Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Pluto Link for sharing this page on Facebook
(also see: Pictures of Pluto / The Rotation of Pluto / The Discovery of Pluto)
See The Rotation of Pluto for additional images posted by NASA on Nov 20, 2015
(Kerberus and Styx had not been discovered when this page was originally posted;
images and discussion of them will be added in the "near" future)

     With a radius of 375 miles (see occultation discussion below), Charon is half the size of Pluto, making it the largest moon in comparison to the size of its planet in the Solar System. As a result it might be more proper, presuming Pluto is considered a planet, to call Pluto and Charon a double planet. The image immediately below was taken by the New Horizons spacecraft on July 13, 2015, the day before its flyby of Pluto. Prior to that mission, Charon had never been imaged as anything other than a bright dot. (The original image is actually far more detailed, but is still on the spacecraft's hard drive as of this post, as there wasn't time to transmit the full-quality image prior to the flyby; instead, a "compressed" version was sent to Earth, in case anything went wrong during the flyby.) The image below that shows a closeup taken at a distance of 49000 miles only 90 minutes before closest approach, and its location on the full-Charon image.
Charon as imaged by the New Horizons spacecraft on July 13, 2015
Below, the image above adding a closeup taken 90 minutes before nearest approach
Closeup of a portion of Charon taken 90 minutes before closest approach on July 14, 2015
Below, false-color images of Pluto and Charon taken July 13, 2015, a day before the New Horizons spacecraft passed Pluto. Other than differences in brightness, the colors have nothing to do with the actual appearance of the objects (and even brightness differences are exaggerated). Instead, these are like geological maps, which are intended to show differences in composition and/or geology, and can be used as clues in interpreting more normal-color and black and white images.
False-color images of Pluto and Charon taken by the New Horizons spacecraft on July 13, 2015, a day before it passed Pluto. The colors are like those in geologic maps, and have little or nothing to do with the actual appearance of the planet, save for differences in brightness, which are also greatly exaggerated.

The occultation of July 11, 2005
(diagrams from and summary based on a report by the Paris Observatory)
     It is very rare for an object as small and as far away as Charon to occult (pass in front of and block the light of) a star; but on July 11, 2005 Charon occulted the fifteenth magnitude star UCAC2 2625713. This was the first such occultation observed since April of 1980, and provided an opportunity to more accurately determine the size of Charon, its density (determined from its previously known mass and the newly determined radius), and what (if any) atmosphere it had at the time of the observation.
      In an occultation the occulting body (in this case, Charon) casts a shadow with the light of the star (a very faint shadow, since the star is so faint) along a path which is exactly the same size and shape as the body, as seen from the Earth. If the body is spherical the shape is the same, no matter how it is turned; but if it is irregular the apparent shape may vary according to the way it is turned. Charon was already known to be about half the size of Pluto, and with that large size, should be very nearly spherical as a result of its (admittedly small) gravitational compression.

      The diagram above shows the predicted path of Charon's shadow during the 2005 occultation, and various observatories located along that path. At each location observations were made of the apparent brightness (magnitude) of Charon and the star it was occulting as it swept past the star. The diagram below shows how the results of the three stations which obtained reasonably accurate results were combined to yield the size of Charon.
     The red lines show the length of time that Charon passed in front of the star, converted to a distance measurement by multiplying the time units by the tangential velocity of Charon relative to the Earth (that is, the relative velocity perpendicular to our line of sight). Stations which were further north observed the northern portion of the shadow, while stations which were further south observed the southern portion. The globe fitted over the tracks shows the estimated size of Charon as determined from those tracks. If more stations had observed the occultation the uncertainty of the result could be reduced; but just these three tracks are in sufficient agreement to give a very precise result.

     The diagram above shows the light curve (brightness measurements) of Charon and the occulted star as determined by combining the results from two stations, with "1" corresponding to all of the star's light being visible, and "0" corresponding to none of the star's light being visible. As in the previous diagram time units have been converted to distances. The sudden change in brightness caused by Charon passing in front of the star occurs at a radius of around 600 to 610 kilometers. Previous estimates of Charon's radius ranged from 600 to 650 kilometers, so this is a fivefold improvement in accuracy.
     Using the more accurate radius also improves density estimates, which previously ranged from 1.4 to 1.8 times the density of water, allowing the structure of Charon to be either mostly ice (if the density were lower) or mostly rock (if the density were higher). The new density estimate is 1.65 to 1.75 times the density of water, suggesting a greater proportion of rocky materials than most prior estimates.
     Finally, as already noted it is possible to estimate the atmospheric density of gases surrounding Charon by very carefully examining the light curve just before the star is occulted, and just after the occultation ends. As shown by the shaded areas in the diagram above, gases in the atmosphere of Charon, if present in sufficient amounts, would block part of the light of the star. The light gray area shows the expected extinction (lessening of the star's light) by 110 nanobars of nitrogen, while dark gray area shows the expected extinction by 15 nanobars of methane (a nanobar is a thousand million times less than the atmospheric pressure of the Earth). Essentially no extinction is observed, suggesting that Charon has no atmosphere, or any atmosphere it has is billions of times more rarefied than our own. This is in sharp contrast to Pluto, which has been observed (through similar occultation observations) to have a few ten-thousandths of an Earth atmosphere of nitrogen and methane when it is closer to the Sun (and warmer) than usual. It is also just what we would expect, as Charon's lower mass and gravity allow any vaporized ices to far more easily escape into space.

