Mars has two moons, Phobos and Deimos. The smaller Mars moon Deimos orbits further away from Mars, and is named after a figure in Greek Mythology that represents dread.
As with the other Mars moon Phobos, Deimos has many of the characteristics and shape of an asteroid. In fact, many astronomers think that it was an asteroid in the past, and was captured by Mars millions of years ago.
Deimos measures 12 km across, and orbits Mars at a distance of 23,460 km. While Phobos orbits Mars in just over 7 hours, Deimos takes 1.2 Martian days (about 30 hours). And this is very important. Because it takes longer to orbit than the Martian day length, its orbit is slowly growing, just like our own moon. It’s not going to crash into the surface of Mars, like Phobos will.
If you could stand on the surface of Mars, Deimos would appear almost star like to the unaided eye. At its brightest, it might look like Venus does from here on Earth.
Seen from the surface of Mars, Deimos often passes directly in front of the Sun. It’s too small to cause a total eclipse, but looks like a black dot moving across the face of the Sun. NASA’s Mars Exploration rovers have captured video of this event in the past.
Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.