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Physics 320: Astronomy and
Astrophysics – Lecture XIV
Carsten Denker
Physics Department
Center for Solar–Terrestrial Research
NJIT
Pluto, Solar System Debris,
and Formation
 The
Pluto-Charon System
 Comets
 Asteroids
 Meteorites
 The Formation of the Solar System
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
The Pluto-Charon System
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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)
NJIT Center for Solar-Terrestrial Research
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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 PlutoCharon 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.
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
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.
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Comets
Halley’s 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
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NJIT Center for Solar-Terrestrial Research
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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December 10th, 2003
Comets (cont.)
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Disconnection events
 Long-period comets
100,000 to 1 million yr
Water on terrestrial
planets from comet
 Long-period comets
impacts?
originate in the Oort cloud
Halley: Suisei, Sakigake,
 Inner cloud in ecliptic
Vega 1/2, Giotto (closest
3,000 AU to 20,000 AU
approach 600 km),
 Outer cloud has spherical
Cometary Explorer
distribution 20,000 AU to
100,000 AU
Halley’s size 15 km  7.2
km  7.2 km
 Planetesimals “catapulted”
from Jovian planets to
Mass: 5  1013 kg to 1014 kg
Oort cloud
Halley is a short-period
comet < 200yr (Kuiper belt  Random motion
objects 30 AU to 100 AU)
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Dust Tail
Frad 
S A
c
 Frad 
Fg 
Fg
Frad
cos  (radiation pressure)
S 
c
GM mgrain
r2
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
L  R2

4 r 2 c
4 GM  R3
4
3

with
m


R
grain
3r 2
3
16 GM R  c
3L

 Rcrit 
3L
16 GM  c
  3000 kg/cm3  Rcrit  191 nm
R < Rcrit: net outward force, spiral away from Sun
R > Rcrit: continue to orbit Sun (Poynting-Robertson effect!)
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Hale-Bopp
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Halley’s Comet
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Sungrazer (SoHO/LASCO)
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December 10th, 2003
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
(1:1 resonance group,
Lagrange points L4
and L5)
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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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December 10th, 2003
Asteroids (cont.)
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
Orbital Resonances and Trojans
NJIT Center for Solar-Terrestrial Research
December 10th, 2003
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.
NJIT Center for Solar-Terrestrial Research
December 10th, 2003