Callisto

What is Callisto?

Moons· Diameter 4,821 km. The outermost of Jupiter's four Galilean moons — nearly Mercury-sized, its ancient surface saturated with 4-billion-year-old craters, and possibly hiding an ocean beneath the ice. Watch it trace the slow outer lap in the orbit view.

The orbit of Callisto at this moment: https://nowsky.org/solar/orbit?fw=moon:jupiter:callisto&sp=0.03 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How big is Callisto?

4,821 km.

How far is Callisto from Earth?

1,883,000 km.

What is notable about Callisto?

most heavily cratered body known.

Which planet does Callisto orbit?

Jupiter.

The moon outside the resonance

Io, Europa and Ganymede orbit in a 1:2:4 resonance that keeps their orbits slightly stretched, and the resulting tides heat all three from within. Callisto is outside the resonance. At 1.88 million km from Jupiter, circling every 16.7 days, it feels no tugging from its neighbours, and its interior has remained nearly untouched for four billion years — which is why its craters survive and why it shows no geological activity.

Even so, magnetic measurements by the Galileo spacecraft hinted at a layer of salty water beneath Callisto's ice. If an ocean can persist on the heat of radioactive decay alone, without tides, then oceans inside icy moons are the rule rather than the exception. JUICE, due at Jupiter in 2031, will make 21 close passes of Callisto to test for that layer.

The moon in depth

Callisto is the outermost of the four moons Galileo discovered in 1610. At 4,821 km across it is nearly the size of Mercury, yet with only a third of the mass — a low-density mix of roughly equal parts rock and ice. It orbits 1.88 million km from Jupiter every 16.7 days, far enough outside the planet's radiation belts to enjoy the mildest environment of the four.

Its surface ranks as the oldest and most heavily cratered in the solar system. With almost no geological activity, scars from four billion years ago survive intact, including Valhalla, a multi-ring impact basin 3,800 km wide. Galileo's magnetometer hinted at a salty ocean some 100 km beneath the ice, but unlike Europa's, that ocean probably never touches rock, so its prospects for life are rated low.

Low radiation and abundant ice make Callisto a frequent candidate for a future crewed base in the Jupiter system. Follow Jupiter in the orbit view and you will see Callisto tracing the slowest lap outside Io, Europa and Ganymede; ESA's JUICE, arriving in 2031, will fly past it many times.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.