Alternate View Column AV-50
Keywords: asteroid impacts Kirkwood zones orbital resonances extinctions risk assessment
Published in the January-1992 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 7/14/91 and is copyrighted ©1991 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.
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What are the chances that you will die because an asteroid hits the Earth? The idea is ludicrous, right? The odds must be infinitesimal when compared to, say, the chances of death in an airplane crash or a tornado. We've heard of people who died in tornados or plane crashes, but who was ever killed by an asteroid? Little ones, meteorites, make streaks in the night sky. Bigger ones may occasionally make a hole in a corn field somewhere. But asteroids never kill anyone, do they?
As it turns out, odds are about 2.4 times greater that you will be killed by a asteroid or comet than by an air crash or tornado, both taken together. Asteroid are surprisingly dangerous. If big one hit the Earth at the wrong place, it could kill many people. And a sufficiently big one could kill everyone.
Last June at San Juan Capistrano, California, about 200 scientists met to discuss the dangers and technological fixes for the problem of killer asteroids. Two scientists, Clark R. Chapman and David Morrison, reported application of the techniques of risk assessment to the risk of death from comets and asteroids. Their conclusions are both surprising and frightening. They estimate that over a 50 year period the chances for a given individual of death from asteroid or comet impacts are about 1 in 6,000. This compares to 1 in 20,000 odds for death in an airplane crash, 1 in 50,000 for death in a tornado, 1 in 5,000 for death by electrocution, and 1 in 2,000 for death in a firearms accident.
The probability of death-by-asteroid is so large, of course, is because the calculated odds are the product of the chances of an asteroid strike (small) and the number of deaths predicted to result from it (very large in some cases). We are coming to the realization that both we as individuals and the human species as a whole are running a significant risk of extinction. All of our eggs are in one basket, and the solar system is throwing large rocks at that basket.
Consider some examples. In 1908 Comet Encke was making a close pass near the Earth. It is believed that a 100 meter (m) diameter chunk of ice from Encke broke off and plowed into the atmosphere over the Stony Tunguska River in Siberia. The result was an air-burst explosion liberating the equivalent of 600 Hiroshima-size nuclear bombs, so much energy that sensitive instruments around the world recorded the resulting shock waves. Trees in the Siberian forests were leveled for dozens of miles around, and horses 400 miles away were knocked from their feet. There was no known loss of human life, but this is only because the impact site was so isolated. If the same ice chunk had, by chance, struck over a major population center, Tokyo, or New York, or Bombay, mega-deaths would have resulted.
The Tunguska ice chunk exploded in the atmosphere leaving no crater. An equivalent stony asteroid would make a crater about 20 asteroid-diameters across and about 5 asteroid-diameters deep. The energy liberated increases as the cube of the asteroid diameter. About 5,000 years ago a chunk of rock from space perhaps 50 m across struck near what is now Winslow, Arizona, leaving a crater a kilometer (1 km) across and liberating an energy equivalent to 500 Hiroshima bombs. In Australia an 8 km asteroid made a crater 160 km across, and in Ontario a 10 km asteroid made a crater 200 km in diameter.
Asteroid strikes in this size range have devastating consequences. The work of Luis and Walter Alvarez and their group has produced a growing body of evidence that about 65 million years ago an asteroid about 10 km in diameter struck the Earth and killed most of the plant and animal life on the planet, including all of the dinosaurs. There is little doubt that a similar event today would kill most or all of the human species.
In all there are 131 confirmed asteroid craters on the surface of the Earth, and that number is growing at about 5 per year as more are discovered. Exploration of the solar system with unmanned spacecraft has revealed the dramatically cratered surfaces of many planets and satellites, from Mercury to the moons of Uranus. On a clear night our Moon, which presents a target area 14 times smaller than Earth's to wandering asteroids, shows many thousands of impact scars that have been not erased by weathering, as they have on the Earth.
And there have been near misses. In 1972 an asteroid perhaps 100 meters in diameter, twice the size of the object that produced the Arizona crater, dipped into the upper atmosphere over the northern USA and Canada before continuing on its way through interplanetary space. Thousands saw the spectacular fireball that it made in the sky. In 1989 an asteroid with a diameter of 800 meters crossed the Earth's orbit, passing through the position that the Earth would occupy just 6 hours later. And in January of 1991 an object perhaps 10 meters across missed the Earth by about 200,000 km, about half the distance to the Moon.
In 1932 it was discovered that asteroid number 1862, which was given the name Apollo, had an orbit that periodically crossed the orbit of the Earth. Since that time, many other asteroids with similar orbits have been identified. As a group they are called the Apollo asteroids. It is a fact of orbital dynamics that an object in an Apollo-type Earth-crossing orbit has many chances to strike our planet.
The population of Apollo asteroids is increased because the solar system contains a celestial slingshot that points in the general direction of our pretty blue planet. Once in a while it hurls a killer asteroid in our direction. If you examine the asteroid belt between Mars and Jupiter closely in regions about 2.5, 2.8 and 3.3 astronomical units (Earth-orbit radii) from the Sun, you will find empty zones, stripes where no asteroid orbits exist. These stripes are empty because at just these distances the large gravitational pull of Jupiter has a cumulative effect, producing orbital deflections that build with time. The empty zones are called the Kirkwood gaps. They correspond to distances where there are orbital resonances because the ratio of the asteroid's orbital period to Jupiter's is 1:3, 2:5, or 1:2, respectively.
