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Pluto is the only planet in the solar system still unvisited by a spacecraft. Its status as the only planet in our Sun's family still studied purely by telescope is uniqueand frustratingto planetary scientists trying to uncover its secrets.

Pluto's Strange Orbit

American astronomer Clyde Tombaugh discovered Pluto in 1930. Despite astronomers' best efforts, Pluto's faintness and star-like appearance allowed the planet to keep most of its secrets. For twenty-five years, we could only refine our knowledge of its strange orbit, finding it on old photographs and taking new ones. Pluto's orbit is more eccentric and more tilted (inclined) than any other planet, taking 248.8 years to make one trip around the Sun. At perihelion (closest approach, which last occurred in 1996), it is only 60 percent as far from the Sun as at aphelion (farthest approach). So at perihelion, Pluto is closer to the Sun than Neptune ever gets. Yet, Pluto and Neptune cannot collide for two reasons. First, the relative inclination of the two orbits means their paths do not intersect. Second, Pluto is in a 2:3 orbit-orbit resonance with Neptune. This means that for every two trips Pluto makes around the Sun, Neptune makes exactly three. When Pluto is at perihelion, Neptune is on the other side of the Sun.

The Significance of Brightness Measurements

In 1955, photometry (brightness measurements) of Pluto showed a repetition of 6.38 daysthe length of Pluto's day. Two trends in the evolution of the brightness have since been found. First, its amplitude has increased from about 10 percent to a current value of 30 percent. This tells us that the subsolar point has been moving equatorward, and that the planet's spin axis must be severely tilted. Second, the average brightness has faded over the years, evidence that Pluto's poles are likely brighter than its equator. Decades of photometry have been interpreted to derive maps of Pluto's surface reflectance, or albedo. These are comparable in detail with what the Hubble Space Telescope has been able to reveal.

The Size and Composition of Pluto and Its Moon

Little regarding Pluto's size or composition was known until recently. In 1976 the absorption of methane was discovered in Pluto's spectrum. This implied a bright, icy planet, and therefore a small radius. In 1978 James Christy, then an astronomer at the United States Naval Observatory, discovered Pluto's satellite, which was named Charon. Orbiting Pluto with the same 6.38726-day period as Pluto's spin, Charon was the key to unlocking Pluto's secrets. By timing the orbital period and measuring the estimated separation between the two, astronomers could compute the total mass of the systemabout 0.002 Earth masses. Charon orbits retrograde , and Pluto spins backwards (just like Venus and Uranus).

Charon's orbital plane above Pluto's equator was seen edge-on in 1988. This produced a series of occultations and eclipses of and by the satellite, each half-orbit, from 1985 to 1992. Timing these "mutual events" allows calculation of the radii for both bodiesapproximately 1,153 kilometers (715 miles) for Pluto and 640 kilometers (397 miles) for Charon. The sum is about the radius of the Moon. When Charon hid behind the planet, Pluto's spectrum could be observed uncontaminated by its moon. This spectrum, when subtracted from a combined spectrum of the pair taken a few hours before or after, yields the spectrum of Charon. Pluto's spectrum showed methane frost: the gas we use for cooking is frozen solid on its surface! Charon's spectrum revealed nothing but dirty water ice. (Independent measurements show the amount of methane on Pluto varies with longitude. Bright regions have more methane than dark regions.) When Charon passed between Pluto and Earth, it (and its shadow) selectively hid different portions of its primary . Interpretation of these measurements is complicated but has allowed refined albedo (or reflectivity) maps of one hemisphere of Pluto to be extracted.

Surface and Atmospheric Readings

The surface temperature of Pluto is currently under debate. Two results have been published: about 40°K (-233°C; -388°F) and about 55°K (-218°C; -361°F). The first value is similar to the temperature on Triton, Neptune's largest moon; the latter is more consistent with Pluto's lower albedo. In either case, it is very cold. Water ice on Pluto is harder than steel is at room temperature! Misconceptions exist about how dark it would seem for an astronaut on Pluto. Despite the planet's remote distance, the Sun would appear to have the brightness of about 70 full Moons on Earth. Combine this with the bright, icy surface and one would have no problems navigating the surface.

On June 9, 1985, Pluto passed in front of a star. Rather than blinking out, the starlight gradually dimmed due to refraction by an atmosphere. Too dense to be methane alone, the atmosphere was suspected to contain nitrogen and carbon monoxide. Both have since been identified on Pluto's surface, with nitrogen comprising about 97 percent of the ground material. From details of precisely how the starlight faded, scientists believe there is a temperature increase close to the surface, much like on Earth. Pluto's atmospheric pressure is only a few millionths that of Earth, and the atmosphere actually may "frost out" with increasing distance from the Sun.

