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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 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 dwarves.
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.
Related Links
More on Pluto:
Education and Activities
Images, Movies and Animation
Source: NASA
Last Updated : 08.20.2003
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Discovered By:
Clyde Tombaugh
Date of Discovery:
1930
Distance from the Sun:
(Semimajor axis of orbit)
5,906,376,200 km
39.48168677 A.U.
Radius:
1,180 km
(0.180 of Earth's radius)
Volume:
0.005 (Earth = 1)
Mass:
0.013 x 1027 g
Density:
2.0 gm/cm3
Surface Gravity:
60 cm/s2
Escape Velocity:
1.2 km/sec
Sidereal Rotation Period:
-6.38718 (retrograde)
Sidereal Orbit Period:
247.92065 sidereal years
Mean Orbit Velocity:
4.7490 km/s
Orbit Eccentricity:
0.24880766
Orbit Inclination:
17.14175 degrees
Equatorial Inclination:
119.61 degrees
Mean Surface Temperature:
40 K
(Note:Pluto's highly elliptical
orbit around our Sun varies from 29.6 AU to 49.6 AU
- averaging 39.6 AU.)
Natural Satellites:
1. Charon
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