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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 was actually closer to the Sun than Neptune, providing rare opportunities to study this small, cold, distant world and its companion moon, Charon.
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. The gravitational influence of the giant planets may have ejected these bodies to much larger distances from the solar system. The recent discovery of several bodies about the size of Charon in the region beyond Pluto has bolstered this theory. These objects are currently referred to interchangeably as trans-Neptunian objects, Edgeworth-Kuiper Disk objects, Kuiper Belt objects, or ice dwarves.
No spacecraft have ever visited Pluto. 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. The moons Nix and Hydra also orbit the same barycenter, but are not large enough to be spherical, and are simply considered to be satellites of Pluto (or, under the alternative viewpoint, of the Pluto-Charon system) Hydra was discovered along with Nix in June, 2005 by the Hubble Space Telescope's Pluto Companion Search Team. Nix follows a circular orbit in the same plane as Charon. Its orbital period of 24.9 days is close to a 1:4 orbital resonance with Charon, but the timing discrepancy is 2.7%, which suggests that there is no active resonance. Hydra orbits the barycenter of the system in the same plane as Charon and Nix, at a distance of about 65,000 km. Unlike other satellites of Pluto, its orbit is only nearly circular; its eccentricity of 0.0052 is small, but significantly non-zero. Its orbital period of 38.2 days is close to a 1:6 orbital resonance with Charon, with the timing discrepancy being 0.3%. Whether this is a true resonance awaits more detailed determinations of its orbit, in particular its rate of precession.
(1) A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. (3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies". The never-before-seen surface of the distant planet Pluto is resolved in these NASA Hubble Space Telescope pictures, taken with the European Space Agency's (ESA) Faint Object Camera (FOC) aboard Hubble. Hubble imaged nearly the entire surface of Pluto, as it rotated through its 6.4-day period, in late June and early July 1994. These images, which were made in blue light, show that Pluto is an unusually complex object, with more large-scale contrast than any planet, except Earth.
The two smaller inset pictures at the top are actual images from Hubble. North is up. Each square pixel (picture element) is more than 100 miles across. At this resolution, Hubble discerns roughly 12 major "regions" where the surface is either bright or dark. The larger images (bottom) are from a global map constructed through computer image processing performed on the Hubble data. The tile pattern is an artifact of the image enhancement technique. Opposite hemispheres of Pluto are seen in these two views. Some of the variations across Pluto's surface may be caused by topographic features such as basins, or fresh impact craters. However, most of the surface features unveiled by Hubble, including the prominent northern polar cap, are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere. Image Credit: Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA
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The Future
Astronauts of America Foundation Title; 'We help launch the future' slogan;
Character and Name Ikky;
and
The Future Astronauts of America
Foundation Logo are
intellectual property of The Future Astronauts of America
Foundation
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 den-sity, 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 tem-porarily 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.
On August 24, 2006, the International
Astronomical Union (IAU) formally downgraded Pluto from an official planet
to a dwarf planet. According to the new rules a planet meets three
criteria: it must orbit the Sun, it must be big enough for gravity to
squash it into a round ball, and it must have cleared other things out of
the way in its orbital neighborhood. The latter measure knocks out Pluto
and 2003UB313 (Eris), which orbit among the icy wrecks of the Kuiper Belt,
and Ceres, which is in the asteroid belt.
