Phobos circles Mars in about 7 hours and 39 minutes—fast enough to complete a little more than three orbits during one Martian day. Mars takes about 24 hours and 37 minutes to rotate once relative to distant stars. Because Phobos goes around the planet faster than Mars itself turns, it has the unusual appearance of rising in the west and setting in the east for many observers on the Martian surface.
Why is Phobos so much faster than Earth’s Moon?
Phobos travels on a remarkably tight orbit, only about 9,400 kilometers (5,800 miles) from Mars’s center on average. Earth’s Moon, by comparison, orbits much farther from its planet’s center. In a stable near-circular orbit, a satellite closer to its planet generally moves around it in a shorter period. Phobos is not racing because it has its own engine; it is falling around Mars under gravity.
A closer orbit is not the only factor: the planet’s mass matters, too. For the same central planet, orbital period increases with distance. That principle explains why Mars’s outer moon Deimos takes about 30 hours and 18 minutes, longer than Phobos’s short circuit.
What do Phobos and Deimos look like?
Both are small, irregularly shaped, cratered bodies rather than the nearly spherical Moon seen from Earth. Phobos is the larger and closer moon; Deimos is smaller and farther out. Neither is remotely comparable to the size of Earth’s Moon.
Why does Phobos rise in the west?
On Earth the sky mostly appears to move east-to-west because our planet spins eastward. Phobos also travels eastward around Mars, but it outruns Mars’s rotation. For someone watching from a suitable location on the surface, this reverses the usual apparent path of a moon: Phobos rises over the western horizon and sets toward the east. It is not evidence that Mars rotates backward.
Is Phobos visible from everywhere on Mars?
No. Its orbit is close to Mars’s equatorial plane and the moon sits so near the planet that it stays below the horizon from some high-latitude locations. Even where it is visible, it appears much smaller in the sky than Earth’s Moon does from our planet.
Could Phobos crash into Mars?
Phobos is slowly spiraling inward under tidal interactions. Over tens of millions of years, it may break apart and create a ring or collide with Mars. Those are long-term scientific predictions, not events expected within a human lifetime. NASA’s Phobos fact sheet explains the moon’s changing orbit and its prominent Stickney crater.
Phobos versus Deimos at a glance
| Property | Phobos | Deimos |
|---|---|---|
| Position | Inner moon | Outer moon |
| Orbital period | About 7 h 39 min | About 30 h 18 min |
| Relative size | Larger | Smaller |
| Surface appearance | Irregular and heavily cratered | Irregular and cratered |
These orbital periods are reported in NASA’s CRISM views of Phobos and Deimos. They describe full revolutions around Mars, not the time either moon remains above a particular observer’s horizon.
Frequently asked questions
Does Phobos orbit Mars three times every 24 Earth hours?
A 24-hour Earth interval contains a bit more than three of Phobos’s roughly 7-hour-39-minute orbits. A full Martian day is slightly longer than 24 Earth hours, so the count is a little higher still.
Does Deimos also rise in the west?
No. Deimos moves around Mars more slowly than Mars rotates, so its apparent motion relative to the surface is different from Phobos’s.
Are the moons made of the same material?
Their surfaces both appear dark and rocky, but their exact origins and compositions remain subjects of research. Their resemblance to some asteroids does not, by itself, prove they were captured asteroids.
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