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Pluto's Moons

Charon's DiscoveryCharon

Charon's discovery was announced by Captain J.C. Smith of the U.S. Naval Observatory on July 7, 1978. It was discovered by James W. Christy of the USNO by looking at photographic plates from the Observatory's 60-inch reflecting telescope at Flagstaff, AZ. It looked like a small pimple growing out of Pluto's side.

It was quickly confirmed to be real by looking at archived photographic plates of Pluto. The images that showed Charon made Pluto look simply slightly elongated, and they had been discarded by other workers as anomalous.

What Charon Can Be Used For

Two pieces of information soon came from Charon. First, once its distance from Pluto and its orbital period were determined, Kepler's Third Law could be used to - for the first time - accurately calculate Pluto's mass. It was originally calculated that the combined mass of Pluto and Charon is 1/430 of Earth's; the current value is about 1/423 of Earth's mass.

The second deals with the dynamics of Pluto and Charon's orbit. Twice throughout Pluto's 248-year period, it and Charon go through a series of transits when one body passes directly in front of the other as seen from Earth. One set of transits happened starting in February 1985, and they continued every 3.2 days until 1990. By very carefully studying the amount of light that was blocked, a map of the dark and light areas of the two bodies could be created. These are the only maps that exist of the two objects.

What Is Known of Charon Today

Originally, Charon was determined by its assumed brightness to be about 10% the mass of Pluto; it is currently believed to have 12% the mass of Pluto, weighing about weighs 1.62x1021 kg. It was also originally believed that it was about 800 km in diameter, and in 1980 this was revised to about 1200 km in diameter when it passed in front of a star on April 7th of that year; it is now believed to be 1186 km (736 miles) in diameter. This gives it a mean density of 1.85 gm/cm3, which is larger than the 1.55 it was originally believed to have.

Pluto and Charon are tidally locked, which means that they always present the same face to each other. In other words, Charon's day is exactly as long as its year, so that as it revolves around Pluto, it rotates at exactly the same rate. The system probably evolved into this situation through a process of tidal evolution. The same thing is very slowly happening to Earth and our moon, where the moon is very slowly retreating from Earth, and Earth's rotation rate is very slowly growing longer; eventually, Earth and the moon will be tidally locked, as well.

Also, like our moon, Charon probably formed in a giant impact event with Pluto in the early days of the solar system. When Charon was first discovered, it was believed that it formed by fission - Pluto spun so rapidly that a piece spun off from it. This theory fell out of favor rather quickly, though.

More Moons

In October of 2005, two candidate moons were discovered in orbit around Pluto. They were provisionally called S/2005 P1 and S/2005 P2, and once they were verified by the Hubble Space Telescope on February 15, 2006, they were re-named Hydra and Nix. If they are confirmed to exist with future observations, then the International Astronomical Union will give them official names, probably based upon minor characters associated with the ancient Roman god of the underworld.

Assuming that the moons are real, they have been observed to have orbital periods much longer than Charon's. They are also in an approximate 6:4:1 resonance (Hydra:Nix:Charon), though the resonance is not exact.

They are rather small when compared with Pluto and Charon, and they orbit about the center of mass of the Pluto-Charon system rather than about Pluto itself. Because of this, the total mass of the Pluto-Charon system, plus the two moons, has been calculated to be 1.4570x1022, which is 3% larger than had been previously determined.

This mass implies that the density of Charon is 1.66±0.06 gm/cm3 and that of Pluto is 2.03±0.06 gm/cm3. The density of Charon was also measured to be 1.71 gm/cm3 based upon a stellar occulation on July 11, 2005, when its radius was also revised upwards by 10 kmThis is less than the previously derived density of Charon, and it is larger than the previously derived density of Pluto. "Official" numbers are still the ones quoted in the tables on this website, which are based upon what NASA provides.

The newly discovered satellites are approximately 100 million times fainter than Pluto. All data on the moons in 2005 were taken with the Hubble Space Telescope.

Based upon further images, it is highly unlikely that Pluto has any more moons that are larger than 10 km.

Data for Pluto's Moon(s)

Discovery Date
Distance from Pluto (103 km)
Orbital Period (days)
Mass (1020 kg)
Radius (km)
Charon 1977; reported July 7, 1978 James W. Christy 19.5714 6.3872304 16.2 603.6
Hydra reported October 2005 Weaver et al. (2006) 64.78 38.2065 0.001-0.09 22-65
Nix reported October 2005 Weaver et al. (2006) 48.675 24.8562 0.001-0.09 22-65

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