Lecture 7
Pluto and Charon
What are they?
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.