The Hubble Space Telescope has been used to measure the size of Charon's orbital radius, about 19,500 kilometers (12,090 miles, or approximately 1.5 Earth diameters). Densities have also been calculated: 1.8 to 2.0 grams per cubic centimeter (112 to 125 pounds per cubit foot) for Pluto and 1.6 to 1.8 grams per cubic centimeter (100 to 112 pounds per cubit foot) for Charon. From the density, scientists can infer the internal composition, a roughly 50-50 mix of rock and ice.

Future Spacecraft Visit?

Efforts to learn more continue. New large Earth-based telescopes equipped with adaptive optics and fast computers will allow the blurring effects of our atmosphere to be nullified, surpassing the resolution of Hubble's rather small 2.4-meter (4.9-foot) mirror. In contrast, the "faster, better, cheaper" policy of the National Aeronautics and Space Administration (NASA) has led to a halt of the Pluto-Kuiper Express spacecraft. A new mission profile, called the New Horizons Pluto-Kuiper Belt Mission, was approved by Congress in 2001. However, funding for this mission is not in the President's proposed budget for 2002. Launch must happen by 2006 or Jupiter will no longer be in position to slingshot the craft towards Pluto with a gravity assist , and the trip to Pluto will take years longer. We will have to wait the better part of a Jupiter orbit (11.8 years) until the geometry repeats itself. By then, Pluto's atmosphere may have frozen out. Until the task is taken seriously, Pluto will remain the only planet unvisited by a spacecraft.

see also Hubble Space Telescope (volume 2); Kuiper Belt (volume 2); Nasa (volume 3); Orbits (volume 2); Planet x (volume 2); Planetary Exploration, Future of (volume 2); Tombaugh, Clyde (volume 2).

Robert L. Marcialis


Binzel, Richard P. "Pluto." Scientific American 262, no. 6 (1990):50-58. Marcialis, Robert L. "The First Fifty Years of Pluto-Charon Research." In Pluto and

Charon, ed. S. Alan Stern and David J. Tholen. Tucson: University of Arizona Press, 1997.

Internet Resources

PLUBIB: A Pluto-Charon Bibliography. Ed. Robert L. Marcialis. University of Arizona. <;umpire/science/plubib_home.html>.


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Pluto, the ninth and farthest planet from the Sun, is one of the least well understood objects in the solar system. It is the smallest of the major planets and has a most unusual orbit. Pluto is only 1,428 miles (2,300 kilometers) in diameter. Since the planet is 3.66 billion miles (5.89 billion kilometers) away from the Sun, it takes almost 250 years for it to complete one revolution around the Sun. However, it takes Pluto only 6.39 Earth days to complete one rotation about its own axis.

In Greek mythology, Pluto is the god of the underworld. The planet was given its name for several reason. First, due to its great distance from the Sun, Pluto is almost always dark. The sunlight it receives is about equal in intensity to moonlight on Earth. Second, Pluto is the mythological brother of Jupiter and Neptune. And finally, the planet's name begins with "PL," the initials of Percival Lowell (18551916), the American astronomer who spent the final years of his life searching for the elusive planet.

The search for Pluto

Shortly after the discovery of Neptune in 1846, astronomers began looking for an even more distant planet. They believed some celestial body existed at the outer reaches of the solar system that caused disturbances in the orbit of Uranus. The gravitational field of Neptune accounted for some of its neighbor's orbital irregularities, but not all of them.

Percival Lowell used traditional mathematical calculations to guess the location of the suspected planet. He also set up a photographic search for it, but all his attempts proved unsuccessful. Pluto was finally discovered in 1930 during a painstaking search of photographic plates by American astronomer Clyde Tombaugh while he was working at the Lowell Observatory in Arizona.

The properties of Pluto

Before Pluto was located, astronomers had expected it to be a large planet about the size of Jupiter, since it was able to influence the orbit of Uranus, located two planets away. At that time, the solar system appeared to fit a neat pattern: small, dense planets were closest to the Sun and giant, gaseous planets were farther away. Pluto broke this pattern: it is a small, dense planet at the farthest reaches from the Sun.

Pluto's orbit also differs from the pattern set by the other planets in the solar system. While the other eight planets orbit the Sun on the same plane, Pluto travels on an inclined orbit that crosses that plane. Its orbitthe most oval in shape of all the planetslies mostly outside of that of its closest neighbor, Neptune. At times, however, it crosses inside Neptune's orbit, bringing it closer to the Sun than Neptune.

Pluto is so distant that no Earth-bound telescope has been able to provide a detailed picture of its surface features. The best image to date was taken by the Hubble Space Telescope (HST) in early 1996, in which the planet looks like a fuzzy soccer ball. The HST only revealed that Pluto has frozen gases, icy polar caps, and mysterious bright and dark spots.

Beyond that, astronomers can rely only on imprecise observations and what is known about the planet's density to paint a more complete picture of the planet. Pluto is probably composed of mostly rock and some ice, with surface temperatures between 350 and 380°F (212 and 228°C). The bright areas on its surface are most likely nitrogen ice, solid methane, and carbon monoxide. The dark spots may hold some form of organic material, possible hydrocarbons from the chemical splitting and freezing of methane.

