Cosmic Scale
Phobos
Moon of Mars · 9,376 km from Mars
How big is Phobos?
Phobos is about 22.2 km across.
See Phobos in 3D, at true scale
Phobos is the larger moon of Mars and orbits closer to its planet than any other moon in the Solar System, only 6,000 km above the surface. It circles faster than Mars turns, so from the ground it rises in the west, twice a day.
At a glance
| Diameter | 22.2 km |
|---|---|
| Mass | 1.06 × 10¹⁶ kg |
| Diameter compared with Earth | 0.002 × Earth |
| Size | About 27 × 22 × 18 km |
| Orbits | Mars, every 7.7 hours |
| Discovered | 1877, Asaph Hall |
| Big crater | Stickney, 9 km wide |
Good to know
- It is spiralling inward by about 2 m every century and will break into a ring or crash in roughly 50 million years.
- Long grooves across it may be stretch marks from Mars's pull.
- Japan's MMX mission plans to bring a sample of it back to Earth.
- Next smaller: Neutron star · 22.0 km
- Next larger: Gaia BH1 · 56.8 km