Physics 320: Astronomy and Astrophysics

1
Physics 320 Astronomy and Astrophysics Lecture
XIV

• Carsten Denker
• Physics Department
• Center for SolarTerrestrial Research

2
Pluto, Solar System Debris, and Formation

• The Pluto-Charon System
• Comets
• Asteroids
• Meteorites
• The Formation of the Solar System

3
The Pluto-Charon System

• Pluto discovered by Clyde W. Tombaugh in 1930
  (15th magnitude)
• 248.5 yr orbital period
• Eccentricity 0.25
• 29.7 AU perihelion (closer than Neptune)
• 49.3 AU aphelion
• 17 inclination to ecliptic
• 3-2 orbital resonance with Neptune (no danger of
  collsions)

• Radius 1160 km
• Its moon Charon discovered in 1978
• Orbit around common center of mass in 6.4 d
• Separation 19640 km (1/20 Earth-Moon distance)
• Reduced mass is 0.24 mass of Earth
• MCharon / MPluto 0.09 to 0.16
• Orbital plane of Pluto-Charon system is inclined
  122.5 with respect to their orbit around the Sun

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Pluto
Pluto is mostly brown. No spacecraft has yet
visited this most distant planet in our
Solar System. The map was created by
tracking brightness changes from
Earth of Pluto during times when it was being
partially eclipsed by its moon Charon. Pluto's
brown color is thought dominated by frozen
methane deposits metamorphosed by faint but
energetic sunlight.
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Pluto-Charon

Pluto is the only planet in our Solar System
remaining unphotographed by a passing spacecraft.
These maps depict the face of Pluto (left) that
always faces Charon, and the face of Charon that
always faces away from Pluto. The Pluto-Kuiper
Express mission is tentatively planned for launch
in 2004 and might encounter Pluto as early as
2012.
6
Comets

• Halleys comet (observed since 240 B.C., 76 yr
  orbital period)
• Nucleus dirty snowball or snowy dirtball?
• Size ? 10 km
• Coma cloud of gas and dust, sublimated ice
• Interaction with sunlight and solar wind creates
  dust (radiation pressure) and ion (magnetic
  field) tail up to 1 AU length

• A hydrogen gas halo envelopes the coma
• Tails are always directed away from the Sun (ion
  trails are straight, dust tails are curved)
• Dust grains scatter light, tail appears
  white/yellow
• Blue ion tail CO ions absorb UV radiation and
  reradiate at 420 nm
• Composition 80 H2O, 10 CO, 3.5 CO2, few
  (H2CO)n, 1 CH3OH

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Comets (cont.)

• Disconnection events
• Water on terrestrial planets from comet impacts?
• Halley Suisei, Sakigake, Vega 1/2, Giotto
  (closest approach 600 km), Cometary Explorer
• Halleys size 15 km ? 7.2 km ? 7.2 km
• Mass 5 ? 1013 kg to 1014 kg
• Halley is a short-period comet lt 200yr (Kuiper
  belt objects 30 AU to 100 AU)

• Long-period comets 100,000 to 1 million yr
• Long-period comets originate in the Oort cloud
• Inner cloud in ecliptic 3,000 AU to 20,000 AU
• Outer cloud has spherical distribution 20,000 AU
  to 100,000 AU
• Planetesimals catapulted from Jovian planets to
  Oort cloud
• Random motion

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Dust Tail
R lt Rcrit net outward force, spiral away from
Sun R gt Rcrit continue to orbit Sun
(Poynting-Robertson effect!)
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Hale-Bopp
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Halleys Comet
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Sungrazer (SoHO/LASCO)
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Asteroids

• Minor planets mostly between Mars and Jupiter
• Discovery of Ceres in 1801 by Piazzi
• Combined mass of all asteroids 5 ?10?4 M?
• Orbital resonances with Jupiter
• Kirkwood gaps
• Trojan asteroids (11 resonance
  group, Lagrange points L4 and L5)

• Hirayama families (originally single asteroid
  that suffered a catastrophic collision)
• Collision speeds of up to 5 km/s
• Composition is a function of the distance from
  the Sun (volatiles (water) vs. refractory
  compounds (silicon))
• Metal rich asteroids from larger parent asteroids
  with chemical differentiation

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Asteroids (cont.)
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Orbital Resonances and Trojans
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Lagrange Points
The Italian-French mathematician Lagrange
discovered five special points in the vicinity of
two orbiting masses where a third, smaller mass
can orbit at a fixed distance from the larger
masses. The Lagrange Points mark positions where
the gravitational pull of two large masses
precisely equals the centripetal force required
to rotate with them.