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How many moons does Mars have? Good question. Considering the fact that Mars is half the size of Earth, you might be surprised to know that Mars actually has two small, asteroid-sized Moons: Phobos and Deimos.
The larger moon is Phobos, and it orbits closer to the surface of Mars. It measures just 22.7 km across, and orbits at an average distance of 9,377 km above Mars. When you consider that the Earth’s moon is more like 384,403 km away from the planet, this is really, really close. Because it’s orbiting this close, Phobos is able to orbit Mars about twice a day. You would see it cross the sky in only 4 hours or so.
Mars’ second moon is Deimos, and it’s even smaller, measuring just 12.6 km across. But it orbits Mars much further away; at a distance of 23,460 km. Deimos takes 30.35 hours to complete an orbit around Mars.
Astronomers think that both of Mars’ moons were once asteroids that were captured in the distant past. In fact, Phobos won’t be orbiting Mars much longer… it’s going to crash. Because Phobos orbits Mars faster than the planet rotates, it’s slowly spiraling inward. In the next 10-50 million years or so, it’ll get so low that the Martian gravity will tear Phobos into a pile of rocks. And then a few million years later, those rocks will crash down on the surface of Mars in a spectacular string of impacts.
So, how many moons does Mars have? The answer is two.
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.