orbit:    5,913,520,000 km (39.5 AU) from the Sun (average)
        diameter: 2274 km
        mass:     1.27e22 kg

    In Greek mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name
    (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and
    perhaps because "PL" are the initials of Percival Lowell.

    Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had
    predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error,
    Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto
    anyway.

    After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the
    discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor
    is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2
    encounter with Neptune is used. There is no tenth planet.

    Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can
    resolve only the largest features on its surface (left and above).

    Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before
    its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many
    transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be
    covered at what times, and watching brightness curves, astronomers were able to construct a rough map of
    light and dark areas on both bodies.

    Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.
    Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful
    measurements of the period and radius of Charon's orbit and Kepler's Third Law), the individual masses of
    Pluto and Charon are difficult to determine because that requires determining their mutual motions around
    the center of mass of the system which requires much finer measurements -- they're so small and far away
    that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157;
    more observations are underway but we won't get really accurate data until the Pluto Express arrives. (It's
    planned launch date is 2001.

    Pluto is the second most contrasty body in the Solar System (after Iapetus). Exploring the origin of that
    contrast is one of the high-priority goals for the proposed Pluto Express mission.

    There are some who think Pluto would be better classified as a large asteroid or comet rather than as a
    planet. Some consider it to be the largest of the Kuiper Belt objects. There is considerable merit to the later
    position, but historically Pluto has been classified as a planet and it is likely to remain so.

    Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (it has been so since 1979 and
    will continue until 1999). Pluto rotates in the opposite direction from most of the other planets.

    Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than
    Neptune's. Its orbital inclination is also much higher than the other planets. Thus though it appears that Pluto's
    orbit crosses Neptune's, they will never collide.

    Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.

    The surface temperature on Pluto is not well known but is probably between 35 and 45 Kelvins
    (-228 to -238 C).

    Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of
    70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices
    of nitrogen with smaller amounts of (solid) methane and carbon monoxide. The composition of the darker
    areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical
    reactions driven by cosmic rays.

    Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon
    monoxide and methane. It is extremely tenuous the surface pressure being only a few microbars. Pluto's
    atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the
    atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to
    space perhaps even interacting with Charon. The Pluto Express mission planners want the probe to arrive at
    Pluto while the atmosphere is unfrozen.

    The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and
    Triton suggest some historical connection between them. It was once thought that Pluto may have once been
    a satellite of Neptune's, but this now seems unlikely. A more popular theory is that Triton, like Pluto, once
    moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and
    Charon are the only remaining members of a large class of similar objects the rest of which were ejected into
    the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another
    body.

    Pluto can be seen with an amateur telescope but it is not easy. Many planet finder charts can show the
    current position of Pluto (and the other planets) in the sky, careful observations over several months will be
    required to actually find it.

    Mass (kg)                                                                  1.29 x 10^22
    Equatorial radius (km)                                             1,160
    Diameter (km)                                                           2300
    Mean density (kg/m^3)                                             2030
    Escape velocity (m/sec)                                           1100
    Equatorial surface gravity (m/sec^2)                         0.4
    Equatorial escape velocity (m/sec)                           1,220

    Mean distance from the Sun (km)                            5,913,520,000
    Average distance from Sun (AU)                             39.53
    Rotation period (length of day in Earth days)          6.3872
    Revolution period (length of year in Earth years)    248.54

    Obliquity (tilt of axis in degrees)                                122.5
    Mean orbital velocity (km/sec)                                   4.74
    Orbit inclination (degrees)                                         17.148
    Orbit eccentricity (deviation from circular)                0.2482
    Tilt of axis (degrees)                                                   122.52

    Mean temperature (K)                                                37

    Visual geometric albedo (reflectivity)                        0.3

    Atmospheric components                                          perhaps nitrogen and methane
    Surface materials                                                        perhaps methane ice

    Discovered by                                                             Clyde W. Tombaugh
    Date of discovery                                                        February 18, 1930

    Magnitude (Vo)                                                           15.12
 

Pluto's Satellite 

    Charon ( "KAIR en"  ) is Pluto's only known satellite:

        orbit    Pluto: 19,640 km distance from
        diameter: 1172 km
        mass:     1.90e21 kg

    Charon is named for the mythological figure who ferried the dead across the River Styx into Hades (the
    underworld).