Pluto's atmosphere is probably made of nitrogen, carbon monoxide, and methane. At Pluto's perihelion (pronounced pear-a-HEE-lee-an; the point on its orbit closest to the Sun), its atmosphere exists in a gaseous state. For most of its orbit, the atmosphere is frozen.

Its only moon

Much of what is known about Pluto was learned following the 1978 discovery of Pluto's moon, Charon (pronounced Karen, and named for the mythological character who transported the dead to the underworld). Prior to Charon's discovery, astronomers thought Pluto and its moon together were one larger object. Charon has a diameter over half that of Pluto, making it the largest moon relative to its planet in the solar system. For this reason, some astronomers consider the two bodies to be a double planet.

The origin of Pluto

Most theories regarding Pluto's origin connect the planet with Neptune's moon Triton. This is because Pluto, like Triton, rotates in a direction opposite that of most other planets and their satellites.

One theory is that Pluto used to be one of Neptune's moons. Struck by a massive object, Pluto was broken in two, creating Charon. The two were then sent into orbit around the Sun. A more popular theory, however, is that both Pluto and Triton started out in independent orbits and that Triton was captured by Neptune's gravitational field.

Trips to Pluto

More questions about Pluto and Charon were to be answered early in the twenty-first century when the National Aeronautics and Space Ad ministration (NASA) planned to send the first unmanned mission to Pluto and its moon. The Pluto-Kuiper Express, which was scheduled to be launched in 2004, was to have consisted of two spacecraft. They were to arrive at Pluto by 2012. They were expected to encounter Pluto near its perihelion, before its atmosphere froze once again, a seasonal deep freeze that lasts more than 100 years. The spacecraft were to study the atmosphere, surface features, and geologic composition of Pluto and Charon, then fly by Pluto into the Kuiper Disk, a ring filled with hundreds of thousands of small, icy objects that are well-preserved remnants of the early solar system. This ring is located between Neptune and Pluto (sometimes beyond Pluto, depending on its oval orbit), some 3.6 billion miles (5.8 billion kilometers) from Earth.

In September 2000, however, NASA issued a stop-work order on the project because of spiraling costs. The project was then canceled in April 2001 when the 2002 budget issued by President George W. Bush's administration provided no money for it.

[See also Solar system ]


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Pluto The ninth and outermost planet of the solar system, its orbit an average 39.44 AU from the Sun and highly eccentric, sometimes carrying it inside the orbit of Neptune. Its distance from Earth ranges from 4293.7 × 106km to 7533.3 × 106km. Pluto is the smallest planet (much smaller than the Moon), with a radius of 1137 km; volume 0.616 × 1010 km3; mass 0.0125 × 1024 kg; mean density 2050 kg/m3; surface gravity 0.66 (Earth = 1); visual albedo 0.3; blackbody temperature 42.7 K. The atmosphere is very thin, with a surface pressure of about 0.003 bar, and composed of methane and nitrogen. The average surface temperature is about 50 K. It has one satellite, Charon, so large that some astronomers consider Pluto and Charon a minor double-planet system. Pluto was discovered in 1930 by Clyde Tombaugh.


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Pluto Smallest and outermost planet of the Solar System. Independently, William H. Pickering and Percival Lowell calculated the possible existence of Pluto. The planet was eventually located (1930) by Clyde Tombaugh – within 5° of Lowell's predicted position. Pluto seems to have a mottled surface with light and dark regions, and signs of polar caps. The surface is covered with icy deposits consisting of 98% nitrogen, with traces of methane, and also probably water, carbon dioxide and carbon monoxide. Pluto has a single moon, Charon, which is so large that some astronomers consider Pluto/Charon as a double planet.;


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Plu·to / ˈploōtō/ 1. Greek Mythol. the god of the underworld. Also called Hades.2. Astron. the most remote known planet of the solar system, usually ninth in order from the sun, discovered in 1930 by Clyde Tombaugh.


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dis / dis/ inf. • v. (also diss) (dissed, diss·ing) [tr.] act or speak in a disrespectful way toward: he was expelled for dissing the gym teacher.• n. disrespectful talk: the airwaves bristle with the sexual dis of shock jocks.


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Dis in Roman mythology, the ruler of the Underworld, equivalent of the Greek Pluto (see Pluto1) or Hades; the name, as coming from Dives ‘rich’, may be a translation for Pluto.


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Pluto2 (the code-name for) a system of pipelines laid in 1944 to carry petrol supplies from Britain to Allied forces in France. The name is an acronym for Pipe Line Under The Ocean.


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Pluto Roman god of the underworld, equivalent to the Greek god Hades. He ruled over the land of the dead and was also a god of wealth, since his realm contained all underground mineral riches.


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Pluto (or PLUTO) (ˈpluːtəʊ) pipe line under the ocean (conveying fuel to Allied forces, World War II)