Pluto

DISTANCE FROM SUN: 3.7 billion miles (5.9 billion kilometers).
REVOLUTION AROUND SUN: 248 years.
ROTATION: 6.4 days.
DIAMETER: 1,454 miles (2,340 kilometers).
DENSITY: 2x that of water.
MASS: .002 x that of Earth.
SURFACE TEMPERATURE: -387°F (-233°C) to -351°F (-213°C).
Satellites: Charon
Is it a planet or an errant asteroid? The debate over mysterious Pluto has intensified in recent years. If it’s a planet, it is the smallest in the solar system, smaller even than seven of the known moons. Images by the Hubble Space Telescope show that it is most definitely spherical, like a planet, and it has distinct surface features. In fact, maps derived from Hubble data show that Pluto has the highest contrast between light and dark regions of any planet except Earth. And, like a planet, Pluto has a moon.
Currently receding from its closest approach to the sun, Pluto has a very thin atmosphere. At its most distant point, however, as the sun becomes little more than a bright star in the daytime sky, the atmosphere freezes and falls to the surface.
Pluto’s rotation is much like that of Uranus, with its poles lying close to its orbital plane. The planet’s relatively bright south polar region was facing Earth in 1930 when American astronomer Clyde Tombaugh spotted it by comparing two photographic plates. Pluto now appears much dimmer with its equatorial region facing the inner solar system.


PLANET AND MOON IN A PAS DE DEUX
Pluto’s moon Charon is about half the planet’s diameter. It was not until the Hubble Space Telescope took an image in 1994 that the two bodies were clearly defined.
To someone standing on the surface of Charon, Pluto would always appear in the same spot in the sky. The stars would seem to move by, but Pluto would remain at the same altitude, in the same direction with respect to the horizon. The same would be true for an astronaut standing on our moon looking at Earth. But unlike on Earth, where the moon seems to course across the sky, an observer on Pluto would see Charon locked perpetually in place. In the entire solar system, Charon and Pluto are the only two bodies so locked to each other in synchronous orbit.
Hubble images show that Charon has a bluer tint than Pluto, indicating they have different surface compositions. Scientists believe that the two bodies were not formed together.


AN ODD ORBIT
Usually Pluto is the most distant known planet from the sun. But for 20 years of its 248-year orbit, it slips inside the orbit of Neptune. Pluto last crossed Neptune’s path in January 1979, made its closest approach to the sun ten years later, and crossed out of Neptune’s orbit in March 1999.


A STILL-FUZZY SURFACE
Even the best Hubble images still show Pluto’s surface as an indistinct jumble of bright and dark areas. It is clear, however, that the planet has bright polar regions and large spots at mid- and equatorial latitudes. Experts also suspect that bright areas are covered with nitrogen, methane, and carbon monoxide ices, some deposited during the planet’s periodic atmospheric freezes. Darker regions may be the result of photochemical reactions from cosmic rays.
Scientists who developed the first maps of Pluto were quick to notice the “Mickey Mouse” region, which seems to display a round head and two smaller round ears.


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