Callisto (Jupiter IV) is Jupiter's second largest Moon
and the second outward from Jupiter of the original four moons
found centuries ago, but the fourth according to current
knowledge. Its radius is 2410km (.4 Earth), its mass is
1.0x1023kg (0.018 Earth), with a mean density
a little less than twice that of water
and subsurface oceans are suspected.
Callisto's orbital period is 17 days and it is tidally locked.
Flyby observations of Callisto have been made by
the Pioneer, Voyager, Galileo, Cassini,
and New Horizons.
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