What are Phobos and Deimos, Mars's two tiny moons?

They are two small, lumpy rocks circling the Red Planet. Phobos, the larger, is only 22.2 kilometres across and Deimos just 12.6. Asaph Hall discovered both in August 1877, and they are named after the twin sons of the Greek god of war, Fear and Terror.

Phobos is a world of extremes. It orbits only about 6,000 kilometres above the Martian surface, closer to its planet than any other known moon, and it races around Mars in just over 7 and a half hours. From the surface of Mars it would rise in the west, cross the sky in 4 hours 15 minutes or less, and set in the east, twice every Martian day. Deimos is farther out and takes 30.35 hours for a lap.

Phobos is also sinking. Tidal forces pull it closer to Mars by about 2 centimetres a year, and a breakup due to tidal forces is estimated in roughly 30 to 50 million years. Images and models suggest it may be a rubble pile held together by a thin crust.

The strangest part is the human story. Jonathan Swift's Gulliver's Travels of 1726 describes two moons of Mars, more than a century before Hall found them. Most astronomers think he was just following an argument of the time: Earth has one moon and Jupiter four, so Mars by analogy must have two. And Hall nearly gave up the search; he wrote that he might have abandoned it had it not been for the encouragement of his wife, Angeline Stickney.