Callisto

The most cratered world

Callisto is Jupiter's dark, old moon. Its surface is completely covered with craters — there's no empty space left! These craters formed billions of years ago and nothing has changed since. Callisto is far from Jupiter, so there is little radiation — a handy spot for a future astronaut base.

Callisto (radius 2,410 km, Mercury-sized) is the outermost Galilean moon, orbiting every 16.7 days. Its surface is the oldest (4 billion years) and most heavily cratered in the Solar System: no geological activity, no volcanoes, no tectonics. The largest feature is Valhalla, a multi-ring impact basin 3,800 km across. Being far from Jupiter (1.9 million km), it gets little tidal heating and low radiation. Galileo spacecraft data suggest a salty ocean may lie beneath 200 km of ice. In 2003 NASA rated Callisto the best base for crewed missions to the Jupiter system.

Fun facts

• Callisto's surface has barely changed in 4 billion years — a museum of the cosmos!
• Callisto is 1.9 million km from Jupiter — 5 times the Moon's distance from Earth.
• From Callisto, Jupiter looks 10 times bigger than the Moon does from Earth.