Phobos (pronounced /ˈfoʊbəs/ FOE-bəs, or as Greek Φόβος) is the larger and closer of Mars' two small moons, named after the horses of the mythical Roman god of war, Mars. It's shaped something like an apple with a bite out of it.

Both Phobos and Deimos were detected first by American astronomer Asaph Hall, Sr. on August 12, 1877at the U.S. Naval Observatory in Washington, D.C. Through his 26-inch telescope, Hall spotted what he called a "Mars star" moving against the black background of space near the Red Planet. He scribbled in his log that Phobos was "faint and difficult to observe."

Physical characteristics
Phobos is heavily cratered, irregular, nonspherical body around 27 × 22 × 18 km in size. Phobos is one of the least-reflective bodies in the solar system. With the Spectroscope it appears to be similar to the D-type asteroids and is apparently of composition similar to carbonaceous chondrite material. Phobos' density is too low to be solid rock and it is known to have significant porosity. These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface regolith layer lacks water but ice below the surface is present. Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation.

The most prominent surface feature is Stickney crater, named after the discovers wife, Angeline Stickney Hall, Stickney being her maiden name. The impact that created Stickney must have almost shattered Phobos. Many grooves and streaks also cover the surface, the grooves are typically less than 30 m deep, 100 to 200 m wide, and up to 20 km in length. Analysis has revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit and must have been excavated by material ejected into space by impacts on the surface of Mars.

Orbital characteristics
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°.

As seen from Mars' equator, Phobos would be one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes would see a smaller angular diameter because they would be significantly further away from Phobos. Phobos' phases, in as much as they could be observed from Mars, take 0.3191 days (Phobos' synodic period) to run their course, a mere 13 seconds longer than Phobos' sidereal period.

As seen from Phobos, Mars would appear 6 400 times larger and 2 500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere.

Phobos' low orbit is declining by 20 meters per century, and it is likely to be destroyed in around 11 million years. Then it will either impact the surface of Mars or break up into a planetary ring.

Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedos and densities very similar to those of C- or D-type asteroids. Based on this similarity, one hypothesis is that both moons may have been captured into Martian orbit from the main asteroid belt.

The moons both have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a way for circularizing the initially highly-eccentric capture orbit and also adjusting the inclination into the equatorial plane. Most theories have relied upon a combination of atmospheric drag and tidal forces, however the current Mars atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Another hypothesis is that the moons accreted in the present position. Still nnother hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal.

Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9 in 1971, followed by Viking 1 in 1977, Mars Global Surveyor in 1998 and 2003, Mars Express in 2004 and 2008, and Mars Reconnaissance Orbiter in 2007 and 2008. The only dedicated Phobos probes have been the Soviet Phobos 1 and Phobos 2; the first was lost en route to Mars, and the second returned some data and images before failing prior to its detailed examination of the moon. The first robot landing was 2012AD. The first human landing was during the second Mars mission to Mars when Carl Kruger and Maria Alvarez landed on Phobos as part of the scientific mission

Phobos has been constantly inhabited since 2071, when the first Science Station was built, followed by the Mars Industrial Works in 2098 and the UN Defense base in 2113. The gravity cetrifuge was built in 2120.

Named Geological Features
The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler. A number of Craters have been named.
Clustril Character in Gulliver's Travels
D'Arrest Heinrich Louis d'Arrest, astronomer
Drunlo Character in Gulliver's Travels
Flimnap Character in Gulliver's Travels
Gulliver Main character of Gulliver's Travels
Hall Asaph Hall, discoverer of Phobos
Limtoc Character in Gulliver's Travels
Reldresal Character in Gulliver's Travels
Roche Édouard Roche, astronomer
Sharpless Bevan Sharpless, astronomer
Skyresh Character in Gulliver's Travels
Stickney Angeline Stickney, wife of Asaph Hall
Todd David Peck Todd, astronomer
Wendell Oliver Wendell, astronomer

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