Long considered to be the smallest, coldest, and most distant planet
from the Sun, Pluto may also be the largest of a group of objects that
orbit in a disk-like zone of beyond the orbit of Neptune
called the Kuiper Belt
This distant region consists of thousands of miniature icy worlds with diameters
of at least 1,000 km and is also believed to be the source of some comets
Discovered by American astronomer Clyde Tombaugh in 1930, Pluto
takes 248 years to orbit the Sun. Pluto's most recent close approach to
the Sun was in 1989. Between 1979 and 1999, Pluto's highly elliptical orbit brought it closer
to the Sun than Neptune, providing rare opportunities to study this
small, cold, distant world and its companion moon, Charon
Most of what we know about Pluto we have learned since the late
1970s from Earth-based observations, the Infrared Astronomical
Satellite (IRAS), and the Hubble Space Telescope
. Many of the key
questions about Pluto, Charon, and the outer fringes of our solar system
await close-up observations by a robotic space flight mission.
Pluto and and Charon orbit the Sun in a region where there may be a population
of hundreds or thousands of similar bodies that were formed
early in solar system history. These
objects are referred to interchangeably as trans-Neptunian
objects, Edgeworth-Kuiper Disk objects or ice
Pluto is about two-thirds the diameter of Earth's Moon
and may have a
rocky core surrounded by a mantle of water ice. Due to its lower density,
its mass is about one-sixth that of the Moon. Pluto appears to have
a bright layer of frozen methane, nitrogen, and carbon monoxide on its
surface. While it is close to the Sun, these ices thaw, rise, and temporarily
form a thin atmosphere, with a pressure one one-millionth that
of Earth's atmosphere. Pluto's low gravity (about 6 percent of Earth's)
causes the atmosphere to be much more extended in altitude than our
planet's. Because Pluto's orbit is so elliptical, Pluto grows much colder
during the part of each orbit when it is traveling away from the Sun.
During this time, the bulk of the planet's atmosphere freezes.
In 1978, American astronomers James Christy and Robert Harrington
discovered that Pluto has a satellite (moon), which they named
Charon. Charon is almost half the size of Pluto and shares the same
orbit. Pluto and Charon are thus essentially a double planet. Charon's
surface is covered with dirty water ice and doesn't reflect as much light
as Pluto's surface.
No spacecraft have visited Pluto. NASA is currently considering a mission called New Horizons
that would explore both Pluto and the Kuiper Belt region. The earliest it would launch is 2006.
Because Pluto is so small and far
away, it is difficult to observe from Earth. In the late 1980s, Pluto and
Charon passed in front of each other repeatedly for several years.
Observations of these rare events allowed astronomers to make crude
maps of each body. From these maps it was learned that Pluto has
polar caps, as well as large, dark spots nearer its equator.