Clark R. Chapman

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The Threat to Earth from Asteroids and Comets
and Possible Countermeasures

• Clark R. Chapman
• Southwest Research Inst.
• Boulder, Colorado

Managing Global Scale Disasters Western
Psychological Association Irvine, California
12 April 2002
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The Hazard from Asteroids and Comets

• Each year, there is a 1-in-200,000 chance that an
  asteroid or comet more than one kilometer wide
  will strike the Earth.
• 40 percent of these objects remain to be found,
  and could strike without warning, threatening the
  future of civilization.
• This extreme example of a natural disaster (a
  tiny chance of happening, but with huge
  consequences) challenges a rational response by
  citizens and policy-makers.

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The processes that formed the planets 4.6 billion
years ago left smaller comets and asteroids some
of which occasionally cross the Earths orbit and
can strike our planet if it happens to be there
at the same time.
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Small impacts happen frequently
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Larger impacts happen rarely in human history...
This Siberian forest (the size of a major city)
was felled in 1908 by a 15-Megaton asteroid
explosion
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Still larger, globally destructive impacts happen
very rarely...

• Comet Shoemaker- Levy 9 struck the planet Jupiter
  in 1994, blackening parts of its atmosphere
  larger than the whole planet Earth
• But Earth is struck by devastating objects less
  often every million years

Image from Peter McGregor and Mark Allen, ANU
2.3m telescope.
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HUGE impacts have happened before65 million
years ago
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What Do We Know About the Impact Hazard?

• How many asteroids and comets there are of
  various sizes in Earth-approaching orbits (hence,
  impact frequencies are known).
• How much energy is delivered by impact (such as
  the TNT equivalence, size of resulting crater).
• How much dust is raised into the stratosphere and
  other environmental consequences.
• Biosphere response (agriculture, forests, human
  beings, ocean life) to environmental shock.
• Response of human psychology, sociology,
  political systems, and economies to such a
  catastrophe. Thats where YOU come in!

WE KNOW THIS Very Poorly Somewhat Very
Well Very Well
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Classification of Hazards

• High Altitude Disintegration (Brilliant Flash of
  Light)
• Projectile fragments and disperses at high
  altitudes (over 40 km)
• Negligible surface damage
• Local/Regional Effects (Blast Damage, Tsunami)
• Projectile explodes in lower atmosphere or
  craters surface
• Severe localized damage from blast or flooding by
  tsunami
• Global Damage (Environmental Disaster)
• Short-term global-scale climatic changes (e.g.
  impact winter)
• Global loss of food crops leads to large-scale
  famine, disease, and possible breakdown of
  civilization
• Mass Extinction (Environmental Holocaust)
• Severe global environmental destruction (e.g. K/T
  event)
• Many species lost forever all, or nearly all,
  human beings die

Decade
Millennium
Million Years
Hundred Million Years
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Chief Environmental Consequences of Impacts
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Is Civilization Robust or Fragile? A State of
Mind?

• Arguments for Fragility
• Modern people are disconnected from nature,
  survivability
• Technology is ever more specialized
• People are interdependent on distant resources,
  other nations
• If societal breakdown spawns violence, modern
  weaponry is very dangerous
•  Arguments for Robustness
• Technological refugia exist (such as bomb
  shelters)
• Society has become experienced in disaster
  recovery
• Technological know-how has become pervasive
• Historical precedence recovery from WWII was
  rapid

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Risk vs. Scale of Impact

• Annual fatalities peak for events near the
  threshold size, about 2 km
• Orange/yellow zone illustrates our range of
  uncertainties for agricultural disaster due to
  stratospheric dust

Stratospheric Dust
Tsunami
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A Royal Flush

• Odds1 to 649,739

• It is more likely that a mile-wide asteroid will
  strike Earth next year than that the next poker
  hand you are dealt will be a royal flush.

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20th Century Catastrophes We have much more
to worry about!

• Averaged over long durations, the death rate
  expected from impacts is similar to that from
  volcanoes.

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Chances from Dying from Selected Causes (for
U.S.A.)
By terrorism (year 2001)
By terrorism (since 1970)
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Fatality Rates Compared with Accidents and
Natural Hazards
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Mitigation Options

• Spaceguard Survey (ongoing telescopic search)
• 90 of hazardous NEAs may be found by 2010,
  certified as safe.
• Very unlikely one is found that will strike
  Earth within decades.
• Deflect Asteroid away from Earth (nudge it from
  threatening path)
• Spacecraft technologies exist to deliver
  deflection devices to threatening asteroid, given
  years/decades warning and lead-time.
• Low-thrust device options rocket engine, solar
  sail, mass driver, even paint-it-black to take
  advantage of Yarkovsky Effect
• Powerful devices anchored bombs or stand-off
  neutron bomb
• Standard Hazard Mitigation (THIS IS FOR YOU TO
  FIGURE OUT!)
• Extrapolate civil defense/natural disaster
  management from local to world context (e.g.
  store food supplies, evacuate countries around
  ground zero, prepare for post-disaster crisis).

