Pluto

The ninth planet from the Sun, Pluto is one of the least well understood objects in the solar system. It is the smallest of the major planets, and has a most unusual orbit. Pluto's companion moon, Charon, is so large that the pair essentially form a binary system. How the Pluto-Charon system formed and how the system acquired its special 2-to-3 orbital resonance with Neptune are unanswered questions at the present time. We will probably not know more until a planned NASA space mission visits the Pluto-Charon system. At this time, Pluto is the only planet in the solar system that has not been visited by a space probe.

In 2000 NASA canceled the previously planed Pluto Express mission. In order to make progress toward its goal of reaching Pluto with a probe by 2020, NASA scientists and engineers have created the New Horizons mission to be administered by Johns Hopkins University, Applied Physics Laboratory.

The Pluto-Kuiper Belt Mission will be the first reconnaissance of Pluto and Charon. The probe will go on to explore the Kuiper Belt. As of February 2003, the Pluto-Kuiper Belt mission was scheduled to launch in 2006, and to encounter Pluto and Charon as early as 2015. Observations of Kuiper Belt objects might occur approximately 11 years later.

Charon's surface is thought to be composed of water ice, nitrogen ice, and carbon-monoxide ice. Charon probably has a core composed of silicate rock, which is a minor component of the satellite's mass. About the core, is a hypothetical mantle and cryosphere (ice layer) of water ice, nitrogen ice, and carbon-monoxide ice. It is likely that Charon has no internal heat source and that it has no appreciable magnetic field.