NEEP602 Course Notes (Fall 1997)
Resources from Space

Dropped for FALL 97

Lecture #25: Dinosaurs beware! The Asteroids are coming! The Asteroids are coming!

Title: Impact Risk, Consequences, and Alternatives



Figure: Orbits of 100 largest known NEO asteroids and comets (NASA, 1992)

The 130+ known impact craters on the Earth show that hits occur

Distribution of terrestrial impact craters (NASA, 1992)
One, the Ries event 14 million years ago produced a crater now 26 km in diameter.

Quiet Crust Stage impacts on the Moon are known to be "young"
Copernicus: ~900 million years
Tyco: ~100 million years
Figure: Estimated number of Earth Crossing Asteroids (ECAs) (NASA, 1992)

  • The work of Alverez, et al (1980, see also update by Cygan, et al, 1996) strongly indicates that a large impact event wiped out the dinosaurs and many other species 65 million years ago (the K-T event) and focused much more attention on evaluating the risks associated with this identified hazard.
    310 km diameter stucture
    20,000 km3 of melt volume for 10 km object at 15-20 km/sec
    Decrease in solar transmission of 10-20% for 10 yrs.
    Effects of CO2 and SO2 not as extreme as once thought
    This risk is in the "low probability, high consequence, we can do something about it" category.
    Global danger is that stratospheric dust and general atmospheric chemical changes will depress global temperatures and increase acid rain, and ozone depletion long enough to seriously endanger civilization.

    Effects of a > 1 km diameter impacting on the Earth at about 20 km/sec (Silver and Schultz, 1982)
  • > 26 km diameter with 100 times the mass of the impactor ejected
  • Major ejecta to one or more crater diameters
  • Significant ejecta to many crater diameters
  • Tsunami of extraordinary scale if impact is in the ocean
  • Atmosphere
    Large quantities of NO formed in high temperatures of the bow shock wave (1-10% of the atmosphere?), Cl2 and SO2 if impact is in the ocean, and CO2, SO2 and S2 if impact is in carbonate and anhydrite rocks.
    Ozone depletion (NO), acid rain (NO, Cl2, SO2) if impact is in the ocean, and greenhouse gases if impact is in carbonate rocks or in CH4 bearing ocean sediments.
    Reduced sunlight transmission due to NO.
    Large quantities of fine dust (10% of mass of impactor?)
    Complete solar blockage for 3-6 months?
    collapse of photosynthesis
    subfreezing temperatures on the continents
    widespread snow
    Large quantities of CO2 if impact is in carbonates or in the ocean
    rebound may be to a temporary greenhouse situation.
    Large quantities of CH4 if impact is in some ocean sediments (clathrates)

    Large impacts may have modified crustal evolution of the Earth (Glikson, 1995)
    Geochemical signatures (like Ir, etc.)
    Penetration of ocean crust
    Tsunami effects at ocean margins
    Disruption of normal land and ocean geological and biological processes
    Seismicly induced fracturing to activate or reactivate existing magma sources (Decan flood basalts in India seem to coincide with the K-T event
    Release of lithostatic pressure above materials near melting point (Sudbury crater in Canada.
    Mass extinctions
    --several others beside the K-T event now suspected
    Rapid speciation of surviving forms.

For a truly global catastrophe, the minimum mass required at 20 km/sec is on the order of 1010 billion tons or a ground burst explosion approaching 106 megatons tons TNT.
This implies a diameter of the ECA of between 1 and 2 km depending on density and velocity.

Frequency of such ECA impacts (NASA, 1992)
Average interval estimated as 500,000 years.
However, paleontological and geological evidence has not confirmed this, in fact, so far it appears somewhat pessimistic.

With current technology, we could discover and track for orbit determination nearly all asteroids and short period comets >1 km in diameter.
moderate sized ground telescopes
small constellation of small, imaging satellites

The few observers now looking discover several new ECAs/mo out of estimated thousands.

A ground system using current technology would cost an estimated 50 M and $10 M/year for detecting, tracking and coordination.


Should the human species worry about this hazard and the associated risk?

Should a detection and tracking system be considered a high priority along with everything else?

If so, should a continuously upgraded capability be established to deflect a threatening ECA?

Deflection options

rockets and nuclear explosives

long duration low thrust propulsion attached to the ECA



Shoemaker-Levy encounters Jupiter (Hubble views, 1994)



Alverez, L.W., et al, 1980, Extraterrestrial cause for the Cretaceous-Tertiary Extinction, Science, v208, 1095-1108.

Cygan, R.T., et al, 1996Researchers focus on Earth's response to hypervelocity impacts, EOS, 77, 197-198

Glikson, A.Y., 1995Asteroid/comet mega-impacts may have triggered major episodes of crustal evolution, EOS, 76, 49 and 54-55.

NASA, 1992, The Spaceguard Survey, D. Morrison, Chair, Report of the NASA International Near-Earth-Object Detection Workshop, January 25, 1992.

Shoemaker, E.M., et al, 1990, Asteroid and comet flux in the neighborhood of Earth, in Geological Society of Americal Special Paper 247, 155-170.

Silver, L.T., and Schultz, P.H., 1982, Geological Implications of Impacts of Large Asteroids and Comets on the Earth, Geological Society of America Special Paper 190, 527p.

Wetherill, G.W., and Shoemaker, E.M., 1982, Collision of astronomically observable bodies with the Earth, in Silver, L.T., and Schultz, P.H., 1982, Geological Implications of Impacts of Large Asteroids and Comets on the Earth, Geological Society of America Special Paper 190, 1-14.

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