Transcript Ch 8--Asteroids, Meteors, Comets

Ch 8--Asteroids, Meteors, Comets
03 Oct 2000
ASTR103, GMU, Dr. Correll
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What do you think?
• Are the asteroids a planet that was somehow
destroyed?
• How far apart are the asteroids on average?
• Why do comets have tails?
• In which direction does a comet tail point?
• What is a shooting star?
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Early Solar System
• From studying
cratering on the
Moon and the
planets
– First half-billion
year had many
collisions between
planetesimals
– More recently the
collisions are
much less
frequent, but do
occur!
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Asteroids
• Asteroids:
– remnant planetesimals
• also known as “minor
planets”
– mainly concentrated in
the asteroid belt
between Mars and
Jupiter--about 10
million miles average
separation
– Mostly stony or
metallic in composition
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Asteroids
• 1801--Piazza discovers Ceres--the first asteroid
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ASTR103, GMU, Dr. Correll
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Asteroids--Origins
• A destroyed
planet?
– Probably not
• Jupiter’s gravity
creates
“resonances”
which create
bands of stable
orbits and gaps of
unstable orbits
– Kirkwood gaps
discovered 1867
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Asteroids outside the belt
• Gravitational perturbations eject comets from the
belt into elliptical orbits
– Some approach the inner planets
– “Apollo” asteroids cross the Earth’s orbit
• Near-Earth Objects (NEOs)
• Near-Earth Asteroids (NEAs)
• Potentially Hazardous Objects (PHOs)
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ASTR103, GMU, Dr. Correll
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Comets
• Comets--best thought of as
“dirty snowballs” orbiting the
Sun which evaporate due to
solar heating
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ASTR103, GMU, Dr. Correll
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Comets Origins
• Primordial gas, dust,
and ice frozen in
clumps at the outer
limits of the solar
system
– Kuiper belt--out to 500
AU
– Oort Cloud--50,000
AU
• Occasionally
perturbed into
elliptical orbits
approaching the Sun
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Parts of a Comet
• Coma--spherical
cloud of vaporized
gas around the
frozen nucleus
• Tails--trail of debris
– dust tail--dust
particles blown away
by solar radiation
– ion tail--ions carried
away by solar wind
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Parts of a Comet
• Tails always point
away from the Sun!
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ASTR103, GMU, Dr. Correll
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Comets Demise
• Comets eventually
–
–
–
–
breakup into smaller fragments (Comet West below)
evaporate
collide with the sun
collide with other planets
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Comets Demise
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Comets Demise
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Meteoroids, Meteors, Meteorites
• Meteoroids--small rocky
or metallic particles
throughout the solar
system
• Meteors--upon entering
Earth’s atmosphere,
friction causes heat and
disintegration--shooting
stars
• Meteorites--remnants
surviving on the ground
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Meteor Showers/Storms--Earth
occasionally passes through cometary
debris streams leading meteor showers!
Earth’s
Location
every
Nov 17
Leonid Meteor Shower (Comet Temple-Tuttle stream)
UWO Meteor Group
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ASTR103, GMU, Dr. Correll
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Meteorites
• Meteorites--remnants
surviving on the ground
– Mostly remnants of asteroids
(to have survived entry into
Earth’s atmosphere)
• Stony meteorites
• Iron meteorites
– Show melting and wearing
patterns from processing in
asteroids
– Carbonaceous Chondrites
show no melting
• may be primordial
components of solar system
• contain organic material!
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Dino Killers?
• Evidence for large impact
65 Million years ago
• Alvarez theory of K-T
boundary
• Impact crater evidence
found off north shore
Yucatan at Chixulub
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ASTR103, GMU, Dr. Correll
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More Recent Events
•
Meteor Crater
– 30,000 BCE
– 1.2 km crater
– 30 m meteor
•
Tunguska 1908
– ~ 1/2 km size
– 2200 sq km area
•
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ASTR103, GMU, Dr. Correll
Peekskill 1993
– ~1m size
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Planetary Defense?
Easy to study; expensive to carry out!
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ASTR103, GMU, Dr. Correll
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What do you think?
• Are the asteroids a planet that was somehow destroyed?
– No, the gravitational pull of Jupiter prevented the planetesimal in
the asteroid belt to coalesce into planet
• How far apart are the asteroids on average?
– The distance between asteroids averages about 10 million miles
• Why do comets have tails?
– Gas and dust evaporated from the comet nucleus are blow away
by the solar wind and solar radiation pressure, respectively
• In which direction does a comet tail point?
– Gas tails point directly away from the Sun; Dust tails point
indirectly away from the sun in arcs
• What is a shooting star?
– A piece of space debris plunging white hot into the Earth’s
atmosphere--a meteor, not a star
03 Oct 2000
ASTR103, GMU, Dr. Correll
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Questions for Thought
• Explain the origins of comets and describe the
various features seen when near the Sun.
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ASTR103, GMU, Dr. Correll
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