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Deimos (Greek: Δεῖμος, pronounced [ˈðeimon]), is the smaller and outermost of the two natural satellites of the planet Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is not capable of sustaining an atmosphere or water, and is thus uninhabitable. Deimos was discovered by Asaph Hall, III at the United States Naval Observatory in Washington, D.C., on 12 August 1877, at about 07:48 UTC (02:48 EDT). Hall also discovered Phobos on 18 August 1877, at about 09:14 GMT (04:14 EDT).
Deimos is named after Deimos, a figure from Greek mythology who personified dread (deima). The name “Deimos” is pronounced /ˈdaɪmɒs/ DY-mos or sometimes /ˈdiːməs/ DEE-məs . It rhymes with “seismos”.