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Transcript
Ch 8--Asteroids, Meteors, Comets
03 Oct 2000
ASTR103, GMU, Dr. Correll
1
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
2
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
3
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
4
Asteroids
• 1801--Piazza discovers Ceres--the first asteroid
03 Oct 2000
ASTR103, GMU, Dr. Correll
5
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
6
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)
03 Oct 2000
ASTR103, GMU, Dr. Correll
7
Comets
• Comets--best thought of as
“dirty snowballs” orbiting the
Sun which evaporate due to
solar heating
03 Oct 2000
ASTR103, GMU, Dr. Correll
8
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
9
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
10
Parts of a Comet
• Tails always point
away from the Sun!
03 Oct 2000
ASTR103, GMU, Dr. Correll
11
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
12
Comets Demise
03 Oct 2000
ASTR103, GMU, Dr. Correll
13
Comets Demise
03 Oct 2000
ASTR103, GMU, Dr. Correll
14
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
15
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
03 Oct 2000
ASTR103, GMU, Dr. Correll
16
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
17
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
03 Oct 2000
ASTR103, GMU, Dr. Correll
18
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
•
03 Oct 2000
ASTR103, GMU, Dr. Correll
Peekskill 1993
– ~1m size
19
Planetary Defense?
Easy to study; expensive to carry out!
03 Oct 2000
ASTR103, GMU, Dr. Correll
20
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
21
Questions for Thought
• Explain the origins of comets and describe the
various features seen when near the Sun.
03 Oct 2000
ASTR103, GMU, Dr. Correll
22