It has been noted that the first two letters of ''Pluto'' are the initials of Percival Lowell, and Pluto's astronomical symbol (20px|) is a monogram constructed from the letters 'PL'. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, |- ! scope="col" colspan="2" | Name (Pronunciation) ! scope="col" | Discovery Year ! scope="col" | Diameter (km) ! scope="col" | Mass (kg) ! scope="col" | Orbital radius (km) (barycentric) ! scope="col" | Orbital period (d) |- ! scope="row" | Pluto | | 1930 | 2,306 (66% Moon) | 13,050 (18% Moon) | 2,040 (0.6% Moon) | |- ! scope="row" | Charon | , | 1978 | 1,205 (35% Moon) | 1,520 (2% Moon) | 17,530 (5% Moon) | 6.3872 (25% Moon) |- ! scope="row" | Nix | | 2005 | 91 | 4 | 48,708 | 24.856 |- ! scope="row" | S/2011 P 1 | | 2011 | 13–34 | | ~59,000 | 32.1 |- ! scope="row" | Hydra | | 2005 | 114 | 8 | 64,749 | 38.206 |}

Mass of Nix and Hydra assumes icy/porous density of 1.0 g/cm³

Origins [link]

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest member of the Kuiper belt, a somewhat stable ring of objects located between 30 and 50 AU from the Sun. Like other Kuiper-belt objects (KBOs), Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of a comet. If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.

Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon Triton, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object. Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits. The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are believed to have been placed in their current positions by interactions with Neptune's migrating resonances. A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the origin of Jupiter's Trojan asteroids. It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed it into a resonant capture. The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodies which became Triton and Eris.

Exploration [link]

After an intense political battle, a revised mission to Pluto, dubbed ''New Horizons'', was granted funding from the US government in 2003. ''New Horizons'' was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.

In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months before closest approach and will continue for at least a month after the encounter. ''New Horizons'' captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI). The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

''New Horizons'' will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. ''New Horizons'' will also photograph the surfaces of Pluto and Charon.

The discovery of Pluto's two small moons, Nix and Hydra, may present unforeseen challenges for the probe. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for damage that could disable the probe.

Concepts [link]

Classification [link]

After Pluto's place within the Kuiper belt was determined, its official status as a planet became controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. The Hayden Planetarium reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.

In 2002, the KBO 50000 Quaoar was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit of 1800 km on its diameter, nearer to Pluto's diameter of 2320 km, although Sedna's diameter was revised downward to less than 1600 km by 2007. Just as Ceres, Pallas, Juno and Vesta eventually lost their planet status after the discovery of many other asteroids, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Named Eris, it is now known to be approximately the same size as Pluto. This was the largest object discovered in the Solar System since Triton in 1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.

2006: IAU classification [link]

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet': # The object must be in orbit around the Sun. # The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium. # It must have cleared the neighbourhood around its orbit.

Pluto fails to meet the third condition, since its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.

On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor Planet Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia". If Pluto had been given a minor planet name upon its discovery, the number would have been about 1,164 rather than 134,340.

There has been some resistance within the astronomical community toward the reclassification. Alan Stern, principal investigator with NASA's ''New Horizons'' mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons." Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."

Researchers on both sides of the debate gathered on August 14–16, 2008, at The Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet. Entitled "The Great Planet Debate", the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of a planet. Just before the conference, on June 11, 2008, the IAU announced in a press release that the term "plutoid" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be of near-spherical shape.

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References [link]

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Category:Astronomical objects discovered in 1930

Jack Hannah (January 5, 1913 - June 11, 1994) was an animator, writer and director of animated shorts.

Hannah was born January 15, 1913, in Nogales, Arizona. He moved to Los Angeles in 1931 to study at the Art Guild Academy. One of his first jobs was designing movie posters for Hollywood theaters. In 1933, during the Great Depression, Hannah dropped off his portfolio at Walt Disney Studios, and soon afterward was hired as an in-between and clean-up artist, working on Mickey Mouse, Donald Duck, and Silly Symphony cartoons.

Hannah's career as an animator commenced with the short ''Modern Inventions'' (released on May 29, 1937). After thirteen films in that capacity, he was assigned to the story department writing cartoon short continuities, beginning with ''Donald's Nephews'' (released on April 15, 1938). He received writing credit on 21 Disney cartoon shorts.

In 1942 he collaborated with Carl Barks on the first two comic books Barks worked on, ''Pluto Saves the Ship'' and ''Donald Duck Finds Pirate Gold''. Hannah in subsequent years did a handful of other Donald Duck comic book stories but, unlike Barks, he stayed at the studio and eventually was given a chance to be a director. The short ''Donald's Off Day'' (released on December 8, 1944) was the first of 94 films he would direct. These include most of the shorts featuring Donald Duck in the post-war era along with all starring Chip 'n Dale and Humphrey the Bear; he also directed some shorts starring Goofy, Mickey Mouse, Pluto and some minor Walt Disney characters.

After Disney stopped producing animated shorts, Hannah did 14 episodes of the Walt Disney anthology television series (composed of footage from the classic cartoons along with new linking material) and fulfilled his ambition to direct live-action by handling Walt Disney's introductions for the episodes. Hannah hoped to segue into a career in live-action but "Walt had me pegged as an animation director so he balked at the suggestion. We had a few heated discussions and I became aware that I had come to an impasse."

Hannah then went to the Walter Lantz Studio and directed a number of films featuring Woody Woodpecker and some minor characters. Besides directing shorts, Hannah also was Assistant Director for the television series ''The Woody Woodpecker Show'', which began airing on October 3, 1957. "I did more or less the same thing that I did with Walt Disney in directing live-action except Lantz was better at taking direction." His last directing effort was the short ''Charlie's Mother-In-Law'' (released on April 16, 1963). He retired shortly thereafter.

In 1975, Hannah was one of the co-founders, along with T. Hee, of the Character Animation program at the California Institute of the Arts.

Hannah was honored as a "Disney Legend" in 1992. As Carl Barks is credited with creating the personality of Donald Duck's comic book version, Jack Hannah is credited with developing, if not creating, the personality of the animated version. It is for this reason Disney historian Jim Korkis has dubbed him "Donald Duck's Other Daddy."

Hannah died in Burbank, California on June 11, 1994, at age 81.

References[link]

External links[link]

Category:1913 births Category:1994 deaths Category:American animators