Moon of Mars
Phobos
Phobos is a small moon of Mars, shaped like a potato. It whizzes around Mars so fast that it rises twice a day.
Fact file
- Type
- Moon of Mars
- Diameter
- 22.4 km
- Distance from Mars
- 9,376 km
- Orbital period
- 0.3 days
- Temperature
- -40 °C on average (from -112 to -4 °C)
Phobos orbits Mars.
Interesting facts about Phobos
- Phobos orbits Mars faster than Mars rotates: seen from Mars, it rises in the west and sets in the east – twice a day.
- Every century it comes about 1.8 m closer to Mars. In 30 to 50 million years it will break apart and form a ring, or crash into Mars.
- Its largest crater, Stickney, is 9 km wide – almost half the size of the moon. The impact nearly tore Phobos apart.
Frequently asked questions about Phobos
How big is Phobos?
Phobos has a diameter of 22.4 km. That is about 0.6% of the diameter of our Moon.
How far is Phobos from Mars?
Phobos orbits Mars at an average distance of 9,376 km.
How long does Phobos take to orbit Mars?
Phobos orbits Mars once every 7.7 hours.
How cold or warm is it on Phobos?
On Phobos the average temperature is −40 °C. The temperature ranges from −112 to −4 °C.
Formation
Uncertain: either a captured asteroid or – more likely – debris from a huge impact on Mars that clumped together in orbit. Phobos is so porous that it is more a loose rubble pile than a solid rock.
Atmosphere
No atmosphere. Gravity is so weak that a strong jump would almost send a person off into space.
Exploration
Photographed by Viking, Mars Express and the Mars Reconnaissance Orbiter. Japan's MMX mission is to bring soil samples from Phobos back to Earth.
Compared with Earth
At around 22 km across, Phobos would roughly fit between Zurich and Winterthur.
Other moons of Mars
Test your knowledge of Phobos
What are the two moons of Mars called?
Phobos and Deimos. “Fear” and “Terror” – the sons of Ares, the god of war. Both were discovered by Asaph Hall in 1877.
The full quiz with points and badges is in the 3D model under “Explore”.
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