PLUTO AND CHARON

Pluto was discovered on February 18, 1930, making it the last planet found in our Solar System. Pluto is usually farther from the Sun then 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.

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 plans to launch a spacecraft, the Pluto Express, in 2001 that will allow scientists to study the planet before its atmosphere freezes.

Pluto has one satellite named Charon. 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's and Charon's rotational periods and Charon's orbital period are all 6.3872 Earth days. For a time after Charon's discovery, astronomers thought of Pluto and Charon as "twin planets" because they are so closely tied together.

Pluto Statistics

Equatorial radius (km) ................................ 1,160

Mean distance from the Sun (km) ............... 5,913,520,000

Rotational period (days) ............................ -6.3872

Orbital period (years) ............................... 248.54

Atmospheric composition Methane Nitrogen

Charon Statistics

Equatorial radius (km) .................................. 635

Mean distance from Pluto (km) ........................ 19,640

Rotational period (days) ............................ 6.38725

Orbital period (days) ............................... 6.38725

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