Phobos and Deimos are just two of many moons of the solar system. Meet Deimos in color as viewed by the Mars Reconnaissance Orbiter on 21 February 2009.
And here is Phobos image taken by the same spacecraft on 23 March 2008.
These two tiny moons of Mars are alleged to be captured asteroids originating near or far reaches of the solar system. The larger moon, Phobos, orbits so close to Mars that it is probably doomed to be shattered by gravitational tidal forces sooner than Deimos.
Let's review some of Phobos and Deimos parameters, respectively, to appreciate the next image taken by NASA's Mars Exploration Rover Spirit on 4 September 2005.
Mean orbital height (km): 9377 and 23436
Orbital period (h): 7.65 and 30.34
Major by minor axes (km): 26x18 and 16x10
Faster Phobos passing Deimos in the Mars night sky in time lapsed photography captured by Spirit via succession of images at 150-second intervals.
Source for data and images: nasa.gov
Note: Observe variation in the parameters cited, depending on the source.