For many decades, Pluto was thought to be alone out on the edge of our Solar System. But in 1978, Jim Christy discovered that Pluto was not alone. Charon, Pluto's moon, is more than half the size of its parent planet. In fact, many astronomers have referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a center of gravity somewhere between them. Pluto and Charon are locked in a synchronous orbit. Pluto's rotational period is 6.3 days. It also takes 6.3 days for Charon to make one revolution around Pluto. Thus, the two bodies continuously face each other. But these are not the only strange characteristics of this remote planet. Pluto's orbit is so highly eccentric that is actually crosses inside the orbit of Neptune. because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17 degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune. Pluto is the only planet that has not been visited by a spacecraft. It is so tiny, and so far away that even the Hubble space telescope has trouble viewing any surface features on the planet. With a diameter of only 1412 miles, Pluto is the smallest planet in the Solar System. Because of its small size, many astronomers believe that it should be classified as an asteroid instead of a planet. In fact, Pluto represents one of the great mysteries of our Solar System. No one knows why this small, rocky  planet can be found among the giant gas planets of the outer Solar System. Some believe that Pluto and Charon may be asteroids that somehow got locked into an orbit around the Sun. Between 1985 and 1990, the Earth was aligned with the orbits of Pluto and Charon in such a way that a series of eclipses would take place. Astronomers were able to take advantage of this situation to accurately determine the size of both Pluto and Charon, and to create the first image of Pluto showing some of its larger surface features. This revealed that Pluto is the second most contrasty body in the Solar system. Astronomers are hoping that a future space probe may one day be able to show us the surface of this strange and distant world. But until then, every new discovery will bring with it a long list of new questions.