Asteroids and Comets: Target Earth

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Asteroids and Comets Target Earth

• Astronomy and Public Policy
• March 22, 2004
• Kerstin McGaughey, Nai Tai, Brittany Taylor, and
  Jenny Sung

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Introduction

• How often is Earth hit by an asteroid or comet?
• Does this depend on the size of the impacting
  body? How so?
• How do we know? How are these rates determined?

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How Often Are We Hit?

• !EVERYDAY!
• Thousands of meteorites (typically a few
  kilograms in mass) penetrate the atmosphere and
  fall harmlessly to the ground each year.
• Small impacting objects that produce ordinary
  meteors or fireballs dissipate their energy in
  the upper atmosphere and have no direct effect on
  the ground below.
• Only when the incoming projectile is larger than
  about 10 m diameter does it begin to pose some
  hazard to humans.

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Size Does Matter
Three Categories

• Category I 10-M to 100-M
• Category II 100-M to 1Km
• Category III 1-Km to 5-Km

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Category I 10-M to 100-M

• 10-M wide intercept Earth every decade.
• Traveling at 20 km/s Kinetic energy 100
  kilotons of TNT (several Hiroshima-size bombs).
• Turn into fireballs high enough in the atmosphere
  that little damage is caused on the ground.
• Sound can be heard over 600km away.
• 100-M wide intercept Earth several times per
  millennium.
• Traveling at 20 km/s Kinetic energy 100
  megatons of TNT (largest thermonuclear devices).
• The bigger the impacting object, the greater the
  chance for ground damage from fireballs or
  airbursts.

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Category II 100-M to 1-Km

• Usually have an intact landing on Earth.
• Strike once per every 5000 years.
• Damage from atmospheric shock and flying debris
  can be severe.
• 1-km object could envelop whole states or
  countries, with fatalities of tens of millions in
  a densely populated region.
• Results vary depending on whether impacting
  object
• is a comet or asteroid
• Comets water ice and other volatiles
• Asteroids rocky or metallic composition.
• A comet this size probably couldnt survive
  passage through the atmosphere, but may generate
  atmospheric bursts sufficient to produce local
  destruction.

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Category III 1-Km to 5-Km

• Globally Catastrophic impact occurs once every
  500,000 years.
• Dust Blanket (crop failure).
• Nuclear Winter (temperatures drops tens of
  degrees Celsius).
• Acid Poisoning (produced from the burning of
  atmospheric nitrogen in the impact fireball).
• Global Warming (water vapor and carbon dioxide
  released to the stratosphere would produce an
  enhanced greenhouse effect).

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What Is Earths Threshold?
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What Are the Odds of Getting Hit?
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How Are These Rates Determined?

• Estimated Earth-Crossing Asteroid Population
• Estimated that 20 to 40 of all ECAs will
  collide with Earth eventually (Shoemaker, 1990).

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How Many ECAs Are There?
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Conclusions

• Much is still unknown.
• Impact frequency predictions for large asteroids
  are uncertain.

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Internet References

• http//neo.jpl.nasa.gov/stats/
• http//impact.arc.nasa.gov/reports/spaceguard/sg_1
  .html
• http//homepages.wmich.edu/korista/DeepImpact.htm
  
• http//www.astronomy.com/photogallery/gallery_list
  .asp
• http//www.vency.com/asteroids.html
• http//neo.jpl.nasa.gov/neo/neoreport030825.pdf