Deimos

Deimos (from the Greek δειμος or deimos panic) is the outer, and the smaller, of the two moons of Mars.

Discovery
Deimos was discovered by the astronomer Asaph Hall on August 12, 1877, at the United States Naval Observatory in Washington, DC. The astronomer V. Knorre named the satellite Deimos (and also provided the name Phobos for the other satellite that Hall discovered six days later), 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
Deimos orbits Mars at a distance slightly further away than the distance of a synchronous orbit. For that reason, Deimos rises in the east and sets in the west of the Martian sky, about 2.7 days after its rising.

Deimos is not round, but is shaped like a potato, with dimensions 15 x 12.2 x 11 km. Its largest surface feature is a 2.3 km diameter crater. Deimos is heavily cratered but has a smooth-appearing surface. Its surface gravity is very weak, perhaps too weak to retain the ejecta from a crater impact. This ejecta is likely retained around Mars in a ring and redeposited as regolith on the surface of Deimos as it passes.

Deimos' orbit is so little inclined with respect to the ecliptic that it makes daily transits across the Sun.

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

Exploration
The NASA Viking 1 and Visiting mission::Viking 2 orbiters both have taken close-up photographs of Deimos on the way to deliver their respective landing craft to the Martian surface. Since then, several missions have made flybys of Deimos, including the Visiting mission::Mars Reconnaissance Orbiter. The two Visiting mission::Mars Excursion Rovers have taken photographs of Deimos as seen from the surface of Mars.

Related link

 * Deimos by Wikipedia

Déimos