Escaping the ultimate disaster - a cosmic collision.

Large celestial impacts don't happen often, but they can wipe out entire civilizations at once. An expert tells how we could protect ourselves - and even benefit - from these space invaders.

There is something especially horrifying about unanticipated, rare, or unfamiliar lethal hazards, such as the possibility of an asteroid hitting Earth and wiping us out.

Americans know that about 50,000 people will be killed on their highways each year, though usually only one or two at a time. Yet this knowledge does not deter them from driving vast numbers of miles each year. In contrast, a single fatal airline accident involving 100 people takes on a spectacular aspect precisely because the rare fatalities, when they do occur, involve 100 or more deaths. Thus a single airline crash that kills 100 people is 100 times as visible as 10,000 highway crashes that kill 10,000 people.

So it is with the prospect of cosmic collisions, which may kill thousands, millions, and even billions of people in a single event.

Clearly, the actual importance of many threats differs radically from their perceived danger. How, then, would people react to a normal hazard that affects the entire planet, but occurs only very infrequently? Among forest fires and brushfires, earthquakes, lightning, landslides, floods, coastal storms, tornadoes, tidal waves, and impact events (i.e., asteroid, meteorite, or comet collisions), the only hazard that could cause the destruction of human civilization or the extinction of the human species is a large impact.

There is evidence that such catastrophic impacts have happened before on Earth. Paleontologists studying the pattern of appearance and disappearance of species in the fossil record have long been aware of abrupt, devastating global extinction events occurring at the ends of geological ages. In 1981, researchers discovered that a thin, global sediment layer that separates the end of the Cretaceous era (the last period of the age of dinosaurs) from the beginning of the Tertiary era (the start of the age of mammals) contained the unmistakable signature of an asteroid or comet impact.

And what actually happened 8,000-10,000 years ago to end the hunter-gatherer chapter in human history? Quite suddenly, agriculture became common, specialized occupations arose, and cities appeared. Writing was invented, giving rise to record keeping and literacy. And the earliest human records all relate stories of floods that devastated civilization. . . . What did happen then? Was the clock of human history reset to zero by an event (or more than one) that devastated civilization?

The Dangers of Impacts

There is a wide range of lethal consequences of asteroid and comet collisions: The death or injury of individuals struck by a falling meteorite affects probably one to 10 people per century. Villages or cities can be struck by showers of meteorites from high-altitude airbursts about once per century. Also about once per century, iron or other physically strong meteorites may resist atmospheric breakup to strike the surface as a single crater-forming body or as a compact shower of iron shrapnel. And low-altitude megaton air-bursts should also strike at populated areas every century or so, setting fires, shattering windows, and demolishing buildings over an area of hundreds to thousands of square kilometers.

About half of impact fatalities are caused by the smaller, more frequent, localized events. About a quarter of the total deaths arise from tsunamis caused by impacts (once every 10,000 years), and another quarter from continental cratering events and low airbursts.

Every 70,000 to 1 million years, a global billion-casualty killer will strike Earth: Collisions of 10-gigaton objects may occur about every 70,000 years; 100-gigaton explosions occur about once per 250,000 years; and 1,000-gigaton events occur a little more than once per 1 million years. If your projected life-span is about 75 years, that means the probability that you will be killed in a global impact event is between 0.01% and 0.1%. By comparison, the probability that you will be killed in a civil airliner crash is 0.005%.

The long-term average death rate from impacts is 4 billion people per million years, or 4,000 people per year worldwide. The people of the United States make up about 5% of the global population, so the average American death rate from global-scale impacts is about 200 per year. By comparison, the death rate of American citizens from commercial aircraft crashes is 100 people a year.

As we have found with hurricanes, predicting impact events could eliminate much of the horror and lethality associated with them. Cataloging the orbits and properties of Earth-crossing objects is already in progress and could be scaled up at modest expense. If we had a discovery and tracking capability, areas threatened by gigaton impactors could at the very least be evacuated. This would be ineffectual at reducing the cost of physical damage and economic dislocation, but would at least reduce the death toll to near zero.

The problem with finding and tracking these very large bodies is that evacuation does not work; the effects of the disaster are global. The leading cause of death would probably be famine induced by climate change. If such a body hits Earth, there are no places to which refugees can be relocated. Moving away from the computed impact area means selecting a slow death over a quick one, since Earth's ability to support life would be universally diminished.

Finding, tracking, and predicting the orbits of kilometer-sized bodies is neither technically demanding nor fiscally draining; rather, the problem arises when we ask what we would do with the knowledge. We can in fact do nothing meaningful to avoid this threat unless we use space technology to divert or destroy the threatening objects. The prospect of letting one hit our densely populated planet is unacceptable.

What Can We Do?

A search-and-tracking system to find bodies down to 250 meters in diameter in near-Earth space would likely find a number of objects in threatening orbits. With probable lead times of centuries, evacuation plans can easily be made and executed. The impact could then be used by fiscally conservative governments as a sort of instant urban renewal program.

But with so much lead time, given the rate of advance of technology, might it not prove much less expensive and inconvenient to do something to avert the impact of a threatening object? And what exactly should we do?

Why not blow it up?

Bad idea. If we split an approaching one-gigaton object into 10 equal pieces of 100 megatons of energy each, they'd strike Earth like a giant shotgun pattern. The main effect of breaking up the threatening impactor would be to increase the damage done. The disruption of a threatening impactor is clearly not a sensible option unless we are certain that almost all of its fragments can be diverted so as to miss Earth.

But if we have the ability to divert dozens of pieces, why not elect the simpler option of gently diverting the whole thing?

