Deimos

Deimos is the smaller and outermost natural satellite of Mars. It was discovered in 1877 by American astronomer Asaph Hall and named after Deimos, the Greek god of dread, one of the sons of Mars in mythology.

Physical characteristics: Deimos measures roughly 12 kilometers across at its longest dimension and has an irregular, elongated shape. The surface is heavily cratered and dark, with a low albedo similar to carbonaceous asteroids. Its density is low, consistent with a rubble-pile structure, and its gravity is extremely weak by terrestrial standards.

Orbit and rotation: Deimos orbits Mars in a near-equatorial, prograde orbit at a mean distance of about 23,460 kilometers from the planet’s center (roughly 20,000 kilometers above the Martian surface). It is tidally locked, presenting the same face to Mars as it completes an orbital period of about 1.26 Earth days.

Origin: The origin of Deimos is not definitively known. The leading hypothesis is that it is a captured asteroid from the outer solar system, likely a C- or D-type body. An alternative idea suggested formation from debris ejected by a large ancient impact on Mars, but the capture scenario remains the more widely supported explanation.

Exploration: Deimos has been observed by multiple Mars missions that studied the planet from orbit, including Viking, Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. No mission has landed on Deimos. It remains a potential target for future exploration and sample-return efforts, such as JAXA’s Martian Moons eXploration program.