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Headline Mile-Wide Asteroid Will Hit in 2028

• Which is least likely to be correct?
• A. The news report is wrong due to bad or
  hyped journalism.
• B. The scientific forecaster goofed. Were safe.
• C. The astronomers erred. The asteroid is
  tiny most of the world is safe.
• D. An asteroid will hit Earth in 2028.
• The correct answer is D
• A, B, and C are all much more likely to explain
  the headline.

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The Scary Case of 1997 XF11

• In March 1998, head-lines warned of pos-sible
  impact in 2028.
• The next day, old data ruled it outbut the
  prediction was badly mistaken.

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Prediction is the Event

• Scientists who predict think of predictions as
  dry scientific results, with objective
  error-bars.
• Users of such predictions are mobilized into
  action by the prediction.
• The predicted event may not happen as predicted
  it may or may not have consequences. The
  predictions always have consequences.
• Predictions of emotionally laden disasters result
  in subjective, sometimes irrational responses.
• Predictions must be made with social
  responsibility, whether of a potential terrorist
  operation or of an asteroid impact. Astronomers
  are learning!

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The Torino Scale
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How the Torino Scale is Calculated From the
Probability of Impact and Size
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Current Dilemma. (Work is Starting to Fix it)
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Our SwRI White Paper Highlights Lack of
Planning
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Findings Evaluation, Warning, and Mitigation

• Existing structure is disorganized Astronomers
  are just starting to learn how to communicate,
  but relevant agencies (e.g. FEMA) arent prepared
  to listen and act.
• Asteroid deflection scenarios have been
  conceived, but no serious systems engineering or
  planning has been done to deal with various
  possible cases.
• There is no known consideration by civil defense
  and disaster management agencies, let alone any
  assignment of responsibilities to relevant
  agencies.
• No US governmental scientific advisory body has
  formally established the priority that the impact
  hazard should command with respect to other
  national priorities.

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There Are Some Hopeful Signs

• The British government debated the impact hazard
  and has started (a bit) to do something
• The Organization for Economic Cooperation and
  Development (OECD) Global Science Forum decided
  in January 2002 to make NEOs one of its top
  priority projects through 2003
• SAIC and the US Air Force Space Command are
  investigating how to establish a Natural Impact
  Warning Center
• Public interest remains high, even if there is
  very little governmental funding

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Some Salient Facts about the Impact Hazard

• It is not a Deep Impact or Armageddon
  shoot-em-down just before they hit scenario
  (sorry, Benny!)
• For asteroids, orbiting in the inner solar
  system, it is a case of finding them decades in
  advance of an impactwith long lead-times for
  mitigation For every asteroid with lt1 year
  warning time, there are 50 with 5 decades of
  lead-time (but comets are another matter)
• It is one of the few big hazards for which it is
  technologically feasible, with some confidence,
  to stop the catastrophe from happening (by
  deflection)
• Near-miss scares and cries of Wolf! are much
  more likely than an actual catastrophic impact

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Why are the big/rare ones so much more important
than Tunguskas?

• Only asteroids larger than 1 mile across can be
  globally destructive and threaten civilization
• For every devastating 15-MT Tunguska blast,
  there are 100 earthquakes, floods, and typhoons
  that are equally destructive
• Cost-effectiveness drops sharply with size the
  average annual fatalities drop while the costs of
  finding the objects and responding to them rises

But, there are contrary viewpoints and interests
On a politicians watch, why would he/she care
about what might happen decades from now? And
there are Star Warriors in the DoD who would
like to test their inventions and try shooting
down small asteroids. And astronomers who would
love to have more and bigger telescopes.
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Conclusionsand Transition to the Psychological
and Sociological Perspectives

• The impact hazard is REAL but it is VERY
  UNLIKELY to happen during our lifetimes
• Its potential consequences are horrific
  exceeding any other natural hazard and equalling
  all-out nuclear war
• We could avert a threatened impact
• In a post-September 11th world, it is difficult
  (for me) to predict how people might react to
  near-misses, huge-but-low-probability disasters,
  bombs in space, and other impact hazard issues