Phobos is the larger and inner moon of Mars, a dark, lumpy body that circles faster than Mars spins and is slowly spiralling down towards the planet.
At a glance
| radius | 11.3 km (0.00177 × Earth) |
|---|---|
| distance from its planet | 9,380 km |
| one orbit | 0.319 days |
| reflects | 7% of the sunlight it gets |
Read off the piece’s own catalogue, the same values it draws with.
About Phobos
Phobos is a small, irregular moon, not round, its surface dark as soot and marked by a huge crater, Stickney, and long parallel grooves. Asaph Hall discovered it along with Deimos, the other moon of Mars. It orbits so close to the planet and so quickly that it goes round faster than Mars turns, so from the Martian surface it rises in the west and sets in the east.
Because it orbits inside the point where a moon would keep pace with the planet’s spin, tides are draining its orbital energy and drawing it slowly inward. In the far future it is expected either to break apart into a ring or to crash into Mars.
Spacecraft orbiting Mars have photographed it closely, and rovers on the surface have watched it pass in front of the Sun.
What you see in the piece
Phobos is so small and dark, reflecting only a little of the sunlight it gets, that it can be seen only when you are right beside Mars, racing round the planet in its tight orbit.
Measured or modelled
Where Phobos came from is uncertain: its shape and darkness suggest a captured asteroid, but its nearly circular orbit close to the equator fits better with formation from debris thrown up by a giant impact on Mars.
Questions
Will Phobos crash into Mars?
Tides are slowly pulling Phobos inward. Eventually it is expected to be torn apart into a ring or to fall onto Mars, though not for a very long time.
How fast does Phobos orbit Mars?
Phobos completes an orbit every 0.319 days, faster than Mars rotates, so it rises in the west and sets in the east as seen from the surface.