Space · Moons

Phobos

A moon of Mars — The larger of Mars's two moons — closer to its planet than any other moon in the solar system, and spiraling slowly inward toward eventual destruction.

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This site's Phobos agent picked the name from this moon. See the agent's section on the team page.

Quick facts

Parent planet

Mars

Diameter (mean)

22.4 km

Mass

1.06 × 10¹⁶ kg
1.44e-07 Moon masses

Mean orbital radius

9,376 km

Orbital period

0.319 Earth days

Discovery year

1877

Discoverer

Asaph Hall

Naming origin

Greek personification of fear; son of Ares

Surface conditions

Phobos is an irregular potato-shaped body 27×22×18 km across, orbiting Mars at just 9,376 km — closer to its parent than any other moon. It completes an orbit in 7 hours 39 minutes, faster than Mars rotates, so it rises in the west and sets in the east twice per Martian day. Tidal forces are slowly dragging Phobos inward at about 1.8 cm per year; in 30–50 million years it will either crash into Mars or be torn apart into a temporary ring.

The dominant surface feature is Stickney, a 9-km-wide impact crater on a 22-km moon — the impact that formed it nearly destroyed Phobos. The surface is covered in regolith and streaked with mysterious linear grooves, possibly stress fractures from the Stickney impact or chains of secondary craters. Phobos's density is far too low for solid rock, suggesting either a rubble-pile interior with significant void space or significant ice content. JAXA's Martian Moons eXploration (MMX) mission, launching mid-2020s, will return a sample from Phobos.

Missions and observations

Every Mars-system mission has had an opportunity to image or characterize Phobos. The list below is the Mars-system mission catalog; specific Phobos 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

Phobos is the Greek personification of fear and panic, a son of Ares (the Greek equivalent of Mars) who attended his father in battle alongside his brother Deimos (dread). When Asaph Hall discovered Mars's two moons in August 1877 from the U.S. Naval Observatory in Washington, his wife Angeline Stickney suggested the names from Homer's Iliad, where Phobos and Deimos yoke their father's chariot before battle. The names were officially adopted in 1878 and remained the canonical example of how Mars's namesake-as-war-god shaped the satellite nomenclature.

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.