Mars satellite Phobos from the Mars Reconnaissance Orbiter.

Credit: NASA/MRO/LPL/HiRISE

Phobos is a captured rocky asteroid only 22 kilometers in average diameter. Larger than Deimos, Mars other satellite, it orbits the planet in only 7 hours and 39 minutes. That is less than the Martian day, so from Mars surface it appears moving backwards across the sky every 4 hours. Its rotation is synchonized with its orbital motion so that it keeps nearly the same face toward Mars, and the tidal friction is slowly causing it to spiral inward. Phobos is very dark, and spotting it and Deimos from Earth takes the best of telescopes and opportune clear sky.

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