Which is bigger Phobos or Deimos?

Answer 1

Phobos, Mars' closest satellite, come from the Greek word meaning 'fear', whilst Deimos means 'panic', both following on from the Mars/War theme. Orbiting at a meagre 6000km from the surface of Mars, Phobos is the closest moon to its' parent planet in the Solar System (at least that we know of), whilst Deimos is the smallest known solar system satellite.

Phobos and Deimos are both believed to be captured asteroids, due to their irregular shape and small size (Phobos having a 22.2km wide diameter, whilst Deimos is just 12.6km wide!). Both moons are, however, heavily cratered and are believed to be made up of rock and ice. Like our moon they have a coating of dust on their surfaces. Phobos' orbit is such that it rises in the west and sets in the east, often more than once in one day.

However, I'm afraid there isn't realy much future left for Phobos as an independent moon of Mars. This is because in little over 50 million years it is destined to crash into Mars (with a thud) due to the planet's tidal forces lowering its orbit (presently at about 1.8cm/year). Nevertheless, it (and Deimos) have been seen by many spacecraft since it was discovered by Asaph Hall in 1877, such as the Mariner 9, Viking 1, Phobos and Phobos 2, the last of which even detected an outgassing coming from Phobos.

Answer 2

Phobos is the larger of Mars' two moons, with a diameter of about 22.4 km, while Deimos has a diameter of about 12.4 km.