Inner moon of MarsA moon on a collision course

Phobos

Phobos circles Mars faster than the planet rotates and is slowly spiralling toward a dramatic end.

Moon of Mars · about 22.5 km mean diameter · 7.7-hour orbit · target of Japan's MMX mission

Host: Mars
Type: Irregular rubble world
Mean diameter: About 22.5 km
Mean orbital radius: 9,376 km
Orbital period: 7 h 39 min
Rotation: Synchronous
Surface gravity: About 0.006 m/s²
Key feature: Stickney crater

Phobos up close

NASA imagery

Colour view of Phobos from about 6,800 kilometres above its cratered surface — **Phobos from 6,800 kilometres** The Mars Reconnaissance Orbiter captured this colour view of Phobos, showing its dark, battered and irregular surface. Image: NASA/JPL-Caltech/University of Arizona.

A moon rising in the west

Phobos orbits close to Mars and completes three laps in roughly one Martian day. From the planet's surface it rises in the west and sets in the east, opposite to the Sun and the outer moon Deimos.

Tidal forces are lowering its orbit by a few centimetres each year. In perhaps 30 to 50 million years the moon could break apart into a ring or crash into Mars, although the exact fate depends on its internal strength.

Built from what?

Phobos resembles a carbon-rich asteroid, but its orbit and surface materials also allow for formation from debris around Mars after a large impact. Its low density suggests a fractured interior with substantial pore space.

The 9-kilometre-wide Stickney crater dominates one hemisphere. Grooves and parallel chains of smaller pits may have formed from impact ejecta or from material sliding into fractures.

Worth knowing

Phobos is named after the Greek personification of fear, a son of Ares, the Greek counterpart of Mars.
The martian moon's gravity is so weak that a person could jump high enough to enter orbit, ignoring air resistance.
Japan's MMX mission plans to study Phobos and return a sample of its surface to Earth.

Watch and explore

Research & further reading

Mission imagery and measurements are interpreted alongside orbital data. Follow the source links for the current NASA overview and precise satellite parameters.

Mission reference NASA · Phobos ↗ NASA's overview of Phobos and its exploration. Orbital data NASA/JPL satellite elements ↗ Mean orbital elements and current satellite catalogue data.