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    Pluto, moons

    Pluto and its moons
    The two new moons are circled in red, alongside Pluto (white) and Charon (blue)
    Pluto has three known moons: the largest and innermost, Charon, discovered in 1978, and two tiny outer moons, whose existence was announced in 2005. These smaller moons were found using the Hubble Space Telescope and were given the provisional designations S/2005 P1 and S/2005 P2. In 2006, the IAU officially approved the names Hydra and Nix. Both move in retrograde orbits around Pluto, Nix (P2) at an average distance of 49,000 km from the planet (about twice as far out as Charon) and Hydra (P1) at a distance of about 65,000 km (about three times as far as Charon). It appears they share the same orbital plane as Charon, and have orbital resonances of 4:1 and 6:1, respectively, with the big moon. Both Hydra and Nix have diameters in the range 50-150 km in diameter, compared to Charon's 1,200 km, and are thought to have masses less than 0.3% of Charon's (or 0.03% of Pluto's mass). Their discovery makes Pluto the first Kuiper Belt object known to have more than one satellite.

    Hubble's Advanced Camera for Surveys instrument observed the two new candidate moons on May 15, 2005. Although they are some 5,000 times fainter than Pluto, they showed clearly enough in the Hubble images to be seen easily by Max Mutchler, of the Space Telescope Science Institute in Baltimore, who was the first member of the discovery team member to identify the objects. Three days later, Hubble looked at Pluto again and showed the two objects in different positions in their orbits around Pluto. The discovery team then re-examined Hubble images taken of Pluto on June 14, 2002, and confirmed the presence of both Hydra and Nix near the predicted locations based on the 2005 Hubble observations.


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