AST1 Introduction to Astronomy

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Asteroids
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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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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.
• Most of the original asteroids are gone!

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Asteroid Ida and its tiny moon, Dactyl
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Close-up of asteroid Ida

• Only 12 meters across

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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
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Comet West
Comet Kohoutek
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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 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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Known Kuiper Belt Objects
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Kuiper Belt Object 1993SC - these images were
taken 4.6 hours apart
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Pluto The biggest Kuiper belt object?
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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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Henbury, Australia
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Popigai, Russia
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A large asteroids impact with Earth may well
have killed off the dinosaurs
65 million years ago
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Gravity Field of Chicxulub Impact Crater
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Could a killer asteroid strike Earth soon?

• Yes, but it is extremely unlikely
• Extinction-level impacts probably occur only once
  every 50-100 million years (the last big one was
  65 million years ago)
• But
• Smaller impacts happen much more often!

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Homework 2 review

• Good
• Nice sketches!
• Descriptions like a thumbnail
• Most described phases motion
• Not as good
• Not to scale, no time date, stars
• Didnt answer Why?
• Confusion about diurnal motion (east to west) vs.
  lunar orbit (west to east)