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Pluto
Mosaic, true-color image of Pluto based on eclipses of Pluto by its satellite, Charon.
Symbol	♇
Date of discovery	19301930 5690 He 5933 AM
Name of discoverer	Clyde W. Tombaugh
Name origin	Greco-Roman god of wealth and the underworld
Orbital characteristics
Celestial class	Dwarf planet
Primary	Sun
Order from primary	10
Perihelion	4,436,824,613 km29.658 AU 2,756,914,999.528 mi
Aphelion	7,375,927,931 km49.305 AU 4,583,189,132.342 mi
Semi-major axis	5,906,376,272 km39.482 AU 3,670,052,065.935 mi
Titius-Bode prediction	77.2 AU
Circumference	188.925 AU28,262,777,589.75 km 17,561,675,806.882 mi
Orbital eccentricity	0.24880766
Sidereal year	248.09 a90,614.873 da
Synodic year	366.73 da1.004 a
Avg. orbital speed	4.666 km/s16,797.6 km/h 2.899 mi/s 10,437.545 mph
Inclination	17.14175°0.299 rad 19.046 grad to the ecliptic
Rotational characteristics
Sidereal day	-6.387230 da-153.294 h
Rotation speed	47.18 km/h0.0131 km/s 0.00814 mi/s 29.316 mph
Axial tilt	119.591°2.087 rad 132.879 grad
Physical characteristics
Mass	1.305 * 1022 kg0.00218 M⊕ 0.178 M☾
Mean density	2,030 kg/m³2.03 g/ml 126.729 lb/ft³
Mean radius	1,195 km742.539 mi
Surface gravity	0.58 m/s²1.903 ft/s² 0.0591 g
Escape speed	1.2 km/s4,320 km/h 0.746 mi/s 2,684.324 mph
Surface area	17,950,000 km²6,930,533.746 mi² 0.0352 A⊕ 0.473 A☾
Minimum temperature	33 K-240.15 °C -400.27 °F 59.4 °R
Mean temperature	44 K-229.15 °C -380.47 °F 79.2 °R
Maximum temperature	55 K-218.15 °C -360.67 °F 99 °R
Number of moons	3
Color	#EEBB88

Pluto is the second-largest dwarf planet in the solar system. (The recently-discovered Eris is larger.) Its name literally means "god of wealth" and is one of the two names for one of the brothers of Zeus; the other is Hades, or "god of the underworld." (The Romans used only the name Pluto for this particular Greek god.)

According to the International Astronomical Union, it no longer qualifies as a planet because it has not cleared its orbit of other objects.

Contents 1 Discovery 2 The dwarf planet controversy 3 Problems for uniformitarianism 4 Observation and Exploration 5 In popular fiction 6 Satellites 7 References 8 Related links

Discovery

In 1930, Clyde W. Tombaugh at the Lowell Observatory surveyed the sky, looking for a planet beyond Neptune (called "Planet X") that other astronomers had predicted from calculations based on an erroneous value for Neptune's mass. Tombaugh knew nothing of the error, but found an object anyway. This object was later given the name Pluto.

But even he realized that Pluto was not large enough to be the predicted Planet X. Astronomers continued to search in vain for it, until Voyager 2 made its flyby of Neptune, and the Jet Propulsion Laboratory scientists determined from this flyby that Neptune was significantly heavier than previously supposed. Current theory predicts no more planets other than the eight now known, but still allows for many other objects, both in the classic asteroid belt and in the Kuiper Belt, essentially a second belt of asteroids and comets.

The dwarf planet controversy

The discovery of Eris, a scatter-disk body 27% more massive than Pluto, caused a controversy concerning what does, and what does not, constitute a planet. If Pluto were to retain its historical designation of "planet," then Eris would also qualify. But Eris was held not to qualify because it was in a neighborhood with multiple other objects, a situation similar to that of a similar body in the asteroid belt, namely Ceres.

In 2006, the International Astronomical Union passed the following resolution:

“

RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way: (1) A "planet"1 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 neighbourhood 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". 1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies. [1]

”

Under those rather strict criteria, Pluto does not qualify. For that reason, Pluto is no longer considered a planet. It shares the new "dwarf planet" category with Eris and Ceres.

Problems for uniformitarianism

Pluto was initially supposed to have been a moon of Neptune. But with the discovery of Charon in 1978, and the subsequent discoveries of its other moons Hydra and Nix, that theory became far less plausible.

Another hypothesis has held that Pluto entered the solar system from outside, and the Sun captured it. But if such a mass dived into the solar system from outside, then it would have followed a hyperbolic path. That it did not pass back out of the solar system to complete the hyperbola begs explanation.

Observation and Exploration

On January 19, 2006, the New Horizons mission was launched. Pluto is its prime target, but mission planners are considering plans to study at least one other Kuiper belt object.^[2]

In popular fiction

Science fiction author Larry Niven once speculated on an "earlier generation" race of extraterrestrial beings that controlled the galaxy until it suffered mutual annihilation in a war with a revolting slave race. As part of that scenario, a member of that "slaver race" once had to make an emergency "landing" in a crippled spacecraft that could no longer brake to a safe approach speed. He then set a course for the earth, bailed out of his ship, and left the ship's autopilot with orders to crash-land on Neptune. Instead of that happening, the ship struck a moon of Neptune hard enough to knock it out of orbit; that moon became known as Pluto.

Pluto has also been a subject of speculation involving future efforts by humanity to colonize that body, efforts often complicated by the presence of extraterrestrial "campers" or even of pathogens, usually viruses, native to Pluto.

Satellites

Table of satellites, in order from the innermost to the outermost:

Name	Perihadion	Aphadion	Eccentricity	Sidereal month	Inclination	Mass	Sidereal day
Charon	1964000019,640 km1.312853e-4 AU 12,203.73 mi	1964000019,640 km1.312853e-4 AU 12,203.73 mi	00	6.387256.387 da0.0175 a	00 °0 rad 0 grad	1.9E+210.0259 M☾1.9e+21 kg 3.17939e-4 M⊕	551858.4153.294 h6.387 da
Nix	4857800048,578 km3.247239e-4 AU 30,184.97 mi	4877200048,772 km3.260207e-4 AU 30,305.516 mi	0.0020.002	24.856224.856 da0.0681 a	0.0006981317007980.04 °6.981317e-4 rad 0.0444 grad	5.0E+166.804295e-7 M☾5.0e+16 kg 8.366815e-9 M⊕
Hydra (moon)	6445800064,458 km4.308751e-4 AU 40,052.344 mi	6510200065,102 km4.3518e-4 AU 40,452.507 mi	0.0050.005	38.20638.206 da0.105 a	0.003839724354390.22 °0.00384 rad 0.244 grad	5.0E+166.804295e-7 M☾5.0e+16 kg 8.366815e-9 M⊕

References

1. ↑ "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.
2. ↑ Jenner, Lynn. "NASA - New Horizons." NASA, October 9, 2007. Accessed July 3, 2008.

Related links

• Pluto by Wikipedia
• Hamilton, Calvin J. "Entry for 'Pluto'." Views of the Solar System, 2007. Accessed January 21, 2008.

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