If, having been discovered in 1930, Pluto was a late addition to our known solar system, its moon, Charon, is almost brand new, having been found in 1978. Named, fittingly, for the mythological ferryman who rowed the dead across the River Styx to the underworld ruled by Pluto, Charon is a little more than half the size of its parent: 806 miles (1,300 km) in diameter versus Pluto's 1,426 miles (2,300 km). Orbiting 12,214 miles (19,700 km) from Pluto, it takes 6.4 Earth days to make one circuit. Pluto and Charon are tidally locked—forever facing one another—the orbital period and rotation period for both synchronized at 6.4 days. Charon's status is undetermined as a satellite of a "dwarf planet." If it were orbiting the Sun (and not Pluto), it would fit the definition of a dwarf planet.
Pluto and its moon Charon were imaged with the Hubble Space Telescope (HST) in 1994. The image was taken when Pluto was 2.6 billion miles (4.4 billion km) from Earth—nearly 30 A.U. away.
(Image from R. Albrecht/ESA/ NASA)
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