Phobos

Mars's potato-shaped bigger moon

Phobos is the bigger of Mars's two tiny moons. It isn't a ball but a potato: it is too small for gravity to squeeze it round. Phobos is so close to Mars that it crosses the sky three times a day! One day it will crash into Mars or break up into a ring.

Phobos is an irregular body 27×22×18 km across. It orbits only 6,000 km above Mars (the Moon is 60 times farther from Earth) every 7 h 39 min — faster than Mars spins, so in the Martian sky it rises in the west and sets in the east. Its surface has the 9 km Stickney crater and mysterious parallel grooves. It is probably a captured asteroid or debris from a giant impact. Its orbit drops 2 m per century: in ~50 million years Phobos will crash into Mars or become a ring. Japan's MMX mission plans to bring back samples.

Fun facts

• Phobos means 'fear' in Greek — a son of Mars, the god of war.
• On Phobos you would weigh less than 1 kg — you could run and jump into orbit!
• Phobos crosses the Martian sky 3 times a day and partly eclipses the Sun each time.