Phobos, one of Mars' two moons, is larger than the other, but its diameter is only about 27 x 22 x 18 kilometres (17 x 14 x 11 miles). Because it is so small, its gravity is not strong enough to give it a round shape like Earth's Moon, giving it a rough, irregular appearance.

Phobos is on a collision course with Mars, but this is a process that will take quite a long time to materialise. The satellite is getting closer to Mars by about 1.8 metres (six feet) every century. If it continues at this rate, in about 50 million years Phobos will either collide with Mars or break apart to form a ring.

This image was taken by the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter, which has been studying Mars since 2006.

Image description:

Mars' moon Phobos stands against the darkness of space. The moon is brownish-red in colour and its surface is rough, with many large and small craters. On its right side, next to the particularly large Stickney Crater, a white patch stands out.

Did you know that this potato-like satellite is named after the mythological god of fear Phobos?