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Why is Charon visible from only one hemisphere of Pluto?
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Pluto
Pluto as observed by the New Horizons spacecraft, July 13, 2015.
Charon is visible from only one hemisphere of Pluto because of gravitational (tidal) interactions that have synchronized Charon’s orbital period with the rotation period of Pluto.1
Because these periods are identical, Charon remains fixed above the same location on Pluto’s surface, never rising or setting, much like a geostationary orbit around Earth. Consequently, the moon is permanently locked to one side of the dwarf planet, making it impossible to observe from the opposite hemisphere.21