PLUTO
Pluto is normally the furthest (known) planet out from the Sun. It is really a double planet - it is tidally locked with its satellite Charon, which means they always have the same sides facing each other.
Pluto's orbit is very eccentric: it is inclined at an angle of 17 degrees to the plane which the other planets orbit on. For part of its orbit, Pluto is actually closer to the Sun than Neptune. If the Earth's orbit was as elliptically shaped as Pluto's, it would approach within 12 million miles of Mars at its farthest point from the Sun and as near as 2 million miles from Venus at its nearest approach to the Sun.
The Facts
Mass (kg): 0.0125x1024
Equatorial Radius (km) 1195
Mean Density (kg/m3): 1750
Length of Day (hours): 153.3
Period of Revolution about Sun (days): 90,465
Acceleration due to Gravity (m/s2): 0.58
Mean Orbital Velocity (km/s): 4.72
Inclination of Axis (degrees): 122.5
Mean Distance from the Sun (AU): 39.48
Other Stuff
The Discovery of Pluto
Clyde Tombaugh discovered Pluto on February 18, 1930. Pluto is named after the Greek god of the underworld, a name it got after the naming of the planet was opened up as a competition. Tombaugh discovered Pluto during a systematic search for a new planet, imaginatively called Planet X, which Percival Lowell thought was causing Neptune to stray off course. Tombaugh took slides of the region the new planet was thought to be in, in pairs. He then had the painstaking task of comparing the pairs of slides to see if anything had moved against the background of stars.
What Do We Know?
Pluto has a bright surface, as it is covered in various types of ice. It is cold enough to have methane frozen on its surface, a feature that was discovered in 1976 by looking at the absorption lines in Pluto's spectrum. The discovery of frozen methane showed that Pluto was a small but bright planet, rather than a larger darker one. If it was larger and had methane frozen on it, it should be brighter than it is. Light reflected from the surface of Pluto takes 5 hours 40 minutes to reach Earth. Pluto's surface, the coldest of any planet, is too frigid for an atmosphere. The estimated surface temperature of -230°C would freeze any atmosphere and keep it on the surface. The Sun is so far away that it appears no larger than Jupiter does from Earth at its closest point. It is very hard to resolve any surface detail of Pluto. It's like trying to read the writing on a golf ball 33 miles away. The Hubble Space telescope has taken pictures of Pluto, which show it to have a northern polar cap, and various brighter and darker spots. These are probably not permanent - as Pluto moves nearer and farther from the sun the ices will melt and refreeze on the surface.
Charon
Observations of Pluto in 1978 showed it to have a bulge, which appeared to rotate approximately every six days. At first people wondered whether the telescope had somehow been knocked to blur the images of Pluto, but all the other stars around were still round, which they wouldn't have been if the picture had been blurred. James Christy discovered Pluto's satellite Charon, named after the mythological being that ferried dead souls across the river Styx to the underworld. This was confirmed when Charon began to eclipse Pluto every 6.4 days.
The eclipsing of Pluto and Charon meant that it was possible to determine their relative sizes. Charon is the largest satellite in relative to its primary in the solar system. The next largest is the Earth's moon.
When Will We Know More?
Just before Clyde Tombaugh died, NASA asked his permission for a space probe to fly past Pluto to gather more information on his planet. This mission, called the Pluto Kuiper Express, will hopefully be launched in 2006, even though its original launch was scheduled for 2004.
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