Comets and Asteroids

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Comets and Asteroids

• Paul J. Thomas
• Department of Physics and Astronomy
• University of Wisconsin - Eau Claire

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Asteroids
Ida and Dactyl, from the Galileo spacecraft
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The Asteroid Belt
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Comets
Comet Hale-Bopp
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The Kuiper Belt and Oort Cloud
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The Comet Nucleus
Halley
Tempel 1
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Deep Impact Mission
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Surprises from Comet Tempel 1

• Nucleus seems to be composed of several
  fragments.
• Water vapor vents appeared all over the nucleus,
  as expected, but carbon dioxide only vented from
  certain parts.
• Dust only came from the carbon dioxide vents.

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Surprises from Comet Tempel 1

• There are some very smooth areas on the nucleus,
  as if something had flowed there. However, the
  comet's gravity is extremely weak.
• There are circular areas with raised rims that
  resemble impact craters. But the surface gets
  vaporized every time the comet's orbit takes it
  close to the sun.

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Organic Molecules in Comets and Asteroids

• Carbonaceous chondrite asteroids are 3-5 organic
  by mass.
• 3 of organic asteroidal carbon is amino acids.
• Kerogen is similar to chondritic organics.
• Halley dust is 30 organic by mass.
• Halley gas is 14 organic by mass.

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The Early Terrestrial Environment

• Complex terrestrial organisms 3.5 Gya.
• Probable biologically mediated C12/C13
  fractionation 3.8 Gya.
• Heavy Bombardment ended 3.8 Gya.

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Meteorites through the roof
Sylacauga, AL, 1954
Freehold Township, NJ, 2007
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Comet Shoemaker-Levy 9
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Asteroid 1997XF11

• 2 km diameter asteroid.
• Brian Marsden (Harvard) calculated 2028 close
  approach of 40,000?180,000 km!
• Paul Chodas and Don Yeomans (JPL) revised this to
  80,000?2,500 km.
• Still, it will almost certainly hit Earth
  eventually...

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Podkammenaya Tunguska Area

• 2200 km2 area of felled trees.
• No central crater, but stand of trees stripped of
  bark and leaves.

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The Tunguska Epicenter
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Tunguska Blast Site

• Butterfly pattern consistent with incidence
  angle of ?45º, airburst height of 10 km (Zotkin
  and Tsikulin, 1966 Korobeinikov et al., 1976)

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Tunguska Then and Now
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15 MT Airburst Models
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Meteor Crater, Arizona

• 50,000 years old.
• 15 MT event.
• Iron asteroid.

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Airbursts
Tunguska, 1908
Once in 300 years
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Crater Cluster on Venus

• 25.6 latitude 336.0 longitude.
• 1.5 km diameter.
• Appears to have been formed by four fragments of
  a single object.

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Small Terrestrial Impacts
Once in 10,000 years
Barringer Meteor Crater, 50,000 y
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Large Terrestrial Impacts
Once in 100,000 years
Gosses Bluff, 143 My
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Civilization-Ending Impacts
Vredefort, 1,970 My
Once in a million years
Manicouagan, 210 My
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Extinction-Level Events
Once in a 100 million years
Chicxulub, 65 My
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Near Earth Objects (NEOs)

• Near Earth Asteroids (NEAs)
• 5083 discovered so far
• 730 with diameters gt 1 km
• 900 Potentially Hazardous Asteroids (PHAs)
• Comets
• Much higher impact speeds, impossible to
  catalogue all objects

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Spacewatch Telescopes
0.9 and 1.8 m telescopes discover 20 Near Earth
Asteroids/year
Spacewatch telescope, Kitt Peak, Arizona
1994 GK
1994 GL
1994 XM1
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One thing Armageddon got right

• Mr. President, NASAs budget for hunting the sky
  for asteroids is only a million dollars a year
  and - excuse my language - its a real big-assed
  sky out there.
• NASAs Executive Director, Dan Truman
• (Billy Bob Thornton)
• Opening weekend grosses
• for Deep Impact 41.1 million
• for Armageddon 34.8 million

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The Threat is Real

• In any year
• Probability of 1/100,000 of Earth being hit,
  without warning, by an undiscovered NEO gt1 km in
  diameter.
• Probability of 1/1,000 to 1/100 of Earth being
  hit, without warning, by an undiscovered NEO 60
  m in diameter.
• These unknown NEOs currently pose a far greater
  risk than any known NEO.
• Rick Binzel, Ted Bowell, Clark Chapman, Paul
  Chodas, Paolo Farinella, Al Harris,
  Andrea Milani, David Morrison, Steve Ostro, Don
  Yeomans