In 1978, 48 years after the discovery of Pluto, the U.S. Naval
Observatory scientists James Christy and Robert Harrington found Charon while trying to refine the orbital parameters of Pluto. Charon is only one-fifth as bright as Pluto, but their close mutual orbit had prevented the discovery of Charon's existence. Charon is named, in part, for the mythological figure who ferried the dead to Hades and is sometimes pronounced "KARE-en." But because Christy also named the moon in honor of his wife, Charlene, many planetary scientists pronounce the moon's name with a soft "ch" sound ("SHAHR-en").
Charon is the largest moon in relation to its planet in the solar system; the two form a double-dwarf planet system more than a planetary system with one moon. The most recent measurement for the semimajor axis of the Pluto-Charon orbital system is 11,580 miles (18,636 km), with an error of plus or minus five miles (8 km) on the measurement. Charon's orbit has an eccentricity of 0.0076, a slight but significant deviation from circularity. The tidal interactions between a planet and its moon normally have the effect of rapidly damping away any eccentricity (note that the eccentricities of the inner moons of Jupiter and Saturn are zero out to three decimal places).That Charon's orbit has an eccentricity may indicate that the system was recently disturbed by interacting with a larger mass or by an impact with the last several million years.
The Jet Propulsion Laboratory has produced what may be the best estimate for Charon's radius: 364 miles plus or minus eight miles (586 km plus or minus 13 km). Other estimates for Charons' radius range from 360 to 391 miles (580 to 630 km).Though, as discussed above, the mass of the Pluto-Charon system is well known, its division between Pluto and Charon is not.
The consensus value seems to be that Charon contains about 11 percent of the total mass of the system, making its mass about 3.36 x 1021 pounds (0.15 x 1022 kg). Based on the best estimate for its radius and its mass, the density of Charon is between 95 and 114 pounds per cubic foot (1,500 and 1,800 kg/m3).The ratio of masses is between 0.084 and 0.157, and a closer flyby mission is needed before their masses can be better calculated.
In truth, Charon does not strictly orbit Pluto: Charon's mass is close enough to Pluto's that they together orbit the center of mass of their combined system (in a system such as Mars and Phobos, Mars's mass is so much larger than Phobos's that their combined center of mass is well within Mars, and Mars's orbit is little affected by Phobos. For Pluto and Charon, their masses are similar enough that each body is affected by their mutual orbit. The orbital period of the two-body system has been exactly measured and found to be 6.387223 days, with a margin of error of plus or minus 0.000017 days.
Pluto and Charon are locked in a mutual synchronous orbit. The Moon always keeps the same face to the Earth, but the Earth turns independently. In the Pluto and Charon system, each body keeps the same face to the other at all times. Therefore, Charon is either always visible or never visible, depending on where one is on the surface of Pluto. The same is true for a viewer looking from Charon at Pluto: Pluto is either always or never visible, depending only on the viewer's location.
The mutual orbit of Charon and Pluto has an obliquity of about 120 degrees to the ecliptic plane and is thought to vary from about 103 to 128 degrees over a period of about 3 million years.This estimate is a result of numerical modeling of the torque of the Charon-Pluto system, not of observations.
Charon's surface lacks the strongly contrasting patches that cover Pluto.While Pluto's brightness varies by 30 percent over their six-day rotation, Charon's only varies by about 4 percent.Water ice seems to cover Charon's surface, and the moon's composition is thought to be similar to Saturn's icy moons, for example, Rhea. The water ice on Charon's surface is cold enough to attain the crystal structure of ice II. Unlike Pluto, Charon seems to have little or no nitrogen, carbon monoxide, or methane. Spectra from Charon are well fit by ammonia
(NH3) and ammonia hydrate (NH3°2H2O). Charon's albedo is about 0.4, in contrast to Pluto's albedo of about 0.7, further indicating that the two bodies have different compositions.
Charon and Pluto have different surfaces and are thought to be made of different materials. Though it was first suggested that Pluto and Charon began as a single body that was reformed into two by a giant impact (in the way the Earth and Moon were thought to be), their very different compositions make this unlikely. As discussed later, there are a large number of binary systems in the Kuiper belt, and Pluto and Charon simply seem to be the most massive. In this respect, as well as in their orbits and appearances, Pluto and Charon seem average denizens of the Kuiper belt.
On May 15, 2005, two additional moons of Pluto were seen in images from the Hubble Space Telescope. The two small bodies, named Nix and Hydra, orbit Pluto at about two and three times the distance of Charon. Nix and Hydra are thought to have diameters of less than about 88 miles (140 km) and 105 miles (170 km), respectively, but they are so small and distant that little is known about them.
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