Mars has two moons, Deimos and Phobos. Unlike our Moon, which is pretty big and spherical, the Martian moons are tiny and irregular. Deimos is a little over a dozen kilometers wide, and Phobos is more like 20 km across.
Phobos is in a very low orbit around Mars, just about 6,000 km above the planet’s surface. It orbits so rapidly that it goes around Mars faster than the planet spins, so it rises in the west and sets in the east! That would be weird to see.
Mars Express is a European Space Agency spacecraft that’s in a highly elliptical orbit around Mars, and the dance of orbital mechanics has it passing relatively close to Phobos three times per year. Just a few weeks ago, on 17 November 2019, it passed the wee moon at a distance of about 2,400 km. As it passed, it took a series of images from different angles, which makes for a dramatic animation of the encounter:
Animation of flyby images of the Martian moon Phobos by Mars Express.
Whoa. At the beginning of the video we’re looking down on the huge impact crater Stickney, which is about 9 km in diameter. You can ...