Moons · Space Codex
Phobos
A dark, lumpy moon 27 km long that circles Mars three times a day, faster than the planet turns.
Mars's doomed inner moon
At a glance
Key points
It is spiralling inward and will crash or break into a ring in about 50 million years.
- Laps Mars three times a day
- Falls about 1.8 m closer to Mars every century
- JAXA's MMX will return a sample in 2031
A moon on borrowed time
Phobos orbits inside the distance where an orbit keeps pace with Mars's rotation, so the planet's tides drag it slowly inward. Seen from Mars it rises in the west and sets in the east twice a day. Long parallel grooves across its surface may be the first stress fractures of a moon being pulled apart.
Surface
Stickney crater, about 9 km wide, spans a third of the moon's 27 km length; the impact that made it nearly shattered Phobos. Its regolith is dark grey and slightly red, streaked with landslides that run down the crater walls.
Exploration
Mariner 9, Viking, Mars Express and Mars Reconnaissance Orbiter have all imaged Phobos. JAXA's Martian Moons eXploration (MMX) is scheduled to launch on 19 October 2026, survey both moons from 2027 and return a Phobos sample to Earth in 2031.
What we still don't know
Whether Phobos is a captured asteroid or debris from a giant impact on Mars. Its dark colour suggests the first; its near-circular, equatorial orbit suggests the second. A sample should settle it.
Missions
- 2005–
Mars Reconnaissance Orbiter
NASAActiveHiRISE camera and relay for surface missions.
- Launch 19 Oct 2026
H3 F10 from Tanegashima at 19:41 UTC (window to 7 Nov). Phobos operations 2027–2030, sample return 2031.
First-hand · our 3D model
How we render it
Phobos is a displaced triaxial mesh at its measured 27 × 22 × 18 km proportions, long axis towards Mars, wrapped in a map from the USGS Mars Express mosaic. It is charcoal-dark, true to its albedo of about 0.07, so at default zoom it is a small dark speck.
It is drawn at about 2.3% of Mars's radius, enlarged from its true 0.33% so it stays visible. It orbits in Mars's equatorial frame, inclined 1.1°, with a 7.6-hour period kept in true ratio to its neighbours, and keeps one face towards Mars.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
- MarsCold desert with an ancient wet past
- DeimosMars's small, smooth outer moon
- Martian Moons eXploration (MMX)JAXA's Phobos sample return
Sources
- NASA Science: Phobos (opens in a new tab)
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.