Pluto

Pluto was discovered on February 18, 1930, making it the last planet found in our Solar System. Pluto is usually farther from the Sun than any of the nine planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249-year orbit. Pluto made its closest approach during 1989 and will remain within the orbit of Neptune until March 14, 1999.

Pluto's orbit is also highly inclined -- tilted 17 degrees to the orbital plane of the other planets. Observations also show that Pluto's spin axis is tipped by 122 degrees. Ground-based observations indicate that Pluto's surface is covered with methane ice and that there is a thin atmosphere that might freeze and fall to the surface as the planet moves away from the Sun. NASA had planned to launch a spacecraft, the Pluto Express, in 2001 that will allow scientists to study the planet before its atmosphere freezes but due to funding problems the mission looks in doubt. If the Pluto Express mission does go ahead it won't be until 2004 and will reach Pluto in 2017.

Pluto has one satellite named Charon [SHAR-on], named after the boatman in Greek mythology who operated the ferry across the River Styx to Pluto's realm in the underworld. Charon was discovered in 1978. Its surface composition seems to be different from Pluto's. The moon appears to be covered with water-ice rather than methane ice. Its orbit is gravitationally locked with Pluto, so both bodies always keep the same hemisphere facing each other.

Pluto Statistics:

Time taken to orbit the Sun in Earth years: 248
Average orbital velocity in miles/second (km/sec): 2.9 (4.7)
Angle between orbit of planet and orbit of Earth: 17.2º
Time taken to rotate on axis in Earth time: 6 days 9hrs
Tilt of axis to perpendicular of orbital plane: 50º ?
Average distance from Sun in astronomical units*: 39.5
Average distance from Sun in miles (km): 3666 million (5900 million)
Average surface temperature in degrees Celsius: -230º ?
Density relative to water: 1.1
Equatorial diameter relative to Earth diameter**: 0.2
Equatorial diameter in miles (km): 1429 (2300)
Mass relative to Earth: 0.002
Escape velocity in miles/second (km/sec): 0.6 (1.0)
Number of known moons: 1
Number of known rings: 0
Date of discovery: 1930
* One astronomical unit is 92,956,000 miles (149,598,000km)
** Earth's diameter is 7,926 miles (12,756km)