On June 22, 1978, Pluto’s moon Charon was discovered by United States Naval Observatory astronomer James Christy (born September 15, 1938).

Today’s (June 22) story of what happened this day in Science, Technology, Astronomy, and Space Exploration history.

Discovery of Charon

On June 22, 1978, the American astronomer James Walter “Jim” Christy had been examining highly magnified images of Pluto on photographic plates taken with the 1.55-meter (61 in) telescope at the United States Naval Observatory Flagstaff Station (NOFS) two months prior.

Then, he noticed a periodic elongation, a small “bulge” on the images of Pluto. The most possible explanation was that Pluto had a moon. The bulge was confirmed on plates dating back to April 29, 1965.

Subsequent observations determined that the bulge was indeed due to a smaller accompanying body (a moon). The periodicity of the bulge corresponded to Pluto’s rotation period, which was previously known from Pluto’s light curve. This indicated a synchronous orbit, which strongly suggested that the bulge effect was real and not spurious.

Discovery of Charon (Pluto bulge)
Charon’s discovery at the U.S. Naval Observatory Flagstaff Station as a time-varying bulge on the image of Pluto (seen near the top on the left, but absent on the right). Negative image.

The International Astronomical Union formally announced Christy’s discovery to the world on July 7, 1978.

Now we know that Pluto has a total of 5 moons, and with a mean radius of 606.0±0.5 km (about 376 miles), Charon is the largest of them. Pluto’s other moons are Hydra (discovered in 2005), Kerberos (discovered in 2011), Nix (discovered in 2005), and Styx (discovered in 2012).

James “Jim” Christy proposed the name Charon after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto’s underworld. Apart from the mythological connection for this name, Christy chose it because the first four letters also matched the name of his wife, Charlene.

Discovery of Charon: Jim Christy shows Pluto bulge photo
40 years after his important discovery, James “Jim” Christy holds two of the telescope images he used to spot Pluto’s large moon, Charon, on June 22, 1978. A close-up photo of Charon, taken by the New Horizons spacecraft during its July 2015 flyby, is displayed on his computer screen. Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Art Howard/GHSPi


At half the size of Pluto (mean radius: 606.0±0.5 km or about 376 miles, Pluto’s radius is 1,188.3 km or 738 miles), Charon is the largest of Pluto’s moons and the largest known satellite relative to its parent body.

Pluto-Charon is our solar system’s only known double planetary system. The same surfaces of Charon and Pluto always face each other, a phenomenon called mutual tidal locking.

We see only one side of our moon due to tidal locking, but the Earth is not tidally locked to the moon (it is not a mutual tidal locking), so an observer on the Moon can see all of the Earth’s surface as the planet rotates. But, on Pluto, an observer will see always the same face of Charon and vice-versa.

In other words, Charon neither rises nor sets, but hovers over the same spot on Pluto’s surface.

Charon orbits Pluto every 6.4 Earth days. The distance between Pluto and Charon is 19,640 km (12,200 miles).

Charon has no significant atmosphere, while the dwarf planet it orbits, Pluto, does. Pluto has a thin, tenuous atmosphere that expands when it comes closer to the Sun and collapses as it moves farther away – similar to a comet.

Charon, moon of Pluto
Three years after NASA’s New Horizons spacecraft gave humankind our first close-up views of Pluto and its largest moon, Charon, scientists are still revealing the wonders of these incredible worlds in the outer solar system. Marking the anniversary of New Horizons’ historic flight through the Pluto system on July 14, 2015, mission scientists released the highest-resolution color images of Pluto and Charon. These natural-color images result from refined calibration of data gathered by New Horizons’ color Multispectral Visible Imaging Camera (MVIC). The processing creates images that would approximate the colors that the human eye would perceive, bringing them closer to “true color” than the images released near the encounter. This image was taken on July 14, 2015, from a range of 46,091 miles (74,176 kilometers). This single-color MVIC scan includes no data from other New Horizons imagers or instruments added. The striking features on Charon are clearly visible, including the reddish north-polar region known as Mordor Macula.

Based on observations made by NASA’s New Horizons spacecraft, the mass ratio of Charon to Pluto is 0.1218:1. This is much larger than the Moon to the Earth: 0.0123:1. Because of the high mass ratio, the barycenter (center of mass) is outside of the radius of Pluto. That’s why the Pluto-Charon system has been referred to as a “double planetary system”.

Size comparison of Earth, Moon, and Charon.
Size comparison of Earth, Moon, and Charon.

Characteristics of Charon

  • Mean radius: 06.0±0.5 km (about 376 miles) (0.095 Earths, 0.51 Plutos). The radius of Pluto: 1,188.3 km (738 miles)
  • Mass: 1.586±0.015)×1021 kg (2.66×10−4 Earths) (12.2% of Pluto)
  • Density: 1.702±0.017 g/cm3
  • Composition: 55% rock to 45% ice (± 5%), whereas Pluto is about 70% rock
  • Surface gravity: 0.288 m/s2 (Earth: 9.807 m/s2, Pluto: 0.62 m/s2)
  • Escape velocity: 0.59 km per second (0.37 mi/s), Earth: 11.2 km/s (6.96 mi/s), Pluto: 1.21 km/s (0.75 mi/s)
  • Temperature: -220 °C (53 K)
New Horizons poster
Artist’s conception depicting New Horizons spacecraft near Pluto. Pluto’s moon Charon, also known as (134340) Pluto I is in the background. New Horizons is an interplanetary space probe that was launched as a part of NASA’s New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by S. Alan Stern, the spacecraft was launched on January 19, 2006, from Cape Canaveral Air Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a speed of about 16.26 kilometers per second (58,536 km/h; 36,373 mph). Its primary mission was to perform a flyby study of the Pluto system in 2015. It has a secondary mission to fly by and study one or more other Kuiper belt objects (KBOs) in the decade to follow. It is the fifth of five artificial objects to achieve the escape velocity that will allow them to leave the Solar System (others being Pioneer 10 – launched in 1972, Pioneer 11 – launched in 1973, Voyager 2 – launched in August 1977, and Voyager 1 – Launched in September 1977). Image: NASA Goddard Media Studios


M. Özgür Nevres

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