Phobos
What is Phobos?
Moons· Diameter ~22 km. The larger, inner moon of Mars, shaped like a potato. It circles just 6,000 km above the surface every 7 hours 39 minutes, so in the Martian sky it rises in the west and sets in the east.
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Orbit and size
| Size | 27 × 22 × 18 km |
|---|---|
| Distance from Mars | 6,000 km above the surface |
| Orbit | 7 h 39 min |
| Fate | crashes into Mars or forms a ring in ~50 Myr |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Where Phobos came from, and the answer MMX will bring back
Where Phobos came from is still an open question. One idea is that it was an asteroid from the asteroid belt captured by Mars's gravity; its dark colour points this way. The other is that it formed from debris thrown out when a large body struck Mars long ago; its low density and an orbit lying neatly over Mars's equator point that way.
Japan's MMX spacecraft aims to settle the question. It is planned to launch in October 2026, reach Mars in 2027, land on Phobos to collect at least 10 g of surface material, and return it to Earth in 2031. Whether the sample resembles Martian rock or an asteroid should reveal where Phobos came from. The surface is also expected to contain material blasted off Mars, so the sample may let scientists study Martian soil too.
Missions to Phobos have had many failures. In 1989 the Soviet Phobos 2 lost contact a few days before it was to release landers onto the moon. In 2011 Russia's Phobos-Grunt, launched to bring back a sample, never left Earth orbit and fell back. In 2010 Europe's Mars Express passed within 67 km of the surface and measured Phobos's mass precisely. Its density turned out to be only about 1.9 times that of water, supporting the idea that the moon is full of empty spaces.
The moon in depth
Asaph Hall of the United States discovered Phobos in 1877, a few days after he found Deimos. It measures 27 × 22 × 18 km and is not round but lumpy, like a potato. It orbits only 6,000 km above the Martian surface, closer to its planet's surface than any other moon in the solar system. That height is a little more than the distance from London to New York (about 5,570 km).
Phobos goes around Mars in 7 hours 39 minutes, much faster than Mars turns once, in 24 hours 37 minutes. So from the ground on Mars, Phobos does the opposite of the Sun and stars: it rises in the west and sets in the east, crossing the sky about twice a day. Each crossing takes a little over four hours.
Phobos is slowly getting closer to Mars. Tidal effects from Mars's gravity bring it 1.8 cm nearer every year. At this rate, in about 50 million years it will either be torn apart by Mars's gravity into a ring or crash onto the surface. Following Mars in the orbit view shows Phobos racing around far inside the orbit of Deimos.
Frequently asked
- How far is Phobos from Earth?
- The distance from mars of Phobos is 6,000 km above the surface.
- Which planet does Phobos orbit?
- Phobos orbits Mars.
- In one line, what is Phobos?
- Moons — The larger, inner moon of Mars, shaped like a potato.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.