Deimos is the smaller outer moon of Mars (the inner moon is Phobos.) Deimos is composed of rock rich in carbonaceous material. Although the surface is noticeably smoother than Phobos it is still cratered. The two largest craters each measure about 3 kilometers in diameter.
In Greek mythology, Deimos was the twin brother of Phobos. The name Deimos (Dread or Terror) comes from Book XV of the Iliad. The name was suggested by Henry Madan (1838–1901) and was originally spelled Deimus.
Deimos was discovered by Asaph Hall III at the United States Naval Observatory in Washington, D.C. on August 12, 1877. Hall was deliberately searching for moons around Mars and also discovered Phobos six days later, on August 18, 1877.
Deimos has an albedo, spectra, and density similar to those of a C- or D-type asteroid, and most likely comprised of carbonaceous material. Deimos is 56% of the size of Phobos and highly non-spherical.
The craters Swift and Voltaire are the only two named features of the moon, and were named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.
Deimos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. No landings on Deimos have been made.
In 2017, Francisco Arias proposed a technique dubbed 'Sandbraking' which would use sand from Deimos (or Phobos) for aerobraking. Using this concept, a small amount of chemical fuel brought from Earth could be used to lift a larger quantity of sand from the surface of the moon(s), which then could be used for a Mars descent manuever.