Phobos
23 kmacross
Phobos is 23 km across and orbits Mars at 9,376 km, once every 7.7 hours. A lumpy, cratered rubble pile just 23 km across, orbiting Mars closer than any other moon orbits its planet.
▶ Visit Phobos in 3D
Why this moon matters
Phobos is less a world than a captured pile of rubble, 23 km across, whirling around Mars just 6,000 km above its surface — closer than any other moon in the solar system. Mars’ tides are dragging it inward by about 1.8 metres per century; within roughly 50 million years it will shatter into a ring. Its largest feature, the 9 km crater Stickney, is from an impact that nearly destroyed it.
It circles Mars faster than Mars spins — from the surface it rises in the west, twice a day. It is spiralling inward, doomed to break into a ring within ~50 million years.
Phobos by the numbers
Diameter
23 km
Distance from Mars
9,376 km
Orbit period
7.7 hours
Rotation
tidally locked
Surface gravity
0.006 m/s²
Temperature
−40 °C day side
Orbital speed
2.1 km/s
Sunlight travel time
13 minutes
In the interactive view you can fly to Phobos directly and ride along as it circles Mars.
Keep going
✓ Facts verified July 2026
Every figure on this page is a real, rounded value checked against primary sources. Found something out of date? See how we keep it accurate.