Charon (moon)

From Freepedia

**Charon**
Discovery
Image:Plutoncharon.jpg Pluto and Charon
Discovered by	James W. Christy
Discovered on	June 22, 1978
Orbital characteristics
Semimajor axis	19,410 km
Eccentricity	0.0002
Orbital period	6.387 d (6 d 9 h 18 min)
Inclination	115.60° (to the ecliptic) 0.00° (to Pluto's equator) 122.54° (to Pluto's orbit)
Is a satellite of	Pluto
Physical characteristics
Mean diameter	1205 km ± 1 km
Mass	1.58×10²¹ kg
Mean density	1.73 g/cm³ ± 0.08 g/cm³
Surface gravity	0.291 m/s²
Rotation period	synchronous
Axial tilt	zero
Albedo	0.37
Atmosphere	none

Charon (shair'-un or kair'-un, Greek Χάρων) is the only known satellite of Pluto. It was discovered by astronomer James Christy on June 22, 1978 by carefully examining highly magnified images of Pluto on photographic plates taken a couple of months before and noticing that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965. It received the temporary designation S/1978 P 1, according to the then-recently instituted convention. It is not to be confused with the similarly named Chiron, another object in the outer solar system with an orbit between those of Saturn and Uranus.

Christy named it after the Greek mythological figure Charon but pronounced it differently. The "ch" at the beginning of the moon's name is soft so it sounds like "Sharon," after the astronomer's wife Charlene, nicknamed Char, which both have soft ch sounds. The mythological figure's name is pronounced with a hard "ch" sound like the modern letter "k", like "ch" in Christy's name. The name "Charon" was officially accepted by the IAU in 1985, as it was very fitting. In mythology Charon was the ferryman of the dead: a figure with close ties to the god Pluto.

The discovery of Charon allowed astronomers to more accurately calculate Pluto's mass and size. Charon revolves around Pluto in 6.387 days, the same period as Pluto's rotation. The two objects are gravitationally locked (tidal locking) so they each keep the same face towards the other.

Charon's diameter is 1,205 km (748.8 miles), just under half the size of Pluto. It has 1/7th the mass of Pluto, and a surface area of 4,560,000 km². Unlike Pluto, which is covered with nitrogen ice, the Charonian surface appears to be coated with water ice.

Due to the unusually small difference in size between it and Pluto, Pluto and Charon are sometimes considered to be a double planet. They are also sometimes thought of as not a planet and a satellite, but as the first two Trans-Neptunian objects.

Simulation work published in 2005 by Robin Canup suggested that Charon could have formed by a giant impact around 4.5 billion years ago, with a Kuiper belt object between 1600 and 2000 kilometres in diameter striking Pluto at a speed of 1 kilometre per second and Charon coalescing from the ring of debris kicked up by the collision. There is less evidence for this theory than there is for the similar theory regarding Earth's Moon, however, most notably a lack of information regarding the composition of the two bodies.

On July 11, 2005 Charon occulted the magnitude 14 star 2UCAC 2625 7135. The event was seen by the 6.5 metre Clay Telescope at the Las Campanas Observatory in Chile. It was the first Charon occultation since 1980. Dr. Bruno Sicardy's team at the Paris Observatory in France used information from this occultation to determine Charon's gravity, radius, and density to unprecedented precision.

The moon, along with Pluto, is planned to be visited by the New Horizons mission in 2015.