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Space Topics: Pluto and Charon

Pluto's Moons

Click to enlarge > The Pluto system on May 15, 2005 This and another photo from the Hubble Space Telescope have revealed that Pluto has two more, small moons, now named Nix and Hydra, in addition to the previously known Charon. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

Pluto is the first trans-Neptunian object discovered to have more than one body orbiting it.  There are now four known bodies in the Pluto system, including the giant moon Charon and two other objects discovered in 2005, Nix and Hydra.  Charon is so massive relative to Pluto that the two actually mutually orbit a point that is outside Pluto's interior, like a dumbbell spinning in space.

Pluto

2,306 +/- 10 kilometers diameter

Charon

Diameter: 1,205 +/- 2 kilometers (52% of Pluto!)
Orbit: 19,571.4 +/- 4 kilometers from Pluto's center (only 8.6 Pluto diameters)
Orbital period: 6.3872304 +/- 0.0000011 days (same as Pluto's rotation rate)

Nix (S/2005 P2)

Diameter: Around 40 to 130 kilometers (10 to 15% smaller than S/2005 P1)
Orbit: 48,675 +/- 121 kilometers from Pluto's center
Orbital period: 24.8562 +/- 0.0013 days (just about 4 times Charon's)

Hydra (S/2005 P1)

Diameter: Around 45 to 160 kilometers
Orbit: 64,780 +/- 88 kilometers from Pluto's center
Orbital period: 38.2065 +/- 0.0014 days (just about 6 times Charon's)

Click to enlarge > Orbits of Pluto and its moons as seen from Earth As seen from Earth, the orbits of Charon, S/2005 P1, and S/2005 P2 appear elliptical. The dots representing Pluto and the moons are not drawn to scale. Credit: NASA / SwRI / JHU APL

Note: most of the figures on this page come from a paper submitted to Astrophysics in 2005 by M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young, and S. A. Stern: "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2"