Don't Forget Deimos

Next year, Russia plans to launch a mission to Phobos, the largest moon of Mars. This spacecraft, called Phobos-Grunt, will be the first spacecraft to land on this moon, and also the first mission to return samples of Phobos to Earth.

Phobos is an interesting object, and it's been studied a lot by orbiting spacecraft. The moon is in a fairly low orbit around Mars, which makes it easy to see from spacecraft placed in orbit around Mars. Earlier this year, the Mars Express orbiter got close enough to feel its gravitational tug, allowing us to explore the mass and density of this world without even landing.

This upcoming mission to Phobos is the culmination of decades of close study of this strange, irregular moon, which could be a captured asteroid or an aggregation of rock that formed in Martian orbit.

But Phobos has a twin. It's the small moon of Deimos, which lurks in a much higher orbit. Deimos has also been photographed regularly, but it's still relatively unknown.

Let's size up the two moons for comparison. Phobos is roughly 28 kilometres long and 23 kilometres wide, extremely small for a planetary moon. Deimos comes in at 16 by 12 kilometres.

The size and shape of both moons are similar, which could suggest a common origin. But this is not necessarily the case.

Deimos is harder to explore because of its small size and its distant orbit, which places it a long way from most spacecraft that lurk around Mars. A Viking orbiter made a close approach in the 1970s, but the current orbiters around Mars can only observe from a fairly discreet distance.