Madsen Pirie

It is rather awe-inspiring that we can send a craft to the surface of Mars and photograph the moons Deimos and Phobos passing each other in a dark sky.  The moons were discovered in 1877, though both Swift and Voltaire had written in fiction that Mars had two moons, and each writer has a crater on Deimos named after him in honour of their lucky guess.  Phobos is the larger one (22.2km avg) and nearer to the planet.  It whizzes round Mars in 11 hours, and has been dubbed “the mad moon of Mars.”  Anyone on Mars sees it at about a third of the size of Earth’s moon.  Deimos is smaller (12.6km avg) and more distant, orbiting just outside synchronous orbit and taking 2.7 days to set.  Both moons always present the same face to Mars. Phobos appears to be made of material similar to that found on the surface of Mars, and may have coalesced in orbit from ejected material.  Deimos is thought likely to have been captured from the asteroid belt.  It is highly likely that voyagers from Earth will visit both of these little moons some day, perhaps using one or both as stations from which to transport vessels to and from the surface before the journey back to Earth.

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