Pluto is the smallest planet in the solar system and is usually the furthest from the Sun (Neptune is sometimes closer). The name is derived from the Roman mythological god of the underworld, Hades since Pluto is so far from the Sun and receives so little light.
Because Pluto is so far away, it was not discovered until 1930 by an American astronomer. To this day, it remains the only planet not yet visited by a spacecraft. Currently, NASA scientists plan to send the Pluto Express to rendezvous with the planet sometime in the year 2007. Much of the data acquired of Pluto came from extensive astronomical observations that revealed its orbit, relationship with Charon (its only satellite) and spin axis. The Hubble Space Telescope also played a major role in determining Pluto's planetary characteristics.
To this date, Pluto's radius is not precisely determined. It is estimated to be 1137 with an error of + or - 8 (one percent). Pluto's equator is almost at right angles to the plane of its orbit, much like Uranus. On the surface, the temperature is a bitter 35 to 45 Kelvins (-228 to -238 degrees Celsius).
- Introduction to Pluto