Pluto

In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto. Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015. Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies. In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 60 and 200 kilometers in diameter. Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent. Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty.

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