Which of Jupiter's Galilean moons is farthest from the planet?

Callisto is the Galilean moon farthest from Jupiter. Its average orbital distance is about 1.88 million kilometres, well beyond Ganymede, Europa, and Io.

The four Galilean moons were discovered by Galileo Galilei in 1610. Their order outward from Jupiter is Io, Europa, Ganymede, and Callisto. This arrangement is useful to remember because the moons also differ dramatically in their environments: Io is intensely volcanic, Europa has a prominent ice-covered ocean, Ganymede is the largest moon in the Solar System, and Callisto is the outermost of the group.

Callisto’s distance gives it a comparatively quiet relationship with Jupiter. Unlike the three inner Galilean moons, it is not part of the same orbital resonance chain and experiences far less tidal heating. Its heavily cratered surface is considered among the oldest and most preserved in the Solar System. Callisto is also tidally locked, meaning it rotates once for every orbit and keeps nearly the same face toward Jupiter. Its orbital period—and therefore its rotation period—is about 16.7 Earth days.