The Moons of Pluto

Charon, in Greek mythology the ferryman of the dead, was discovered on June 22, 1978, by astronomer James Christy and with an estimated diameter of 1,207 kilometers (750 miles)is the largest of Pluto's moons. It has an unusual relationship with its primary in that both keep the same face turned toward the other during rotation (tidally locked). The surface seems largly covered by water ice, the mean surface temperature a frosty -220 degrees Celsius. It appears to have no atmosphere and its physical makeup is largly a matter of conjecture. [1] Upon its discovery Charon replaced Earth's moon as the largest moon relative to its primary in the Solar System.

The 3 minor moons [2] orbiting Pluto are much smaller without the mass necessary to assume a spherical shape:

Nix named after the Greek goddess of night and mother of Charon, was discovered in June 2005 by the Hubble Space Telescope's Pluto Companion Search Team (from images taken in May) and has an estimated diameter of 46 to 137 kilometers (29 to 85 miles), its physical makeup is unknown, its atmosphere non-existant.

Hydra, in Greek mythology a many headed monster and guardian of the underworld, was also discovered in June 2005 by the Hubble Space Telescope's Pluto Companion Search Team (from images taken in May) has an estimated diameter of 61 to 167 kilometers (38 to 104 miles), its physical makeup is unknown, its atmosphere non-existant.

S/2011 P1 or P4 (interim) was discovered on June 28, 2011 by the Hubble Space Telescope's Pluto Companion Search Team while searching for a possible ring system. It was verified as a new moon on July 20, 2011, has an estimated diameter of 13 to 34 kilometers (8 to 21 miles) its physical makeup is unknown and like its companion moons it has no atmosphere.

[1] The New Horizons spacecraft will come within 27,000 km of Charon (16,800 miles), its closest approach July 14, 2015, continue into the Kuiper Belt, then finaly exit the Solar System in 2029.

[2] The presence of minor moons suggests the possibility of a closely confined or tenuous ring system.

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