How big is Charon?

Charon is the largest of Pluto's five moons, and it is about 1,212 kilometers (753 miles) across. That is a little more than half the width of Pluto itself. No other known moon is so large compared with the world it orbits.

Size

The best measurement of Charon's size comes from NASA's New Horizons spacecraft, which flew past Pluto and Charon on July 14, 2015. Scientists traced Charon's edge in the spacecraft's images and found a mean radius of 606 kilometers, give or take 1 kilometer. That makes Charon 1,212 kilometers across.

Charon is round. The same study found no measurable flattening at its poles, less than half a percent. Earlier measurements from Earth, made by timing how long Charon blocked the light of a distant star, had given a similar radius.

Size comparisons

If Pluto were shrunk to the size of a basketball, Charon would be a ball about 12 centimeters (4.8 inches) across, circling only about 2 meters (6.5 feet) away. If Earth were shrunk to a basketball instead, the Moon would be a tennis ball about 7 meters (24 feet) away.
The Moon is about 2.9 times as wide as Charon, and Earth is about 10 times as wide.
Quaoar, the next smaller round world in the explorer, is about 1,090 kilometers across, so Charon is about 11% wider.
A belt of canyons on Charon is more than 1,600 kilometers (1,000 miles) long, about four times the length of the Grand Canyon.

Locked face to face

Charon orbits Pluto once every 6.4 days, and Pluto spins once in the same time. As a result, the same side of Charon always faces Pluto, and the same side of Pluto always faces Charon. This is called mutual tidal locking. Seen from Pluto, Charon never rises or sets but hovers over the same spot on Pluto's surface.

The two are only about 19,600 kilometers (12,200 miles) apart, about eight times Pluto's width. The Moon, by comparison, is about 30 Earth widths from Earth. Because Charon is so big and so close, Pluto and Charon are sometimes called a double dwarf planet.

Discovery and name

James Christy and Robert Harrington discovered Charon in June 1978 at the U.S. Naval Observatory in Flagstaff, Arizona. They were not looking for moons but trying to pin down Pluto's orbit. Christy noticed that Pluto looked stretched in some images, and that the bulge moved around Pluto every 6.39 days. He found the same stretching in older images, and further images confirmed that it was a moon.

Christy named it after Charon, the mythological ferryman who carried souls across the river Acheron, one of the rivers around the underworld of the god Pluto. He also chose it because its first four letters matched the first four letters of his wife's name, Charlene.

A cracked, reddish surface

New Horizons found a far more varied world than many scientists expected, with mountains, canyons and landslides. In color, Charon's most striking feature is a reddish region around its north pole, informally named Mordor Macula.

A belt of fractures and canyons runs across the whole face of Charon, twice as deep as the Grand Canyon in places. The plains south of it have fewer craters, so they are younger. One idea being discussed is that an ocean inside Charon froze long ago, and the change in volume cracked the crust open.