Callisto is the second-largest of Jupiter's moons, and it is about 4,820 kilometers (3,000 miles) across. That is its mean diameter, almost the same as the width of the planet Mercury. Callisto's surface is covered in craters, and NASA describes it as the most heavily cratered in the Solar System.

Size

NASA gives Callisto's mean radius as 2,410.3 kilometers, so its mean diameter is about 4,820 kilometers. The mean diameter is the width of a perfect sphere with the same volume. NASA also gives the distance around Callisto's equator as about 15,144 kilometers (9,410 miles), which works out to the same width.

NASA's Jet Propulsion Laboratory lists the same radius, known to within 1.5 kilometers. Only two moons in the Solar System are larger than Callisto: Jupiter's Ganymede and Saturn's Titan.

Size comparisons

Mercury is only about 1% wider than Callisto, but it has about three times Callisto's mass.
Callisto is about 1.4 times as wide as the Moon.
Ganymede, Jupiter's largest moon, is about 9% wider than Callisto.
Callisto is a little under two-fifths as wide as Earth.
If Jupiter were shrunk to the size of a basketball, Callisto would be a bead about 8 millimeters wide, circling about 3.2 meters (10.6 feet) from the ball's center.

An ancient, cratered surface

Callisto's icy, rocky surface is about 4 billion years old, and it is covered in craters of many shapes and sizes. The craters are still visible, so scientists think Callisto has little geologic activity. It has no active volcanoes or shifting plates to wear the craters away. Bright spots across the surface are thought to be crater peaks capped with water ice.

A possible hidden ocean

Data from NASA's Galileo spacecraft in the 1990s suggest that Callisto may have a salty ocean about 250 kilometers (155 miles) below its surface. Later research suggests the ocean could lie deeper below the surface than first thought, or might not exist at all. If it does exist and touches rock, it could offer some of the conditions that life needs.

The European Space Agency's Juice spacecraft, launched in 2023, is due to reach Jupiter in July 2031. It will fly past Callisto, Europa and Ganymede, and study their icy shells and the oceans that may lie beneath them.

Sources

Jovian Satellite Fact Sheet. NASA, 2023.
Planetary Satellite Physical Parameters. NASA Jet Propulsion Laboratory.
Callisto: Facts. NASA.
Juice factsheet. European Space Agency.
Mercury Fact Sheet. NASA, 2024.
Moon Fact Sheet. NASA, 2024.
Earth Fact Sheet. NASA, 2024.
Jupiter Fact Sheet. NASA, 2024.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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