Pluto is so far away that little is known of its physical nature. Until the late 1970s, studies of sunlight reflected from its surface suggested a rotation period of nearly a week, but measurements of its mass and radius were very uncertain. All this changed in July 1978, when astronomers at the U.S. Naval Observatory discovered that Pluto has a satellite. It is now named Charon, after the mythical boatman who ferried the dead across the river Styx into Hades, Pluto's domain. The discovery photograph of Charon is shown in Figure 13.22(a). Charon is the small bump near the top of the image. Knowing the moon's orbital period of 6.4 days, astronomers could determine the mass of Pluto to much greater accuracy than had previously been possible. It is 0.0021 Earth masses (1.3 10²² kg), far smaller than any earlier estimate—more like the mass of a moon than of a planet. In 1990 the Hubble Space Telescope imaged the Pluto—Charon system (Figure 13.22b). The improved resolution of that instrument clearly resolved the two bodies and allowed even more accurate measurements of their properties.

Figure 13.22 (a) The discovery photograph of Pluto's moon, Charon. The moon is the small bump on the top right portion of the image. (b) The Pluto—Charon system, to the same scale, as seen by the Hubble Space Telescope. The angular separation of the planet and its moon is about 0.9".

The discovery of Charon also allowed astronomers to measure Pluto's radius very precisely. Pluto's angular size is much less than 1", so its true diameter is blurred by the effects of Earth's turbulent atmosphere. (Sec. 5.3) However, Charon's orbit has given astronomers new insight into the system. By pure chance, Charon's orbit over the 6-year period from 1985 to 1991 (less than 10 years after the moon was discovered) happened to be oriented in such a way that viewers on Earth saw a series of eclipses. Pluto and Charon repeatedly passed in front of each other, as seen from our vantage point. Figure 13.23 sketches this orbital configuration. With more good fortune, these eclipses took place while Pluto was closest to the Sun, making for the best possible Earth-based observations.

Figure 13.23 The orbital orientation of Charon produced a series of eclipses between 1985 and 1991. Observations of eclipses of Charon by Pluto and of Pluto by Charon have provided detailed information about the sizes and orbits of both bodies.

Basing their calculations on the variations in reflected light as Pluto and Charon periodically hid each other, astronomers computed their masses and radii and determined their orbit plane. Additional studies of Pluto's surface brightness indicate that the two are tidally locked as they orbit each other. Pluto's diameter is 2270 km, about one-fifth the size of Earth. Charon is about 1300 km across and orbits at a distance of 19,700 km from Pluto. If planet and moon have the same composition (probably a reasonable assumption), Charon's mass must be about one-sixth that of Pluto, giving the Pluto—Charon system by far the largest satellite-to-planet mass ratio in the solar system.

As shown in Figure 13.24, Charon's orbit is inclined at an angle of 118° to the plane of Pluto's orbit around the Sun. Since the spins of both planet and moon are perpendicular to the plane of Charon's orbit around Pluto, the geographic "north" poles of both bodies lie below the plane of Pluto's orbit. Thus, Pluto is the third planet in the solar system (along with Venus and Uranus) found to have retrograde rotation.

Figure 13.24 Charon's path around Pluto is circular, synchronous, and inclined at 118° to the orbit plane of the Pluto—Charon system about the Sun, which is itself inclined at 17° to the plane of the ecliptic.

The known mass and radius of Pluto allow us to determine its average density, which is 2100 kg/m³—too low for a terrestrial planet, but far too high for a mixture of hydrogen and helium of that mass. Instead, the mass, radius, and density of Pluto are just what we would expect for one of the icy moons of a jovian planet. In fact, Pluto is quite similar in both mass and radius to Neptune's large moon, Triton. The planet is almost certainly made up mostly of water ice.

Spectroscopy reveals the presence of frozen methane as a major surface constituent. Pluto is the only planet in the solar system on which methane exists in the solid state, implying that the surface temperature on Pluto is no more than 50 K. Pluto may also have a thin methane atmosphere, associated with the methane ice on its surface. Recent computer-generated maps have begun to hint at surface features on Pluto (see Interlude 13-1). Similar studies indicate that Charon may have bright polar caps, but their composition and nature are as yet unknown.