Callisto has more craters than any other world we know. Its surface has barely changed in about 4 billion years.
One impact left Valhalla, a bullseye of rings about 3,800 km across.
Callisto orbits outside Jupiter's fiercest radiation, so NASA studies have picked it as a possible place for a human base out here.
There may be an ocean under its surface too, though it's buried deeper than Europa's.
If you stood here
Dark, ancient ground pocked with craters as far as you can see, with Jupiter rising and setting in a sky that never brightens.
- Width
- 4,821 km
- Goes round Jupiter
- every 17 days
- One day
- The same: it always shows Jupiter one face
- Temperature
- about −140 °C
- 70 kg on Earth weighs
- 9 kg here
