Pluto

Once known as the smallest, coldest, and most distant planet from the Sun, Pluto has a dual identity, not to mention being enshrouded in controversy since its discovery in 1930. Pluto is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant region consists of thousands of miniature icy worlds with diameters of at least 1,000 km and is also believed to be the source of some comets.

Pluto has three known moons, Hydra and Nix, besides its companion moon, Charon. At about 1,186 km (737 miles), Charon's diameter is a little more than half of Pluto's.

Pluto and and Charon orbit the Sun in a region where there may be a population of hundreds or thousands of similar bodies that were formed early in solar system history. These objects are referred to interchangeably as trans-Neptunian objects, Edgeworth-Kuiper Disk objects or ice dwarves. Pluto is about two-thirds the diameter of Earth's Moon and may have a rocky core surrounded by a mantle of water ice. Due to its lower density, its mass is about one-sixth that of the Moon. Pluto appears to have a bright layer of frozen methane, nitrogen, and carbon monoxide on its surface. While it is close to the Sun, these ices thaw, rise, and temporarily form a thin atmosphere, with a pressure one one-millionth that of Earth's atmosphere. Pluto's low gravity (about 6 percent of Earth's)causes the atmosphere to be much more extended in altitude than our planet's. Because Pluto's orbit is so elliptical, Pluto grows much colder during the part of each orbit when it is traveling away from the Sun. During this time, the bulk of the planet's atmosphere freezes. In 1978, American astronomers James Christy and Robert Harrington discovered that Pluto has a satellite (moon), which they named Charon. Charon is almost half the size of Pluto and shares the same orbit. Pluto and Charon are thus essentially a double planet. Charon's surface is covered with dirty water ice and doesn't reflect as much light as Pluto's surface. Because Pluto is so small and far away, it is difficult to observe from Earth. In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years. Observations of these rare events allowed astronomers to make crude maps of each body. From these maps it was learned that Pluto has polar caps, as well as large, dark spots nearer its equator. The duo's gravity has locked them into a mutually synchronous orbit, which keeps each one facing the other with the same side. Many moons - including our own - keep the same hemisphere facing their planet. But this is the only case in which the planet always presents the same hemisphere to its moon. If you stood on one and watched the other, it would appear to hover in place, never moving across the sky.

Charon was discovered in 1978, while two additional moons Hydra and Nix, were discovered in 2005.

In Greek mythology, Charon was the boatman who carried the souls of the dead to the underworld - a kingdom that in Roman mythology was ruled by the god, Pluto. The U.S. Naval Observatory's James Christy suggested the name after he found the moon in 1978.

Seven years later, Charon and Pluto began a five-year period of eclipsing each other from Earth's point of view. That was lucky for us, because it enabled scientists to measure the diameters and masses of both objects as each passed in front of the other.

Charon appears to be covered by water ice, which differs from Pluto's surface of frozen nitrogen, methane, and carbon dioxide. One theory is that the materials that formed Charon were blasted out of Pluto in a collision. That's very similar to the way in which our own moon is thought to have been created.

NASA launched its New Horizons spacecraft to Pluto and Charon in January 2006, and it should arrive in 2015, becoming the first spacecraft to visit them. In preparation, the New Horizons project is organizing a search for additional moons of Pluto, using ground-based telescopes and possibly the Hubble Space Telescope.