    (Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his
    wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHRen").

    Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since
    the images of Charon and Pluto were blurred together.

    Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a
    distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a
    planet and a moon.

    Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its
    mass and density are also poorly known.

    Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they
    both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very
    interesting.)

    Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to
    Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice.

    Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not
    been resolved.
 
    It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.

    It is doubtful that Charon has a significant atmosphere.
 

Charon Statistics

            Discovered by                                                     J. Christy
            Date of discovery                                                1978

            Mass (kg)                                                             1.77e+21
            Equatorial radius (km)                                        635
            Mean density (gm/cm^3)                                    1.83
            Mean distance from Pluto (km)                          19,640
            Escape velocity (m/sec)                                     610

            Rotational period (days)                                      6.38725
            Orbital period (days)                                            6.38725
            Mean orbital velocity (km/sec)                            0.23
            Orbital eccentricity                                               0.00
            Orbital inclination (degrees)                               98.80

            Visual geometric albedo                                     0.5
            Magnitude (Vo)                                                    16.8
 
 

Clearest View 

This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope. The image was taken by the European Space Agency's Faint Object Camera on February 21, 1994 when the planet was 4.4 billion km (2.6 billion mi) from Earth. Hubble's corrected optics show the two objects as clearly separate and sharp disks. This has allowed astronomers to measure (to within about 1 percent) Pluto's diameter of 2320 km (1440 mi) and Charon's diameter of 1270 km (790 mi). The Hubble observations show that Charon is bluer than Pluto. This means that both worlds have different surface composition and structure. A bright highlight on Pluto suggests it has a smoothly reflecting surface layer.

    Planned Launch Date: 2001
    Launch Vehicle: Delta or Russian Proton
    Planned on-orbit mass: <100 Kg
    Power System: Radioisotope Thermal Generators (RTGs) of 65 W

    Originally designated the Pluto Fast Flyby (PFF), the Pluto Express mission is planned to be a two
    spacecraft mission designed to make studies of the planet Pluto and its satellite Charon. Its major science
    objectives are to: (1) characterize the global geology and geomorphology of Pluto and Charon; (2) map the
    composition of Pluto's surface; and, (3) determine the composition and structure of Pluto's atmosphere.
    Intended to reach Pluto as quickly as possible (before the tenuous Plutonian atmosphere can refreeze onto
    the surface as the planet recedes from the Sun), the two Pluto Express spacecraft will arrive one year apart
    after 6-9 years of travel, depending on the ultimate mass of the spacecraft. Studies of the double-planet
    system will begin 12-18 months prior to closest approach. The overall structure of the spacecraft is an
    aluminum hexagonal bus with no deployable structures. Power will be provided by radioisotope thermal
    generators (RTGs) similar in design to those used on earlier missions (e.g., Galileo). Communications will
    be via a fixed, 1.47 m high-gain antenna employing an X-band uplink receiver and downlink transponder.
    Pointing control will be maintained by a wide-field star tracker and a set of three solid-state rate sensors.
    The on-board computer is a 1.5 MIPS RISC-based system capable of processing a science data stream
    of 5 Mbps. The solid-state data storage system is capable of storing 400 Mb in both compressed and
    uncompressed formats. Strawman experiments for the spacecraft include a visible-light imaging system, an
    infrared mapping spectrometer, an ultraviolet spectrometer, and a radio occultation experiment utilizing an
    ultrastable oscillator (USO) and the on-board telecommunications system.

    Prototype instruments were delivered from JPL and other investigators in 1994. They were able to
    demonstrate that science goals can be met with an ultraviolet/visible/infrared radio science flight instrument
    total weighing less than 7 Kg, and using less than 6 W. A potential cooperative effort with Russia may lead to
    the inclusion of Zond probes, to study the Plutonian    atmosphere.

    The Pluto Express spacecraft are intended to be considerably less massive and smaller in size than
    previous missions. A comparison diagram of the original Pluto Fast Flyby design with Voyager, Galileo,
    Magellan, and Cassini gives an idea of the difference.