Roman god of war and Venus, the god of love.

The composition of Phobos is carbon-rich porous rock, similar to that of D-type asteroids. As with other objects of this size, it is irregularly shaped yet its surface is less reflective than other bodies found in the Solar System. Phobos has a heavily cratered exterior, with the largest crater, measuring nine kilometres, known as Stickney Crater. This crater is named after Angeline Stickney Hall, the wife of Asaph Hall. All of the other craters found on Phobos are named after characters and places from Gulliver’s Travels and also astronomers who have studied Phobos in depth. Kepler Dorsum is named after astronomer Johannes Kepler and is the only ridge with a name.

The surface of Phobos is also marked with a series of grooves and ridges. Most measure less than 30 metres in depth, 100 to 200 in width and up to 20 km in length. There is evidence of fine regolith at least 100 metres deep on the surface of Phobos. Regolith is the accumulation of dust, soil and other such particles.

Phobos lies close to Mars’s equatorial plane. It orbits Mars in just over seven hours and 40 minutes, meaning that it goes round the planet three times a day. This is quicker than Mars rotates as it orbits below the synchronous orbit radius. Phobos’s orbital radius is getting smaller and it is likely that it will eventually collide with Mars in about 50 million years.

Many space missions en route to Mars have photographed Phobos, beginning in 1971. The Russian space probes, Phobos 1 and Phobos 2, were launched in 1988 but were unsuccessful in returning data. A further attempt in 2011 failed when the probe Fobos-Grunt crashed down to Earth in January 2012. The Mars Global Surveyor has, however, observed the surface of Phobos in detail.

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