Chapter 8 Vagabonds of the Solar System

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Chapter 8 Vagabonds of the Solar System
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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?

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Most asteroids orbit the Sun between Mars and
Jupiter
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In general, asteroids are small
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Asteroids are found by looking for moving objects
(streaks) in long exposure photographs
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Asteroid Ida and its tiny moon, Dactyl
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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

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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 Earths diameter.
• The combination of the pull of the Suns gravity
  and Jupiters gravity keeps the asteroids
  stirred up enough to keep anything from
  coalescing.

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Jupiters gravity creates wide gaps in the
asteroid belt
Kirkwood Gaps
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Asteroids exist outside the asteroid belt
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Asteroids exist outside the asteroid belt

• Trojan asteroids in front and behind Jupiter
• Apollo asteroids which cross Earths 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

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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

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The Kuiper Belt of comets spreads from Neptune
out 500 AU from the Sun
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Kuiper Belt Object 1993SC - these images were
taken 4.6 hours apart
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Comet Kohoutek and Comet West
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Comets lack tails until they enter the inner
solar system
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Comets often have two tails a thin ION tail
and a curving DUST tail
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Anatomy of a comet
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15 km long by 8 km wideComet Halley nucleus
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Comets dont last forever

• Fragmentation of Comet West shortly after passing
  near the Sun in 1976
• (sequence of photos is from March 8 to March 24)

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Comet orbits are altered by gravitational
interactions with planets
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Small rocky debris peppers the solar system

• meteors
• falling stars
• shooting stars
• bolides
• fireballs
• each are caused by small rocks colliding with
  Earths atmosphere and heating up due to friction
  with the air

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Meteorites are space debris that land intact
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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

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Impact craters and meteor showers mark remnants
of space debris on Earth
Arizona crater is some 50,000 years old
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The 1908 Siberean Tunguska mystery provides
evidence of catastrophic collisions
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A large asteroids impact with Earth may well
have killed off the dinosaurs
some 65 million years ago
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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 Earths atmosphere -- a
  meteor.

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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.