Pluto and Charon: Introduction

• Neptune discovered from irregularities in motion of Uranus
• Was there remaining irregularity in Uranus’ motion that would imply another planet?
  • Percival Lowell began search in ~1906
  • Pluto discovered 1930 only 6° from predicted position (just a lucky coincidence!)
  • Initial observations and recent discovery of Charon show Pluto cannot disturb Uranus.

Pluto and Charon

• Pluto was discovered in 1930 as a result of a concentrated search for a ninth planet.
• Extreme orbital characteristics:
  • largest orbit of the planets - semimajor axis a = 39.4 AU
• highest inclination (i = 17°.15)
• highest eccentricity (e = 0.249)
• • aphelion distance = 49.2 AU
  • perihelion distance = 29.6 AU
  • Neptune’s distance from Sun = 30.1 AU
  • From 1979 – 1999 Neptune is farther from the Sun than Pluto!

Pluto Does Not Cross Neptune’s Path!

Pluto and Charon

• Double nature discovered in 1978
• Orbital period P = 6.4 days
• Brightness variations imply that period of rotation is the same as period of revolution for both Pluto and Charon (i.e., tidally locked)

Eclipses are Valuable
Eclipse-Based Maps

• Pluto and Charon mutually eclipse one another
• Eclipses can be used to measure the sizes of Pluto and Charon.
• Kepler’s Third Law can be used to measure the masses of Pluto and Charon.
• Eclipses can be used to map the surfaces of Pluto and Charon.

Masses and Sizes

Property                     Pluto                Charon

Mass  (Earth=1)                0.0022                0.0003
Diameter (km)                 2300                   1300
Density (g/cm³)                 1.8                    1.2
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Composition

• Densities of Pluto and Charon give clues to composition
  • Pluto a mixture of rock and ice
  • Charon mostly ice - density close to that of water

Surface Composition

• Pluto is surprisingly bright compared to other bodies with icy surfaces.
  • Long exposure of methane ice to solar UV radiation normally turns it dark.
  • Thus, Pluto’s methane must partially vaporize at perihelion, renewing surface ice. Due to high eccentricity of orbit:
    • Perihelion: 29.6 AU: T = 51 K
    • Aphelion: 49.2 AU: T = 40 K

Pluto - Kuiper Express

• Only planetary system not yet visited by spacecraft

• Planned mission: The Pluto/Kuiper Express

  • Launch late in 2004
  • Reach Pluto in 2012
  • Go on to study objects in the Kuiper belt.
  • Unfortunately, was put on "hold" by NASA a few months ago.

• Objectives:

  • characterize the global geology and geomorphology ofPluto and Charon;

  • map the composition of Pluto's surface

  • determine the composition and structure of Pluto's atmosphere. 

• The mission is intended to reach Pluto before the tenuous Plutonian atmosphere can refreeze onto the surface as the planet recedes from the Sun. Studies of the double-planet system will begin 12-18 months prior to closest approach.  Most aspects of the mission are still under study and subject to change. 

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