Transcript The Asteroid/Comet Impact Hazard: An Extreme Low

The Asteroid/Comet Impact Hazard:
An Extreme Low-Probability High-Consequence Hazard
Poster Presentation, 28th Hazards Research & Applications Workshop, 13-16 July 2003, Boulder, Colorado USA
Clark R. Chapman
Southwest Research Inst., 1050 Walnut St., Boulder CO 80302
Summary: The Asteroid/Comet Impact Hazard
While the chances of a mile-wide asteroid or comet striking the Earth during the 21st century is very small (about 1
in 10,000), they are not zero and the consequences could threaten the existence of civilization as we know it. Impacts
by much smaller asteroids occur much more frequently. An impact that is more likely than not to occur during this
century could have very serious consequences. In the fear-enhanced, post-September 11th world, responses by
security or military agencies to an unpredicted impact, or even to a "near miss," could be far more dangerous than the
direct effects of the explosion. For example, it is not certain that all nations threatened by nuclear attack would be able
to reliably determine that an asteroid explosion in the skies above was not a nuclear attack requiring retaliation.
Analysis of the impact hazard, as the end-member of a spectrum of natural hazards, provides some insight to the
psychology, politics, and practicalities of dealing with an unusual, dangerous hazard. Impacts, even relatively modest
ones, have some features in common with other natural disasters and/or human-caused accidents or terrorist/military
actions: the manifestation of disaster generally has familiar attributes, including earthquake, tsunami, wind, shock
wave, fire, falling debris, crumbling structures, etc. On the other hand, normal warning systems (e.g. missile-attack
warnings, Pacific tsunami warnings) are not necessarily tuned to recognize the signatures of impacts, and the suite of
potential impact damages is complex and without precedence, requiring some modifications in generic disaster
response protocols. The unusual nature of this hazard is psychologically difficult for public officials to deal with.
Unlike most natural hazards, there are options for mitigating the effects of an impact, or even avoiding the impact
altogether. Telescopic search programs have found more than half of all Earth-approaching asteroids larger than 1 km
diameter. There are proposals for searches that could find 90% of objects down to only a few hundred meters in size.
When an object is found, its orbit can be determined accurately, and any possible Earth impact can be precisely
predicted…or, much more likely, ruled out. If an asteroid is found headed for Earth, we will likely have decades to
plan for the impact. And there are various space-borne technical options for actually changing the trajectory of the
asteroid so that the impact won't happen at all. There are serious policy issues, however, concerning even the
possibility of moving an asteroid. For instance, a low-thrust, long-duration "push" over the course of a year (as
advocated by the B612 Foundation [http://www.b612foundation.org]) might move the nominal impact point over a
country that was previously safe, before eventually changing the trajectory enough to avoid a collision anywhere on
Earth. Also, for a sufficiently large asteroid or comet with short warning time, the only technology powerful enough to
intervene might involve currently prohibited nuclear weapons in space.
We are
Here!
Asteroid Belt
Sun
NEOs
[email protected]
The Torino Scale. Trying to explain natural hazards to the general
public in an understandable way has previously resulted in the creation of
the Richter Scale (now “Magnitude” scale), the Saffir-Simpson hurricane
scale, and numerous other such devices. Two currently relevant ones are
shown below: (a) the much lampooned “terrorism scale” of the Homeland
Security Dept. and (b) the U.S. Forest Service’s signs you can see on
roads to the west of Boulder about fire danger.
The Torino Scale was developed to create some public awareness of
how seriously to take a future news story such as this: “Astronomers say
there is a 1-in-200,000 chance that a 500-meter sized asteroid will strike
Earth on April 1st, 2028.” The scale itself is a linear, 0-to-10 scale, with
associated colors and words (left). Some critics of the scale have
advocated that it be 2- or 3-dimensional, but even educated lay people
rarely comprehend 2-D graphs. A 2-dimensional plot (lower left) of the
Torino Scale is a technical definition of how the Torino Scale values for a
predicted potential impact are calculated from two quantities: the impact
energy and the probability of impact. The technical version is not
intended for public presentation, but for use by scientists and science
communicators.
The processes that formed the planets 4.6 billion years ago left many small
remnant objects: comets (beyond the outer planets) and asteroids (in a “belt”
between the orbits of Mars and Jupiter). There are collisional, chaotic, and thermal
processes that cause some smaller asteroids and their meteoritic fragments to
leave the asteroid belt and orbit the Sun among the terrestrial planets. Some of
them occasionally cross the Earth’s orbit and can strike our planet...if the Earth
happens to be there at the same time. Impact velocities are typically 20 km/sec.
Take-Away Message: Asteroid and comet impacts may seem to be exotic, but the threat is real and
How Big, How Often?
Impacts range from tiny “shooting stars” (upper left) that you can see
on any clear night to (clockwise) “dinosaur killers” that have only one
chance in a million of striking Earth during the 21st century.
potentially as serious as nuclear war. On the other hand, the chances of an impact disaster with more fatalities
and consequences than large earthquakes, storms, and floods are small. Astronomical predictions are precise,
so advance warning to store up food or evacuate ground zero is possible. And technology exists to actually
move a threatening asteroid off its collision course. Impacts are an interesting “end-member” natural hazard.
A useful approach to communicating about
the impact hazard is to compare with other
hazards that people think more about.
Many people fear animals like sharks and
snakes, yet the
annualized death toll
from asteroid impacts is
greater than most of
those hazards. On the
other hand the impact
hazard is insignificant
compared with disease,
famine, war, and even
common natural and
man-made hazards (like
automobile accidents
and floods).
Conclusions
OVER KASHMIR?
OVER ISRAEL?
HOW WOULD THE
GENERALS
RESPOND?
By terrorism
(mostly due to
Sept. 11th
attacks)
Different scales and frequencies of impacts of practical concern: rare, civiliza-
tion-threatening impacts by multi-kilometer asteroids (top) to frequent mediascares or hype of inconsequential impacts (bottom). For each scenario, I show
asteroid diameter and chance of happening somewhere on Earth this century.
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