Catastrophic Impacts and Mass Extinctions
- Earth impacts happen. Solar system formation is a messy process. Even 4.5 billion years later a lot of debris remains.
- The impact of a small asteroid (a kilometer
across) deposits the energy of a full scale nuclear war (1 million megatons) -
- Objects of this size punch through the Earth's atmosphere as if
it wasn't there at all.
- The Earth has been cratered as much, if not more, than
- Such an impact would trigger global fires and fill the atmosphere
with dust which would take years to settle out.
- The change in global temperature would lead to the collapse
of the ecosystem and widespread extinctions.
- Such an event probably led to the extinction of the Dinosaurs at the
end of the Cretaceous period.
- The evidence lies in:
- In a more positive light, this event may have
permitted the emergence of the mammals as the
predominant species on the Earth.
- Cosmic forces have influenced the progress of the
evolution of life on Earth.
- According to the fossil record, substantial changes to the
ecosystem (specifically the extinction of a significant number
of species) occurs every 50 million years or so.
- This rate is consistent with the expected rate
of arrival for 1-10 kilometer asteroids.
- Smaller objects -- 100 meters to 1 kilometer in size --
can also do substantial damage and arrive more frequently.
- Such an object may moderately alter the climate and disrupt agriculture.
- If the object landed in an ocean it would create enormous
tidal waves which would destroy coastal cities (where most people live).
- The subsequent consequences would be global economic and social devastation. This is a civilization disrupting event.
- Such impacts happen every 100,000 years. You
have a 1-in-1000 chance of experiencing such an
event in your lifetime!
- These events really happen:
- Meteor Crater, Arizona (~50,000 B.C.) -- An iron asteroid
50 meters wide with a mass of 300,000 tons impacted the Arizona desert
releasing the energy of a 20 megaton explosion, creating a crater more
than a kilometer wide.
- Tunguska, Siberia (1908) -- An object estimated at 30 meters in diameter exploded due to atmospheric stresses setting fire to and leveling trees
over a area 40 kilometers in diameter below.
- Western U.S. (1972) -- An estimated 10 meter wide object glances
off of the Earth's atmosphere in a near-miss.
- Eastern U.S. (1992) -- A half-meter-sized asteroid streaks across the
night sky along the East Coast. A surviving fragment damages a car in Peekskill, N.Y.
- Jupiter (1994) -- Amazed Earthlings watch as more than a dozen kilometer-sized fragments of a disrupted comet fall one-by-one into Jupiter. Any
one such impact would have devastated Earth.
- Park Forest, Illinois (2003) - thousands of fist sized and larger stones pummel a Chicago suburb.
- The Sun (ongoing) -- Comets impact the Sun several times a year.
- Special issues...
- The manner in which solar systems form suggests that
impacts are a factor on planets everywhere in the universe.
- Debris naturally remains behind after the planets are
- Cleanup of the Solar System is a gradual process. Impacts were
more frequent in the distant past, but continue today.
- Is life on Earth fortunate in that it has not been wiped
out completely by a major impact?
- Has the timing of major impacts on Earth been just
right to advance evolution toward intelligence? or has the timing
here set us back?
- Today we have the technology to:
- Survey the solar system and identify potential threats to the Earth.
- The largest objects (>1 km) are the brightest and most easily cataloged (and there are fewer of them).
- Smaller objects are harder (and thus more costly) to find.
- Identify potentially hazardous objects decades in advance of a near-encounter with Earth.
- Nudge offending objects into a more benign orbit with a nearby thermonuclear explosion.
- Deflection means changing the velocity of an asteroid by about 1 mm/sec (if you can do it 10 years in advance of a collision with Earth).
- Even the gravitational tug of a spacecraft hovering over a 200-meter diameter asteroid for a year could do the trick.
- Still, we can always be blindsided by a comet or an undetected
- Threatening asteroids are often discovered after they have passed Earth.
- Comets only become detectable once they get about the Earth's distance
from the Sun.
- Making a committment to assess the impact threat thoroughly requires public
support for spending a good fraction of a billion dollars.
- Making the case to do so depends on the public's perception of risk.
- In terms of simple probabilities, death via asteroid impact
has a similar or greater likelihood than many common risks -- HOWEVER, actual
events are infrequent. Civilization has yet to record one.
- A couple of projects promise to survey the population of hazardous asteroids within a decade.
- A current goal is to catalog 90% of the threatening asteroids greater than 1km in size by 2010.
- Note that a 100-meter diameter object can be quite devastating. For every 1-km diameter asteroid there are several hundred of these smaller asteroids.
- Currently there are about 6000 "near-earth" objects (NEO's) known, of which about 1000 are "potentially hazardous asteroids" (they can make close approaches to the Earth).
- Currently there are NO known objects that pose a significant threat.
- For a well-illustrated presentation outlining the impact hazard risks see
Clark Chapman's on-line lecture.
Revised November 10, 2003