One of a pair of irregular, small moons orbiting Mars, Deimos was discovered by the American astronomer Asaph Hall in 1877 along with its fellow satellite, Phobos.
Both moons are grey in colour, cratered and generally similar in appearance to asteroids that orbit between Mars and Jupiter. One theory is that both moons were once asteroids that were captured by Mars's gravitational force, but this has not been confirmed.
Photo: Deimos taken by the Mars Reconnaissance Orbiter (NASA/JPL-Caltech/University of Arizona)
Sir Patrick Moore explains how the asteroid belt between Mars and Jupiter was discovered. He also talks about Bode's Law, an 18th century empirical rule that was once thought to predict planets' orbits. It is now discredited by astronomers, including Sir Patrick.
The 19th and early 20th century American astronomer Asaph Hall discovered Mars' small moons, Phobos and Deimos. Observations of the moons' orbits allowed others to calculate the Red Planet's gravity, which is just under half that of Earth's.
Deimos ( /ˈdaɪməs/ DY-məs; also /ˈdiːməs/ DEE-məs; Greek: Δείμος; also DAY-moce or DEE-moce) is the smaller and outer of Mars's two moons (the other being Phobos). It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is Mars II.
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