Pluto, which was discovered in 1930, is but a dot of light in even the largest Earth-based telescopes. Pluto is 2/3 the size of Earth's moon but 1,200 times farther away, which makes viewing surface detail as difficult as trying to read the printing on a golf ball located thirty-three miles away (more info). The adjacent movie made from recent Hubble Space Telescope computer enhanced images (Ref) indicates that we are finally beginning to resolve some detail on the surface of this distant planet.
Pluto is on a highly elliptical orbit at an average separation of almost 40 A. U. from the Sun, with an orbital period of 248 years. Since the planet was only discovered in 1930, we have observed only a portion of its orbit so far. Further, the orbit is tilted by about 17 degrees relative to the plane of the ecliptic, much more than for any other planet. Its equatorial radius of 1150 km is only 20% of that of the Earth, and its mass is only 0.0025 that of the Earth. Thus, it is by far the smallest planet, either in mass or diameter. Its period of rotation appears to be almost 6 1/2 days.
