National Aeronautics and Space Administration Logo
Follow this link to skip to the main content NASA Banner
Solar System Exploration
Planets
Facebook Twitter YouTube Facebook Twitter YouTube Flickr iTunes
Follow Us
Charon: Overview
   Back to Pluto   Overview   Gallery   Facts & Figures   Education   News 
Fuzzy image showing Pluto and Charon
Charon was discovered when an observant astronomer noted changes in Pluto's shape in images.

Charon is almost half the size of Pluto. The little moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 19,640 km (12,200 miles).

The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is more neutral grey than Pluto (which has a red tinge), indicating that they have different surface compositions and structure.

Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto -- this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Pluto's rotation is retrograde: it rotates backwards, from east to west (Uranus and Venus also have retrograde rotations).

Discovery
Charon was discovered in 1978 when sharp-eyed astronomer James Christy noticed images of Pluto were strangely elongated. The blob seemed to move around Pluto. The direction of elongation cycled back and forth over 6.39 days - Pluto's rotation period. Searching through their archives of Pluto images taken years before, Christy found more cases where Pluto appeared elongated. Additional images confirmed he had discovered the first known moon of Pluto.

How Charon got its Name
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.

Just the Facts
Orbit Size (semi-major axis):  17,536 km
Mean Radius:  603.6 km
Volume:  921,162,612 km3
Mass:  1,546,625,822,956,300,000,000 kg
Resources
People Spotlight
Mihaly Horanyi Mihaly Horanyi
"I study cosmic dust, and dusty plasmas in space and in the laboratory." Read More...
Headlines
Science & Technology Features
14 Jan 2014: 
Awards and Recognition   Solar System Exploration Roadmap   Contact Us   Site Map   Print This Page
NASA Official: Kristen Erickson
Advisory: Dr. James Green, Director of Planetary Science
Outreach Manager: Alice Wessen
Curator/Editor: Phil Davis
Science Writer: Autumn Burdick
Producer: Greg Baerg
Webmaster: David Martin
> NASA Science Mission Directorate
> Budgets, Strategic Plans and Accountability Reports
> Equal Employment Opportunity Data
   Posted Pursuant to the No Fear Act
> Information-Dissemination Policies and Inventories
> Freedom of Information Act
> Privacy Policy & Important Notices
> Inspector General Hotline
> Office of the Inspector General
> NASA Communications Policy
> USA.gov
> ExpectMore.gov
> NASA Advisory Council
> Open Government at NASA
Last Updated: 12 Aug 2013