Space · Moons

Deimos

A moon of Mars — The smaller of Mars's two moons — so distant and small that from the Martian surface it looks like a bright star, not a disk.

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Quick facts

Parent planet

Mars

Diameter (mean)

12.4 km

Mass

1.48 × 10¹⁵ kg
2e-08 Moon masses

Mean orbital radius

23,463 km

Orbital period

1.263 Earth days

Discovery year

1877

Discoverer

Asaph Hall

Naming origin

Greek personification of dread; son of Ares

Surface conditions

Deimos is the smaller, more distant of Mars's moons — 15×12×11 km, orbiting at 23,463 km (about 2.5 times Phobos's orbital radius). Its longer orbital period (30 hours) means it slowly drifts west across the Martian sky over several days, while Phobos races east in hours. From the Martian surface Deimos would appear as a bright star, not a resolvable disk.

The surface is smoother than Phobos's because dust from impacts has filled in older craters rather than escaping the moon's weak gravity. Two craters — Swift and Voltaire — are named for authors who wrote of two Martian moons before they were discovered (Jonathan Swift in Gulliver's Travels, 1726; Voltaire in Micromégas, 1752). Both fictional accounts predicted two moons with approximately the orbital characteristics later observed, an oft-cited coincidence in the history of science.

Missions and observations

Every Mars-system mission has had an opportunity to image or characterize Deimos. The list below is the Mars-system mission catalog; specific Deimos encounters are documented in mission archives.

Mission	Year at Mars	Status
Mariner 4 NASA	1965	Completed
Mariner 9 NASA	1971	Completed
Viking 1 NASA	1976	Completed
Viking 2 NASA	1976	Completed
Mars Pathfinder & Sojourner NASA	1997	Completed
Mars Global Surveyor NASA	1997	Completed
Mars Odyssey NASA	2001	Active
Mars Express ESA	2003	Active
Spirit & Opportunity NASA	2004	Completed
Mars Reconnaissance Orbiter NASA	2006	Active
Phoenix NASA	2008	Completed
Curiosity NASA	2012	Active
MAVEN NASA	2014	Active
Mangalyaan ISRO	2014	Completed
ExoMars TGO ESA/Roscosmos	2016	Active
InSight NASA	2018	Completed
Perseverance & Ingenuity NASA	2021	Active
Tianwen-1 & Zhurong CNSA	2021	Completed
Hope UAE	2021	Active

Naming etymology

Deimos is the Greek personification of dread and terror, twin brother of Phobos (fear), both sons of Ares attending him in battle. Angeline Stickney suggested both names to her husband Asaph Hall in August 1877; the IAU formally adopted them the following year. The Deimos/Phobos pairing is the cleanest example of consistent thematic naming in the satellite catalog — both moons named for the two divine attendants of their parent body's namesake, in the same year, by the same astronomer.

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.