Reference image
Object guide
Phobos is Mars' larger, closer moon. It orbits so close that it rises and sets rapidly and is slowly spiraling inward; tidal forces may eventually break it apart or cause it to impact Mars. Phobos is approximately 22.534 km across.
How to observe Phobos
Phobos is resolved as a separate world mainly through imaging or spacecraft data; visually, its relationship to mars is the key observing context.
Scientific context
Useful scale comparisons are 0.002 Earth diameters, 0.001 g surface gravity, 0.001 Earth years per orbit. These values are rounded from the catalogue measurements shown on this page.
- Radius
- 11.267 km
- Diameter
- 22.534 km
- Mass
- 10,659,000,000,000,000 kg
- Mean density
- 1.779 g/cm³
- Surface gravity
- 0.0057 m/s² (0.001 g)
- Orbital period
- 0.3189 days (0.001 years)
- Parent body
- mars
0.002 Earth diameters0.001 g surface gravity0.001 Earth years per orbit
Source: NASA Science (checked 2026-08-09). Values are rounded catalogue measurements.
Interactive orbit model
Aug 13, 2026, 22:251×
Calculated positions / separate exploration scales