Callisto is the outermost Galilean moon, a dark, battered ball of ice and rock saturated with craters. While its siblings were volcanically or tectonically remade, Callisto has barely changed in four billion years - it is the Solar System’s best-preserved fossil surface.
Its surface is about 4 billion years old - among the oldest terrain in the Solar System.
Valhalla, its largest impact basin, spans roughly 3,800 km of concentric rings.
It orbits outside Jupiter’s harshest radiation belts, making it a candidate site for a future crewed base.
What a dead world teaches us
Callisto orbits far enough from Jupiter to escape the tidal kneading that heats Io and Europa, so its interior never got the energy to reshape the surface. Every crater since the era of heavy bombardment is still there, one on top of another. When a surface is saturated with craters like this, it is a clock: it records the entire impact history of the outer Solar System.
Physical characteristics
Diameter
4,821 km
Mass
1.080e+23 kg
Surface gravity
1.24 m/s²
Density
1.83 g/cm³
Tidally locked
Yes
Mean temperature
-140 °C
Geometric albedo
0.22
Atmosphere and composition
Atmosphere
An extremely tenuous exosphere of carbon dioxide and oxygen.
Composition
A jumbled mix of rock and water ice, only partially separated into layers - a sign its interior never fully melted.
Discovery
Discovered by
Galileo Galilei
Year
1610
How
one of the first astronomical telescopes
Missions
Voyager 2NASA · Flyby probe · 1977 · active
Launched 16 days before its twin, Voyager 2 is the only spacecraft ever to visit all four giant planets - Jupiter, Saturn, Uranus and Neptune. It entered interstellar space in 2018 and is still returning data.
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
GalileoNASA · Orbiter · 1989–2003 · completed
The first spacecraft to orbit an outer planet, Galileo circled Jupiter from 1995 to 2003 and dropped a probe into its atmosphere. In 2003 it was deliberately flown into Jupiter so it could never crash into - and contaminate - Europa’s possible ocean.