Atlas · Mars
The moons of Mars
Mars has two small, lumpy moons. Phobos is so close that it goes round faster than Mars turns, so from the ground it rises in the west and sets in the east.
Phobos and Deimos
Mars and the orbits of its two moons, to scale. Phobos is less than two Mars-widths above the surface.
The edge of the drawing is 25,500 km from the centre of Mars.
How to read it
You are looking straight down on Mars’s north pole. Mars is drawn to scale with the orbits. The moons are not: at this scale most would be far too small to see, so each is a dot.
- Solid dot. A spacecraft has mapped the whole surface.
- Half dot. A spacecraft has seen part of it.
- Ring. Never seen as more than a point of light.
- Pale line. Goes round the usual way, the way the planets go round the Sun: anticlockwise on these drawings.
Each orbit is a slowly turning ellipse fitted to positions from NASA’s Jet Propulsion Laboratory for the years 2000 to 2040. The list gives how far out each fit can be. Touch a row to pick out its orbit.
Phobos and Deimos
2 moons
Both were found in 1877. Phobos is slowly falling inwards and will break up or hit Mars in some tens of millions of years.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Phobos. Partly seen by spacecraft.drawn to within 0.32° | 9,375 km | 7.7 hours | 1.1° | 22.2 km | 1877 | Fly there |
| Deimos. Partly seen by spacecraft.drawn to within 0.40° | 23,500 km | 30.3 hours | 2.7° | 12.4 km | 1877 | Fly there |
Where this comes from
- NASA/JPL Solar System Dynamics, planetary satellites: orbits fitted to JPL Horizons positions, 2000 to 2040
- NASA/JPL, planetary satellite physical parameters
- NASA/JPL, planetary satellite discovery circumstances
- NASA, the moons of Mars
Orbit size is the average distance from the centre of Mars. Tilt is measured from Mars’s equator for the close moons and from the plane of the planets for the far ones, and above 90° means the moon goes round backwards.