Pluto and its Moons: Charon, Nix, and Hydra
NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
Average Distance from the Sun: 5.900 billion km (39.439 AU)
Size (Equatorial Diameter): 2 284 km (0.179 x that of Earth)
Mass: 1.20 x 1022 kg (0.002 x that of Earth
Length of Day (Solar Rotation Period): 6.75 Earth days
Length of Year (Sidereal Orbital Period): 247.7 Earth years
Temperature: -230°C
Gravity: 0.4 m/s2 (0.04 x that of Earth)
Pluto was discovered by Clyde Tombaugh in 1930. Until 2006 it was thought of as the ninth and smallest planet. However, it is now considered to be a dwarf planet, because it lies within the Kuiper Belt and therefore has not cleared the neighbourhood around its orbit.
Pluto was long considered to be different to the eight major planets, being neither a terrestrial planet nor a gas giant. It is also very small, there are seven moons in the Solar System which are larger than Pluto, and its orbit is more elongated and tilted than those of the planets. At its closest it is only 4 425 million km from the Sun, at its farthest it is 7 735 million km from the Sun. From January 1979 through to 11 February 1999 Pluto was closer to the Sun than Neptune. The next time this occurs will be in the early part of the 23rd century.
Surface and Atmosphere
Very little is known about Pluto as no spacecraft has visited the distant dwarf planet. However, one is on its way. In January 2006, NASA launched its New Horizons spacecraft to Pluto which will reach Pluto and the Kuiper Belt in July 2015.
Scientists currently believe that Pluto is similar in appearance and composition to Triton (Neptune's largest moon). It has a rocky core that is surrounded by a mantle of water-ice and methane-ice. Above this mantle is a thin haze layer less than 50 km thick and above this an atmosphere of methane up to 300 km in thickness. This thin atmosphere may freeze and fall to the surface as the dwarf planet moves away from the Sun.
On the surface of Pluto the Sun would look like a bright star, equal to the light of a full moon on Earth.
Moons
Pluto has one major moon, Charon, and two smaller moons.
Charon (diameter: 1192 km) was discovered in 1978 by James Christy and is half the size of Pluto. The pair orbit around a centre of mass that lies between the two bodies and for this reason some scientists refer to Pluto and Charon as a double planet rather than as a dwarf planet and moon.
It takes Charon just over six days to orbit Pluto, or roughly the same amount of time as it takes Pluto to rotate. This means that Charon always remains above the same spot on Pluto.
Pluto's two smaller moons were discovered using the Hubble Space Telescope in May 2005. The moons, Nix and Hydra, are only 45 to 160 kms in diameter. They are about 5,000 times fainter than Pluto and are about two to three times farther from Pluto than Charon.
Rings
None
Mythology
Pluto was named after the god of the dead and ruler of the Underworld to both the Romans and Greeks. To the Greeks he was the son of Cronos and brother of Zeus. His realm was known as Hades.