The result of Jupiter's additive pushes is chaos. In a Kirkwood gap the asteroid's orbit is hypersensitive to small variations in orbital parameters, and it is propelled into another orbit in some unpredictable direction. There is a good chance that it will make the long dive into the inner solar system and become an Apollo asteroid. Computer simulations of this mechanism account for about the number and direction of chondritic meteorites that are observed to fall on the Earth. The Kirkwood gaps of the asteroid belt thus form a sort of celestial slingshot pointed at the Earth, with a good supply of ammunition on hand from the asteroid belt itself. It is estimated that about 500 asteroids with diameters of 1 km or more are presently in Apollo-type orbits. There are perhaps a dozen with diameter of 5 km or more, the size of large mountains. The latter are in the dinosaur-killer class and could do for us what the Alvarez asteroid did for the dinosaurs.
From a broader viewpoint, asteroid strikes might not be altogether bad. I suggested in a previous AV Column [Analog 01/86, "The Pump of Evolution"] that the average rate of irregular extinctions of species from asteroid strikes might be peculiar to our solar system and might have driven the progress of evolution on Earth at a much faster rate than might be the case for an otherwise earth-like planet that lacks such a "pump". Asteroid bombardment of Earth from the Kirkwood gaps might be the reason we are here at all.
Our civilization has shown an increasing unwillingness to assume risk. There are growing demands for "absolutely safe" pharmaceuticals, automobiles, nuclear power plants, and working environments, as certified by some burgeoning federal bureaucracy. If each of us is running a lifetime risk of 1 in 6,000 of being killed by a rock from space, can that risk be considered acceptable? Let's assume that it is not. Then what can be done to reduce the risk to an acceptable level?
The first thing required is information. Up to now most of the Apollo asteroids that have made close passes to the Earth have been detected after they have gone by. NASA currently spends less than $1 million per year in searching for Earth-crossing asteroids. A single 36-inch telescope at Kitt Peak equipped with a specially designed electronic detector is used in this search. The largest Earth-crossing asteroid that has been tracked so far is about 10 kilometers in diameter, just the size of Alvarez's dinosaur killer. But there are many more out there about which we know nothing. And the resonances in the Kirkwood gaps are always adding more. Clearly we need to increase our investment in the technologies that can locate hazardous asteroids months or years before they come our way. A well-conceived and well-funded program for placing special asteroid-finding telescopes at sites on the Earth, in space, and perhaps on the Moon could locate perhaps 95% of the Earth-crossing asteroids in the next decade. Then we could at least make a reasonable assessment of risk.
But there must also be a way of dealing with an asteroid hazard, once it is detected. Can killer asteroids be deflected from incipient collisions with the Earth? In works of science fiction about asteroid and comet strikes, the traditional method of dealing with the problem is to launch nuclear-tipped ICBMs at the incoming menace a few hours before impact, so that the protagonists can save the Earth in the nick of time, as it were. Unfortunately, this is a flawed strategy. It is easy to deflect the dangerous object when it is moving slowly and is far away. It is nearly impossible when the object is fast and close. And a nuclear warhead may do more harm than good. Breaking up the rock into a several smaller rocks, all of which strike the surface, is like MIRVing a nuclear warhead and would probably do more net damage than if the large object had remained in one piece.
On the other hand, early detection of a dangerous asteroid provides a sizable advantage. A small robot space vehicle could be dispatched to rendezvous with the asteroid and to apply the gentle persuasion of a small velocity change to steer it into a safe orbit. This could be done with a small ion engine, or perhaps with a light-sail reflecting sunlight and/or laser pulses beamed from Earth. Nuclear bombs might also do the job, but since the strength and rigidity of the lump of rock to be steered would be unknown, it is likely that a small and continuous applied impulse would be preferable to the large, short impulse from a bomb. Perhaps we could even "tame" a dinosaur-killer asteroid by steering them it into a near-earth orbit from which it could be mined for raw materials for space-based engineering projects. NASA, always on the lookout for expensive new projects to keep its standing army of bureaucrats and engineers at full employment, is studying the asteroid problem. You can expect to hear more soon.
I should also comment on the down-side of asteroid-steering technology. What can be deflected away from the Earth can also be deflected toward the Earth. The nation or group that becomes the master of the technology can not only protect the planet but can also apply persuasive extortion to other nations, selling them "insurance" against asteroid strikes that might otherwise "accidentally" hit their population centers. Niven and Pournelle's Mote in God's Eye described an alien civilization that had made war in just this way.
So it seems to come down to the old problem. We need protection from cosmic accidents. We need a dedicated group who will watch the Apollo asteroids and deflect them away when necessary. But who is to watch the asteroid watchers?
A final note: this column marks an anniversary of sorts, in that it is my 50th Alternate View column for Analog. My first column, on the subject of magnetic monopole induced proton decay, appeared 7 years ago in the July, 1984 issue. Analog has provided me with a wonderful soap box from which to describe new and interesting physics, and I've enjoyed it. I hope you have.
Asteroids and Impacts:
The New Solar System, J. K. Beatty, B. O'Leary, and A. Chaikin, eds., Sky Publishing Company, Cambridge, MA (1981);
William J. Broad, The New York Times, page B5, June 18, (1991);
The Resonances in the Asteroid Belt:
R. A. Kerr, Science 228, 1186 (1985).
This page was created by John G. Cramer on 7/12/96.