by David Jewitt
NATURE - VOL 403
13 JANUARY 2000
from Website
David Jewitt is at
the Institute for Astronomy, University of Hawaii,
2680 Woodlawn Drive,
Honolulu, Hawaii 96822, USA.
e-mail:
jewitt@ifa.hawaii.edu
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Astronomers like to forget that the
roots of their subject lie in ancient superstitions about the
influence of the cosmos on everyday affairs. In fact, astronomy and
astrology were closely intertwined as recently as four centuries
ago, when Tycho Brahe laid the foundations of modern astronomy while
simultaneously maintaining a lucrative business in personal
horoscopes.
Modern astronomers generally scoff at
such superstitious beliefs, so it is somewhat ironic that science
has in the past few decades uncovered compelling evidence for
celestial interference in terrestrial matters. It is now clear that
asteroids occasionally wander from the main belt beyond Mars because
of chaotic instabilities caused by Jupiter. Some of these errant
asteroids strike the Earth with terrible consequences. On page 165
of this issue, Rabinowitz et al.1 report that the number of
threatening near-Earth objects (NEOs) larger than 1 km in diameter
is only half the previous estimates. But we still have no effective
means of detecting them all, and no form of self-defense.
The Earth bears the scars of previous
encounters with NEOs. Hundreds of impact craters, some the size of
small American states, have been discovered on the surface of our
planet. Each was produced by a devastating explosion that must have
been fatal to life in the surrounding areas on scales from local to
global (Fig. 1).
The Cretaceous–Tertiary mass extinction
of 65 million years ago seems to have been triggered by the impact
of an asteroid 10 km in diameter2. Ten thousand people killed by
‘falling stones’ in Shanxi Province, China, in 1490 were possibly
the victims of a much smaller and thoroughly fragmented projectile.
Still more recently, on 30 June 1908, 1,000 square kilometers of
Siberian pine forest in
Tunguska were blown flat by a 10-megaton
atmospheric blast caused by a 70-metre asteroid.
The gradual acceptance of the evidence for impacts by asteroids (and
comets) has led naturally to questions about the magnitude of the
threat posed by NEOs to life on Earth
3, 4. Rabinowitz and colleagues1
provide the most recent and best controlled estimate of the number
of large, potentially Earth-threatening NEOs. They report that there
are nearly 1,000 NEOs larger than 1 km in diameter and that, given
the present rate of discovery, it will take 20 years for 90% of
these objects to be found. Should we worry?
The answer depends on the number of
fatalities to be expected, but also on personal assessments of risk.
The number of NEOs found by Rabinowitz et al. is within a factor of
two of previous estimates based on less controlled samples, so
published estimates of impact mortality are essentially unchanged.
Considering events of all energies there is about 1 chance in 20,000
of being killed by an impact during the course of a human lifetime4,
similar to the likelihood of being killed in an airplane accident.
The perception of risk from impacts is
smaller than for being killed in a plane crash because planes crash
at a steady rate with (relatively) few deaths per event, whereas
lethal impacts are rare but kill a lot of people. At the very least,
the potential consequences of impact are large enough to cause
concern. In the past decade, thanks to several reported near-miss
encounters with small objects, the impact threat has become a
subject of intense interest to the general public (spawning the
popular movies Deep Impact and Armageddon).
In 1994, the United States House
Committee on Science and Technology went so far as to order the US
space agency NASA to “catalogue within 10 years the orbital
characteristics of all (Earth-orbit-crossing) comets and asteroids
that are greater than 1 km in diameter”. This particular cut-off
diameter was picked in part because 1-km NEOs are thought to be the
smallest objects capable of wreaking global havoc (for example, by
disrupting the climate and shutting down photosynthesis). Smaller
objects cause regional damage but would be unlikely to precipitate a
major extinction like the Cretaceous–Tertiary event.
Last summer, astronomers devised a new risk-assessment scale,
similar to the Richter scale used for earthquakes, to help the
public understand the hazard posed by a given NEO. The so-called
Torino scale ranges from zero (no chance of a collision) to 10
(certain collision causing global devastation). No known NEO has yet
had a Torino number greater than one. This is just as well because
we presently have no coherent plan of action should a real threat
arise. The simplest option — massive evacuation of the impact site —
would be impractical because of the positional uncertainties and
large numbers of people involved, and would be ineffective because
the damage from large NEOs will be global.
One option that has been discussed is
the thermonuclear destruction of the incoming NEO (a bad idea
because the shower of debris produced by the exploding NEO might be
as damaging as the initial object, and would be radioactive). Given
enough time, the NEO might be deflected from an Earth-intersecting
path by a series of smaller explosions, or by attaching rockets or
solar sails that use radiation pressure from the Sun.
The focus on NEOs larger than 1 km ignores the threat from smaller
but much more numerous objects. The Earth’s atmosphere offers little
protection against objects larger than 100 meters in diameter
4.
These smaller objects outnumber NEOs larger than 1 km by a factor of
100, so they are much more likely to strike in our lifetimes. There
is a 1% chance that the Earth will be struck by a 300-metre NEO in
the next centur 4. Such an impact would deliver a withering
1,000-megaton explosion and cause perhaps 100,000 deaths. If the
impact occurred in or near a densely populated region — the eastern
seaboard of the United States, for instance, or Western Europe or
coastal Asia — the fatalities could easily rise into the tens of
millions.
Neither can we take refuge in the fact that 70% of the Earth is
covered by oceans. Impact-induced tsunamis could wipe out coastal
cities over a wide area. So, to have practical value, surveys should
not be limited to the (observationally easy but numerically rare)
1-km NEOs, but should instead catalogue objects at least down to the
few-hundred-meter size range 5. What is needed is a more ambitious
survey to completely identify the population of small, potentially
threatening NEOs.
The strategy for such a survey has been explored by Alan Harris of
the Jet Propulsion Laboratory 6. He argues that the whole sky must be
surveyed on a monthly basis with a sensitivity about 100 times
greater than current NASA-sponsored surveys. How can this be done? A
large (6–8-metre) telescope is required, with a wide field of view
tiled with CCD (charge-coupled device) optical detectors and
connected to a massive computer array capable of meeting the huge
data-processing demands. The technology exists and tentative designs
are beginning to appear 7-9. Such a telescope, which would have many
applications in other branches of astronomy, is projected to cost
about $100 million (about half the price of a Jumbo jet).
What is missing is any sign that such a facility will be funded by
governments and their agencies. Perhaps astronomers can attract the
interest of private donors in the search for threatening NEOs. If
not, it seems we will have to face the asteroidal impact hazard with
our eyes wide shut.
References
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Rabinowitz, D., Helin, E.,
Lawrence, K. & Prado, S. Nature 403, 165–166 (2000).
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Alvarez, L. W., Alvarez, W., Asaro, F. & Michel, H.
Science 208, 1095–1108 (1980).
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Morrison, D. The Spaceguard Survey: Report of the NASA
International Near-Earth Object Detection Workshop (Jet
Propulsion Laboratory, Pasadena, 1992).
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Chapman, C. R. & Morrison, D. Nature 367, 33–40 (1994).
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Binzel, R. P. et al. From the Pragmatic to the
Fundamental: The Scientific Case for Near-Earth Object
Surveys (1999).
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Harris, A. Planet. Space Sci. 46, 283–290 (1998).
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Dark Matter Telescope
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NASA Infrared Telescope Facility
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A comparative study of current and
planned NEO surveys
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