Phobos

Phobos (from the Greek φόβος or phobos fear) is the inner, and the larger, of the two moons of Mars.

Discovery
Phobos was discovered by the astronomer Asaph Hall on August 18, 1877, at the United States Naval Observatory in Washington, DC. Astronomers were especially excited by the very short sidereal month of Phobos--about one-third of a Martian sidereal day--a phenomenon without precedent at the time.

The astronomer V. Knorre named the satellite Phobos (and also provided the name Deimos for the other satellite that Hall had discovered six days earlier), per a suggestion by Henry G. Madan of Eton, based on the names given in The Iliad for the two servants of Ares, the Greek god of war, named Fear (Phobos) and Panic (Deimos).

Orbital and physical characteristics
Phobos orbits Mars at a distance closer than the distance of a synchronous orbit. For that reason, Phobos actually rises in the west and sets in the east of the Martian sky. Yet because Phobos orbits Mars so closely, it is not visible above the horizon at latitudes higher than 70° north or south.

Phobos is not round, but is shaped like a potato, with dimensions 27 x 21.6 x 18.8 km. Its most remarkable surface feature is Crater Stickney, named for the family of Mrs. Asaph Hall. Stickney has a diameter of 10 km, and its outer rim has pronounced surface striations suggesting a tremendous impact.

Phobos is spiraling in toward Mars at a rate of 18 mm per year. At that rate, if it had 50 million years to wait, it would enter the Martian atmosphere and drop to the surface in a cataclysmic crash, or else disintegrate into a ring.

Origin
The favored theory among conventional astronomers is that Phobos and its companion moon Deimos are captured C-type asteroids. However, that theory is not universally accepted.

The Soviet spacecraft Phobos 2 discovered that Phobos was venting something into space, but all its systems failed before its controllers could determine what that substance was--most commentators believe that it was water.

In the 1950's and 1960's, a number of American and Soviet astrophysicists actually speculated that Phobos was hollow, and even that it was an artificial construct. This speculation had part of its basis on a lack of understanding of Phobos' density and a gross overestimation of the rate of Phobos' orbital decay--5 cm per year instead of the 1.8 cm per year that present observations have established.

Exploration
The NASA Visiting mission::Viking 1 orbiter took the first close-up photographs of Phobos on its way to deliver a landing craft to the Martian surface. Since then, several missions have made flybys of Phobos, including the orbiter that services the Spirit and Opportunity rovers, and the European Space Agency's Visiting mission::Mars Express.