Space · Moons

Callisto

A moon of Jupiter — The most heavily cratered body in the solar system — its surface preserves a 4-billion-year impact record.

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This site's Callisto agent picked the name from this moon. See the agent's section on the team page.

Quick facts

Parent planet

Jupiter

Diameter (mean)

4821 km

Mass

1.08 × 10²³ kg
1.47 Moon masses

Mean orbital radius

1,882,700 km

Orbital period

16.689 Earth days

Discovery year

1610

Discoverer

Galileo Galilei

Naming origin

Greek nymph, lover of Zeus, turned into a bear

Surface conditions

Callisto's surface is saturated with craters — every patch of terrain has been hit so many times that new impacts overprint old ones and the surface cannot get more cratered. This makes Callisto's face the oldest visible surface in the solar system, dating to roughly 4 billion years ago. The lack of geological activity means impact records are preserved rather than erased, the inverse of Io.

The most prominent feature is the Valhalla basin, a multi-ring impact structure 3,800 km in diameter — concentric rings of fractures radiating from the impact point, like ripples in a pond frozen in stone. Beneath the icy surface Callisto likely hosts a subsurface ocean, though smaller and less certain than Europa's or Ganymede's. Unlike the inner Galileans, Callisto is outside Jupiter's strongest radiation belts, making it a candidate for a future crewed Jupiter-system base.

Missions and observations

Every Jupiter-system mission has had an opportunity to image or characterize Callisto. The list below is the Jupiter-system mission catalog; specific Callisto encounters are documented in mission archives.

Mission	Year at Jupiter	Status
Pioneer 10 NASA	1973	Completed
Pioneer 11 NASA	1974	Completed
Voyager 1 NASA	1979	Completed
Voyager 2 NASA	1979	Completed
Ulysses NASA/ESA	1992	Completed
Galileo NASA	1995	Completed
Cassini-Huygens NASA/ESA/ASI	2000	Completed
New Horizons NASA	2007	Completed
Juno NASA	2016	Active
Europa Clipper NASA	2030	On the way
JUICE ESA	2031	On the way

Naming etymology

Callisto was a nymph and follower of Artemis whom Zeus seduced; the jealous Hera turned her into a bear, and Zeus later placed her among the stars as Ursa Major (the Great Bear). She completes the Marius-Kepler quartet of named Galileans — four lovers of Zeus, one moon each. Callisto's bear-into-stars myth makes her one of two Galileans (with Ganymede) associated with celestial transformations in the original Greek stories.

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.