Discovery photos of Charon.
Distorted image (left) shows Charon above Pluto, undistorted image (right) shows it in line with planet.

Comparison of ground and early Hubble Space Telescope photos of Pluto and Charon

Pluto and Charon from the repaired Hubble Space Telescope at maximum elongation (0.9 arc seconds apart). Pluto is on left, 2390 km diameter, Charon on right, 1190 km diameter. The bodies orbit each other at a distance of 19,600 km

Motion of Pluto around the Sun and Charon around Pluto

Gradually changing orientation of orbit of Charon as seen from the Earth

Bottom: Geometry of eclipses and occultations of Pluto and Charon in late 1980s
Top: Change in brightness of Pluto and Charon as a result of eclipses and occultations

One model of internal structures of Pluto and Charon

Relative sizes of largest Kuiper Belt Objects known in 2001, compared to Pluto and Charon
(Calvin Hamilton)

Relative size of various large moons and Pluto. (NASA, Wikipedia Commons)

Pluto's Other Moons
     Hubble Space Telescope photos showing the movement of Charon and two suspected additional moons around Pluto during a three-day period (half the orbital period of Charon) in mid-May, 2005. Since then the objects' status as moons of Pluto has been verified, and their orbital elements fairly well determined. The inner moon is about 30000 miles from Pluto, or a little over twice Charon's distance of 12000 miles, and has an orbital period of about 25 days, or approximately four times Charon's period. The outer moon is about 40000 miles from Pluto, or a little over three times Charon's distance, and has an orbital period of about 38 days, or approximately six times Charon's period. The near commensurability of the orbital periods with Charon (meaning that the orbital periods are the ratios of small whole numbers -- in this case, 4/1 and 6/1) implies that they are locked into their orbits by gravitational interaction with Charon and Pluto. It is suspected that all three moons are the result of a single event, a collision that split the proto-Pluto into two large objects and a number of smaller objects. (H. Weaver (JHU / APL), A. Stern (SwRI), and the HST Pluto Companion Search Team, ESA, NASA, apod051103)

     A more recent image of Pluto and its moons, taken with the Hubble Space Telescope, showing the newly designated names of the smaller satellites, Nix and Hydra. Since the smaller moons are five thousand times fainter than Pluto, the image grossly overexposes Pluto and Charon, causing them to display very noticeable diffraction spikes (an artifact produced by the way that the secondary mirror of the HST is supported), while Nix and Hydra remain merely dots, indistinguishable from points. Their sizes, estimated from their faintness, are about 25 to 80 miles diameter (depending upon its albedo) for Nix, and 30 to 100 miles diameter for Hydra. This is much smaller than Charon's 750 mile diameter, which is more than half Pluto's 1430 mile diameter. The names of the new moons were chosen to match the first letters in New Horizons (the spacecraft now enroute to a June 2015 flyby of Pluto and its moon system), and to fit the Greek mythology associated with planetary names. Pluto was of course the Greek god of the underworld. Nix was the mother of Charon, the ferryman who carried souls across the river Styx to Hades. Hydra was chosen as the name of the third moon because that mythological beast had nine heads, and Pluto is the ninth planet from the Sun. (So even though Pluto has now been (controversially) designated as a dwarf planet, Hydra remains a reference to its one-time status as a "major" planet.) (M. Mutchler (STScI), A. Stern (SwRI), and the HST Pluto Companion Search Team, ESA, NASA, apod060624)

Data Table for Pluto and Its Moons

Diameter = 1433 miles = 0.18 Earth's diameter
Mass = 0.0021 Earth masses
Rotation period = - 6.38723 days (retrograde relative to its orbital motion)
Day length = - 6.38679 days (38 seconds less than the rotation period)

Diameter = 749 miles = 0.52 Pluto's diameter (the largest ratio of any moon)
Orbital size = 12160 miles
Orbital period = 6.389723 days = rotational period of Pluto, so Pluto always keeps one face to Charon
Rotational period = 6.389723 days = orbital period, so Charon always keeps one face to Pluto
Day length = the same as for Pluto

Diameter = 25 to 80 miles (depending upon albedo)
Orbital size = 30250 miles (approximately 2 1/2 times the size of Charon's orbit)
Orbital period = 24.856 days (approximately 4 times the period of Charon's orbit)

Diameter = 30 to 100 miles (depending upon albedo)
Orbital size = 40250 miles (approximately 3 1/3 times the size of Charon's orbit)
Orbital period = 38.206 days (approximately 6 times the period of Charon's orbit)