Pluto
The planet Pluto was discovered by Clyde Tombaugh in 1930 at Lowell Observatory in Flagstaff, Arizona. Long regarded as the last planet of the solar system, she was demoted to the status of dwarf planet in 2006.
Its orbit is highly eccentric and its distance from the sun varies a lot between 30 and 49 astronomical units. Thus, during certain periods, Pluto is closer to the Sun than Neptune, which was the case for example between 1979 and 1999. The orbit is also highly inclined at an angle of 17 degrees to the ecliptic plane.
Spectroscopic observations revealed that the surface of Pluto is covered with ice nitrogen and methane. A very thin atmosphere is present with a pressure 100,000 times weaker than the earth pressure. The average temperature is -220 degrees Celsius.
Charon and the other satellites
Because of its remoteness, Pluto is very difficult to study. It was so until 1978 that the astronomer James W. Christy of the US Naval Observatory discovers that the planet has a satellite, Charon, at a distance of 19 600 km.
Thanks to a very rare phenomenon, the passage from Earth in the orbital plane of the system between 1985 and 1990, astronomers have observed a series of mutual eclipses of the two main bodies. They were able to deduce their size (from Selected by the New Horizons probe): 2370 km in diameter Pluto and Charon in 1208. The separation between the two bodies and represents that almost eight times the diameter of Pluto, a very rare scenario.
The rotation period of the two bodies is the same and equal to the period of mutual orbit, a little over six Earth days. This is an exceptional phenomenon: the two bodies always have the same face and appear stationary in the sky to the other, synchronous rotation generated by mutual tidal forces.
Unlike Pluto, Charon has no atmosphere and its surface would be covered with water ice. The density of Charon suggests that the satellite contains a proportion of ice much larger than Pluto, which could be explained if Charon was formed in a collision from Pluto mantle materials.
In Roman mythology, Pluto was the god of the Underworld, the equivalent of the Greek god Hades. Charon was the ferryman of Hades who with his boat helped the deceased to cross the Styx.
A composite view of Pluto satellites
It was not until the new millennium that other smaller satellites are detected, all by the Hubble Space Telescope: Nix and Hydra in June 2005, Kerberos in June 2011 and finally Styx in June 2012.
Styx is the closest of the four small satellites, about 42,000 km of the central couple. It is also smaller, with a diameter between 10 and 25 km. In Greek mythology, the River Styx was that separated the underworld the world of the living.
Nix is 49,000 kilometers of torque and has a very elongated shape, with 56 kilometers long and 25 kilometers wide, which gives the satellite an irregular and chaotic rotation. Nix was the goddess of the night and the mother of Charon.
Kerberos has a diameter of between 13 and 34 kilometers, and is located in an orbit at 59,000 kilometers of the couple Pluto and Charon. The Kerberos name comes from the famous Cerberus, the three-headed dog who stood guard at the entrance to the underworld.
Finally, Hydra was detected at a distance of 65,000 kilometers. As Nix, Hydra has formed an elongated, 58 by 34 km, and a chaotic rotation. Its name comes from the Hydra killed by Hercules.
In order of increasing distance to the couple Pluto-Charon orbital periods of these satellites is 20 Earth days to Styx, Nix 25, 32 and 38 for Kerberos Hydra.
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