The idea of diverting the course of an asteroid that is several hundred meters in diameter seems breathtakingly ambitious. Yet, human mining activities routinely crush, excavate, and move comparable volumes of rock. There is an important factor that makes this scenario much less daunting: We are merely trying to avoid a single predicted impact with Earth.

Suppose our asteroid-search team finds a 250-meter body that is due to hit Earth dead center a few hundred years from now. This same body has probably been crossing Earth's orbit for 10 million to 100 million years without an impact. If we can just ease it by Earth without an impact on this one occasion, we may well buy ourselves another 30 million years to figure out what to do the next time it threatens us. So the real problem is not to devise a permanent fix; it is to avoid a specific near-term event.

We might give the asteroid a small sideways nudge so that, when it reaches Earth, it will skim by to one side of the planet rather than strike it directly. Or we could accelerate or decelerate the asteroid along its direction of orbital motion so as to change its orbital period slightly. This would cause the asteroid to cross Earth's orbit a little ahead of or behind the impact schedule it was following, and hence cross Earth's orbit at a point ahead of or behind Earth.

There are many methods available for making such small changes in the velocity of an asteroid. One of the favorite techniques proposed by military experts is to explode a small nuclear warhead well clear of the surface of the asteroid. But simply launching an existing intercontinental ballistic missile at the asteroid would not work: Such vehicles cannot achieve escape velocity to reach an asteroid on its orbit around the Sun. Further, missile guidance systems are designed to operate for the half-hour of an intercontinental trip - not the weeks or months required for the trip to an asteroid. The mission would have to be accomplished by a military warhead combined with a NASA planetary spacecraft bus that provides guidance and power.

Readers concerned about the environmental impact of such an explosion should realize that the asteroid would not be contaminated to any significant degree by radioactive bomb debris, since the surface layer would be boiled off by the blast. The bomb vapor would be swept out of the solar system by the solar wind at a speed of about 600 kilometers per second. The net result of the asteroid deflection is really a twofold benefit to Earth: A devastating impact would be avoided, and there would be one less nuclear warhead on Earth.

Other options for deflecting asteroids and comets include chemical propulsion, electrical propulsion, nuclear thermal propulsion, solar thermal propulsion, and solar sailing.

The choice of flight hardware for an asteroid-deflection mission clearly depends on what technological options are available at the time the problem arises. In general, of course, the more different technologies we have to choose from, the more likely we are to have a good choice. It is quite impossible to guess what the preferred solution will be in the year 2010 or 2050, let alone 2300 or 5875. But if a threatening asteroid were discovered this year and action had to be taken in the next 10 to 20 years, we would be forced to choose quickly. We almost certainly would have to make do with existing, tested technology, such as the nuclear war-head option.

Don't Panic

Our central conclusion is that there is no reason to panic. There is a real threat, confirmed by a wide range of evidence, but we are not helpless in the face of cosmic bombardment. We certainly have no reason to ignore the impact hazard. Once we understand that the threat exists, and once we begin to collect the information we need to deal with the threat intelligently, there is no longer any need to retreat into denial.

First, we have the technical ability to discover and track almost every body that poses a global threat. Within a few decades, a nearly complete catalog of the larger near-Earth asteroids and short-period comets could be compiled for modest cost with known technology.

Second, massive objects found to be on threatening orbits can be deflected using techniques and hardware developed by NASA and the Defense Department.

There is another way of looking at near-Earth bodies: as opportunities rather than threats. A large proportion of the most-threatening objects are also the most-accessible bodies in the solar system for spacecraft missions from Earth. These bodies are the most-promising sources of raw materials for a wide range of future space activities. They may provide the propellants for future interplanetary expeditions, the metals for construction of solar power satellites to meet Earth's energy needs in the third millennium, the life-support materials and radiation shielding to protect space colonies, and the precious and strategic metals needed by Earth's industries.

For instance, the smallest known metallic asteroid, 3554 Amun, contains over $1 trillion worth of cobalt, $1 trillion worth of nickel, $800 billion worth of iron, and $700 billion worth of platinum. The total value of this single small asteroid is approximately equal to the entire national debt of the United States. By comparison, the uncontrolled impact of Amun with Earth would deliver a devastating 7-million-megaton blow to the biosphere, killing billions of people and doing hundreds of trillions of dollars worth of damage.

Thus we come to our final, and most startling, discovery: The stick that threatens Earth is also a carrot. Every negative incentive we have to master the impact hazard has a corresponding positive incentive to reap the bounty of mineral wealth in the would-be impactors by crushing them and bringing them back in tiny, safe packages, a few hundred tons at a time, for use both in space and on Earth.

Remember that we will almost certainly have hundreds to thousands of years of warning time before a threatening global-scale impact. We need not be driven to rash and risky actions taken precipitously under threat of death. We will almost certainly have plenty of time to deal with the problem.

Dealing with near-Earth objects should not be viewed grudgingly as a necessary expense: It is an enormously profitable investment in a limitless future. It is a liberation from resource shortages and limits to growth. It is an open door into the solar system - and beyond.

About the Author

John S. Lewis is co-director of the NASA/University of Arizona Space Engineering Research Center and commissioner of the Arizona State Space Commission. His address is Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721. E-mail jsl@u.arizona.edu.

He is the author of Rain of Iron and Ice: The Very Real Threat of Comet and Asteroid Bombardment (Addison-Wesley, 1996), on which this article is based. His latest book is Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets (Addison-Wesley, 1996); both books are available from the Futurist Bookstore; see page 59 for details.