The Disputed Planet
Pluto, the ninth planet, was the last planet discovered (in 1930),
the first and only planet discovered by an American (Clyde Tombaugh),
and it is the only planet never visited by a spacecraft or probe.
Pluto is usually farther from the Sun than any of the planets;
however, due to the eccentricity of its 248-year orbit, Pluto periodically
crosses inside the orbit of Neptune. Neptune then spends 20 years
as the most distant planet from the Sun. This last occurred from
1979 through 1999, and will occur again in 2226.
Courtesy of NASA JPL
Pluto is the smallest planet in our solar system, and as a matter
of fact, is smaller than our Moon and six other moons (Io, Europa,
Ganymede, Callisto, Titan, and Triton)! Plutos own moon, Charon,
was discovered in 1978 by James Christy at the U.S. Naval Observatory,
in Washington, D.C.
Rocky, icy little Pluto resembles a comet more than it does the
other planets, and its orbit is suspiciously similar to those of
the Kuiper Belt objects, which are comets-in-waiting. Scientists
are still debating if Pluto should be considered a planet at all.
Pluto can be seen with a good amateur telescope, but it may take
careful observations over several months to actually find it. NASA
is developing a mission, called New Horizons, that could provide
a closer view of the mysterious planet by the year 2015. Still in
the planning stages, the mission could launch a spacecraft as soon
as 2006 to study Plutos and Charons surface properties,
geologies, interior makeups, and atmospheres. The craft would then
fly on to study some Kuiper Belt Objects in the year 2026.