Plutos Moons

Pluto possesses three known moons. Charon, by far the largest, is fully half the size of Pluto. It revolves around the dwarf planet—more accurately, the two bodies revolve around a common centre of mass—at a distance of about 19,640 km

(12,200 miles), equal to about eight Pluto diameters. (By contrast, Earth's Moon is a little more than one-fourth the size of Earth and is separated from the latter by about 30 Earth diameters.)

Charon's period of revolution is exactly equal to the rotation period of Pluto itself; in other words, Charon is in synchronous orbit around Pluto. As a result, Charon is visible from only one hemisphere of Pluto. It remains above the same location on Pluto's surface, never rising or setting (just as do communication satellites in geostationary orbits over Earth). In addition, as with most moons in the solar system, Charon is in a state of synchronous rotation—i.e., it always presents the same face to Pluto.

Charon is somewhat less reflective (has a lower albedo—about 0.35) than Pluto and is more neutral in colour. Its spectrum reveals the presence of water ice, which appears to be the dominant surface constituent. There is no hint of the solid methane that is so obvious on its larger neighbour. The observations to date were not capable of detecting ices of nitrogen or carbon monoxide, but, given the absence of methane, which is less volatile, they seem unlikely to be present. Charon's density implies that the moon contains materials such as silicates and organic compounds that are denser than water ice. The disposition of these materials inside Charon is even more speculative than it is for Pluto.

Scientists have exploited the presence of Charon to reveal several characteristics of Pluto that would not otherwise be known, particularly its mass and size. Much of this information was acquired through the extraordinary coincidence that in 1985, just seven years after Charon's discovery, it began a five-year period of mutual eclipse events with Pluto in which the moon alternately crossed the disk of (transited) and was hidden (was occulted, or was eclipsed) by Pluto, as seen from Earth, every 6.4 days. These events occur when Earth passes through Charon's orbital plane around Pluto, which happens only twice during Pluto's 248-year orbit around the Sun. Careful observations of these events allowed determinations of the radii of Pluto and Charon and of the masses of both bodies that were more precise than heretofore possible. In addition, monitoring the changes in the total brightness of the two bodies as they blocked each other permitted astronomers to estimate their individual overall albedos and even to create maps depicting brightness differences over their surfaces.

Pluto's other two moons, called Hydra and Nix (provisionally designated S/2005 P1 and S/2005 P2, respectively, on their discovery), are much smaller than Charon—about 60 and 50 km (37 and 31 miles) in diameter, respectively, if their surface reflectivity is assumed to be similar to Charon's. They revolve around Pluto outside Charon's path in nearly circular orbits (like Charon) and in the same orbital plane as Charon. Based on preliminary observations, the orbital radius of Hydra is about 64,700

km (40,200 miles); of Nix, 49,400 km (30,700 miles). It appears that for every 12 orbits completed by Charon, Hydra makes about 2 orbits (for a ratio of 6:1 in their orbital periods), while Nix makes nearly 3 orbits (for a 4:1 ratio); this also means that the orbital periods of Hydra and Nix are in a 3:2 ratio. These relationships of the orbital periods, which are approximately in the ratios of small whole numbers, suggest that the small moons are in stable dynamic resonances with Charon and with each other—that is, all three bodies pass one another periodically, interacting via gravity in a way that tends to maintain the regularity of their encounters.

discovery of _pluto and its moons_

When Pluto was found, it was considered the third planet to be discovered, after Uranus and Neptune, as opposed to the six planets that have been visible in the sky to the naked eye since ancient times. The existence of a ninth planet had been postulated beginning in the late 19th century on the basis of apparent perturbations of the orbital motion of Uranus, which suggested that a more-distant body was gravitationally disturbing it. Astronomers later realized that these perturbations were spurious—the gravitational force from Pluto's small mass is not strong enough to have been the source of the suspected disturbances. Thus, Pluto's discovery was a remarkable coincidence attributable to careful observations rather

One of the discovery photographs of Pluto's moon Charon, taken at the U.S. Naval Observatory station in Flagstaff, Ariz., in 1978. Charon appears merely as a bulge on the upper right portion of Pluto's silhouette. Official U.S. Navy Photograph than to accurate prediction of the existence of a hypothetical planet.

The search for the expected planet was supported most actively at the Lowell Observatory in Flagstaff, Ariz., U.S., in the early 20th century. It was initiated by the founder of the observatory, Percival Lowell, an American astronomer who had achieved notoriety through his highly publicized claims of canal sightings on Mars. After two unsuccessful attempts to find the planet prior to Lowell's death in 1916, an astronomical camera built specifically for this purpose and capable of collecting light from a wide field of sky was put into service in 1929, and a young amateur astronomer, Clyde Tombaugh, was hired to carry out the search. On Feb. 18, 1930, less than one year after he began his work, Tombaugh found Pluto in the constellation Gemini. The object appeared as a dim "star" of the 15th magnitude that slowly changed its position against the fixed background stars as it pursued its 248-year orbit around the Sun. Although Lowell and other astronomers had predicted that the unknown planet would be much larger and brighter than the object Tombaugh found, Pluto was quickly accepted as the expected ninth planet. The symbol invented for it, B, stands both for the first two letters of Pluto and for the initials of Percival Lowell.

Charon was discovered in 1978 on images of Pluto that had been recorded photographically at the U.S. Naval Observatory station in Flagstaff, fewer than 6 km (3.7 miles) from the site of

One of the discovery photographs of Pluto's moon Charon, taken at the U.S. Naval Observatory station in Flagstaff, Ariz., in 1978. Charon appears merely as a bulge on the upper right portion of Pluto's silhouette. Official U.S. Navy Photograph

Pluto's discovery. These images were being recorded by James W. Christy and Robert S. Harrington in an attempt to obtain more-accurate measurements of Pluto's orbit. The new satellite was named after the boatman in Greek mythology who ferries dead souls to Hades' realm in the underworld.

Prior to the discovery of Charon, Pluto was thought to be larger and more massive than it actually is; there was no way to determine either quantity directly. Even in the discovery images, Charon appears as an unresolved bump on the side of Pluto, an indication of the observational difficulties posed by the relative nearness of the two bodies, their great distance from Earth, and the distorting effects of Earth's atmosphere. Only near the end of the 20th century, with the availability of the Hubble Space Telescope and Earth-based instruments equipped with adaptive optics that compensate for atmospheric turbulence, did astronomers first resolve Pluto and Charon into separate bodies.

A team of nine astronomers working in the United States discovered Pluto's two small moons, Hydra and Nix, in 2005 via images made with the Hubble Space Telescope during a concerted search for objects traveling around Pluto as small as 25 km (16 miles) in diameter. To confirm the orbits, the astronomers checked Hubble images of Pluto and Charon made in 2002 for surface-mapping studies and found faint but definite indications of two objects moving along the orbital paths calculated from the 2005 images.

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