Mars satellite Deimos from the Mars Reconnaissance Orbiter.

Credit: NASA/MRO/LPL/HiRISE

Deimos is a captured rocky asteroid only 12 kilometers in average diameter. Smaller than Phobos, Mars other satellite, it orbits much farther away from the planet with a period of 30 hours. Its rotation is synchonized with its orbital motion so that keeps nearly the same face toward Mars.

Curiosity rover captured a sequence of images of Phobos passing in front of Deimos in this video from NASA.