Mean orbital velocity: 2.9 miles (4.7 km) per second
Inclination of axis: 122.5
Orbital eccentricity: 0.248
Orbital inclination: 17.15
Diameter at equator: 1,430 miles (2,300 km)
0.192 x Earth's diameter
Oblateness: 0
Mass: 1.29 x 10^22 kg
0.002 x Earth's mass
Volume: 1.08 x 10^10 km^3
0.01 x Earth's volume
Density: 2.03 g/cm^3
0.85 x Earth's density
Surface gravity: 0.4 m/s^2
0.04 x Earth's gravity
Escape velocity: 0.7 miles (1.1 km) per second
Mean surface temperature: -382 F / -230 C / 43 K
Maximum apparent diameter as seen from Earth: 0.2"
Albedo: 0.4
Number of known satellites: 1
Mean diameter of Sun as seen from the planet: 49"
Satellite statistics
Charon
Mean distance from center of Pluto: 12,200 miles (19,600 km)
Inclination of axis: 0
Rotation period: 6.39 days
Orbital period: 6.39 days
Orbital eccentricity: low
Orbital inclination to Pluto's equator: 98.8o
Diameter: 745 miles (1,200 km)
Mass: 1.77 x 1021 kg
Density: 2 g/cm3
Escape velocity: 0.37 mile (0.59 km) per second
Planet composition
Little is known about the composition of Pluto, but its low density (about twice that of water) indicates a major constituent must be methane and water ice. The remainder is probably silicate rock, perhaps in a core surrounded by an icy mantle. Although no clear surface detail has been seen on Pluto, there appears to be a dark band near its equator.
Pluto's diameter is about 2,300 km (2/3 the size of our moon); Charon's is approximately 1200 km. The two are so close in size that they are sometimes called a ╥double planet╙.
Together, Pluto and Charon have about 1/5 the mass of our Moon.
Topography
No surface detail has been detected on either Pluto or its satellite, Charon.
Atmosphere
Until 1988 it was not known if Pluto had an atmosphere. In that year, an occultation of a star by Pluto was observed. Because the star gradually disappeared behind Pluto instead of instantaneously, it was determined that Pluto did in fact have atmosphere. This atmosphere blocked part of the star's light before the planet blocked all of it. Had there been no atmosphere, the star would have stayed a constant brightness until it was behind Pluto.
This tenuous atmosphere is probably pure methane; the surface pressure on Pluto is about 100,000 times less than the pressure at sea level on Earth. It is possible that very high in the atmosphere is a ╥smoggy╙ region of ammonia, methane and other hydrocarbons, and carbon monoxide.
Magnetic fields and gravity
There is no reason to believe Pluto has an inherent magnetic field because it probably lacks an iron core. However, it is likely that there are interactions of some sort between the solar wind and Pluto's surface or atmosphere.
Pluto's surface gravity estimated to be around 0.08 times that of the Earth.
Orbit and rotation
It takes 248 years for Pluto to complete an orbit of the Sun. Its equatorial rotation is 6.3 days.
At 17, Pluto has the most highly-inclined orbit of all planets. Pluto also has the most eccentric orbit. This eccentricity takes Pluto inside Neptune's orbit, where it has been since 1977 and will remain until March 1999.
Moons (and/or rings)
The only known satellite is Charon, which orbits Pluto every 6.3 days. Charon appears to be covered in water ice (as opposed to the methane ice that covers Pluto). Charon's orbit is locked with Pluto's, so the same sides of the globes always face each other.
Mythology
Pluto was named for the Greek god of the lower world.
Discovery and exploration
Clyde Tombaugh discovered Pluto in January 1930 based on calculations and predictions made by Percival Lowell at the turn of the century. James Christy first speculated about the existence of Pluto's satellite, Charon, in 1978. The Hubble Space Telescope later verified its existence. Pluto is the only major body in our solar system that has not been visited by an exploratory probe, but NASA is currently exploring the possibility of such a probe.
