Chapter 8
Vagabonds of the Solar System
What do you think?
• Were 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?
Most asteroids orbit the Sun
between Mars and Jupiter
In general, asteroids are small
Asteroids are found by looking for moving
objects (streaks) in long exposure photographs
Asteroid Ida and its
tiny moon, Dactyl
A Short History of Asteroids
• 1000 km Ceres was discovered in 1801
• 600 km Pallas was discovered in 1802
• Juno and Vesta were discovered in the 19th
Century -- all the rest in the 20th Century
• Officially, there are about 7000 known asteroids,
most tiny and less than 100 km across, but as
many as 100,000 might be out there
• Even 100,000 spread out over an 18,000,000 mile
orbit means that they are rather rare
Was the asteroid belt once a planet
that has since been destroyed?
• If all the asteroids were assembled into a
planet, it would have a tiny diameter of only
1500 km, or about 12% Earth’s diameter.
• The combination of the pull of the Sun’s
gravity and Jupiter’s gravity keeps the
asteroids “stirred up” enough to keep
anything from coalescing.
Jupiter’s
gravity
creates
wide
gaps in
the
asteroid
belt
Asteroids
exist outside
the asteroid
belt
Asteroids exist outside the asteroid
belt
• Trojan asteroids in front and behind Jupiter
• Apollo asteroids which cross Earth’s orbit
about the Sun
• Kuiper asteroids (Kuiperoids) exist beyond
the orbit of Neptune
– these Kuiperoids might not be rocky asteroids
at all, but rather, icy comets
Comets seem to come from two
possible places
• Oort Cloud
– Reservoir of long period comets that might only
come through the solar system once in billions
of years and can come from any direction
• Kuiper Belt
– Reservoir of short period comets that come
through the solar system regularly and only
come in along the plane of the ecliptic
The Kuiper Belt of comets spreads from
Neptune out 500 AU from the Sun
Kuiper Belt Object 1993SC - these
images were taken 4.6 hours apart
Comet Kohoutek and Comet West
Comets lack tails until they enter the inner solar system
Comets often have two tails:
a thin ION tail and a curving DUST tail
Anatomy of a comet
15 km long
by 8 km
wide
Comet
Halley
nucleus
Comets don’t last forever
Fragmentation of Comet West shortly after
passing near the Sun in 1976
(sequence of photos is from March 8 to March 24)
Comet orbits are
altered by
gravitational
interactions with
planets
Small rocky debris peppers the
solar system
•
•
•
•
•
meteors
falling stars
shooting stars
bolides
fireballs
each are caused by small
rocks colliding with
Earth’s atmosphere
and heating up due to
friction with the air
Meteorites are space debris that land intact
Meteorite Types
• Stony meteorites
– look much like ordinary rocks
• Iron meteorites
– heavy and composed on iron and nickel
minerals
• Stony-iron meteorites
– contain roughly equal amounts of rock and iron
– rare ones are carbonaceous chondrites which
have never melted and contain amino acids one of the building blocks of life
Impact craters and meteor
showers mark remnants of space
debris on Earth
Arizona crater is some 50,000 years old
The 1908 Siberean Tunguska mystery
provides evidence of catastrophic collisions
A large asteroid’s impact with Earth
may well have killed off the dinosaurs
some 65
million
years
ago
What did you think?
• Were the asteroids a planet that was somehow destroyed?
No, the gravitational pull from Jupiter prevented a planet from ever forming there.
• How far apart are the asteroids on average?
The distance between asteroids averages ten million kilometers.
• Why do comets have tails?
Gas and dust that evaporate from the comet nucleus are pushed away from the Sun
by sunlight and the solar wind.
• In which direction does a comet tail point?
Comet gas tails point directly away from the Sun; comet dust tails make arcs
pointing away from the Sun.
• What is a shooting star?
A shooting star is a piece of space debris plunging through the Earth’s atmosphere - a meteor.
Self-Check
1: Sketch the location of the asteroid belt in the solar
system and explain the nature and origin of the
Kirkwood gaps.
2: Describe the relationships among meteoroids,
meteorites, meteors, and meteor showers.
3: List the principal classes of meteorites; compare
and contrast their compositions and frequencies in
space and on Earth.
4: Compare and contrast asteroids, meteoroids, and
comets in terms of orbital characteristics, chemical
composition, size, and structure.