Charon, a satellite of Pluto

Charon, also known as Charon, was discovered in 1978. Charon was a servant of Pluto in Roman mythology who asked the souls of the dead for money to row them across the River Styx. In 2005, two more satellites of Pluto were discovered: Charon (Nix) and Charon (Hydra). According to the current definition, Charon may be the largest satellite of Pluto, or it may form a double dwarf planet with Pluto.

The New Horizons mission visited Pluto and Charon in 2015.

Charon's name is easily confused with another small body in the solar system (2060) Chiron.

The discovery of Charon

Charon was discovered by astronomer James Christy at high magnification on June 22, 1978. Discovered from photographic plates of Pluto. Christie noticed a tiny bump that appeared periodically. Later, film back to April 29, 1965 also confirmed the bump.

The naming of Charon: Charon's original temporary name was S/1978 P 1, which was given according to the new naming rules established by the conference at that time. Christie later called it Charon, but it was not until 1985 that the International Astronomical Union recognized the name as official. In Greek mythology, Charon is the ferryman of the dead, and is a god closely linked to Hades (Pluto in Roman mythology) in mythology.

Although the traditional English pronunciation of the mythical picture is a hard K sound, Christie changed the pronunciation of the ch to "Char" in the pronunciation of this satellite. Later, his wife Charlene's nickname was "Char "(charcoal gray), and the pronunciation of sh has also become popular in the astronomical community, and everyone uses this pronunciation as usual. In European languages ??other than English, the pronunciation from mythology is still used, and the sound of ch is pronounced.

Charon’s physical properties Charon has a diameter of approximately 1,212 kilometers (750 miles), exactly half the size of Pluto, and a surface area of ??approximately |4,580,000 square kilometers, which is covered with frozen nitrogen and methane. Unlike Pluto, Charon's surface may appear to be frozen, non-volatile water. Charon's surface temperature is about -230°C and its density is 1.63 g/cubic centimeter, indicating that rock accounts for more than half of its composition and ice is slightly less than half. Its surface atmosphere is only about 0.1 millibars, which is one ten thousandth of the concentration of the atmosphere on the Earth's surface. It is so thin that it is almost nonexistent. Scientists are currently working hard to study the surface of Charon to determine whether the satellite has an atmosphere.

Charon’s orbital characteristics: Changes in luminosity on the surfaces of Pluto and Charon confirmed that they were in periods of mutual occultation. Pluto and Charon are tidally locked and orbit each other every 6.387 days, so they always face each other in the same position. The average distance between the two is 19,570 kilometers (12,163 miles). The discovery of Charon allowed astronomers to accurately measure the total mass of the Pluto system, and mutual occlusion could calculate their respective sizes (diameters). However, individual masses could still only be estimated. It was not until the discovery of Pluto's outer satellites in 2005 that According to the orbital details of the outer satellite, Charon's mass is approximately 11.65% of Pluto, and its density is also shown to be 1.65±0.06 grams/cubic centimeter. It is suggested that 55±5% is rock and 45% is ice. Pluto is relatively dense, about 70% rocky. The formation of Charon The results of a simulation conducted by Robin Kaner and published in 2005 suggested that Charon was formed in a large collision about 4.5 billion years ago, much like the conditions of the Earth and Moon. In this model, Charon coalesced from the fragments of a giant Kuiper Belt object that struck Pluto at high speed, destroying itself and blowing away much of Pluto's mantle. But such an impact should make Pluto more rocky than we found, and Charon more icy. Therefore, it is now believed that Pluto and Charon were two different celestial bodies before they collided in orbit. Although this collision was violent, it could only evaporate the ice like methane, but it was not enough to cause disintegration.

The center of mass of the satellite or the dwarf planet Pluto-Charon falls outside these two celestial bodies, so one of them does not really revolve around the other, and the masses of the two can be compared with each other. Therefore, Charon is not suitable as a satellite of Pluto. Instead, they should be double dwarf planets and should be reclassified along with Pluto.

There is a draft at the annual meeting of the International Astronomical Union, the 2006 Planet Redefinition, which proposes that a planet must orbit the sun and have enough gravity to become a sphere. Under such a draft, Charon qualifies as a planet because the draft clearly defines the condition for a satellite as that the center of gravity of the system must be within the main celestial body. However, the final reclassification classified Pluto as a dwarf planet, but there was no formal definition of the planet's satellite, leaving Charon's identity unclear.

(So ??far, Charon has not been included in the recognized list of dwarf planets by the International Astronomical Union)

The planetary standards of the International Astronomical Union stipulate that in a multi-celestial system orbiting a star, if the main body A celestial body is a planet, and a secondary celestial body is also spherical. It can also be called a planet if it meets certain conditions, that is, the center of gravity between the primary celestial body and the secondary celestial body is not inside the primary celestial body. In this way Charon meets this condition.

Charon (Nix) and Charon also orbit the same center of gravity, but they are not large enough to be spheres, so they can easily be identified as moons of Pluto.

(Or, to take an alternative view, a moon of the Pluto-Charon system)