Comet Busters
By Peter Tyson

Fortunately, we have more at our disposal than panic in dealing with an intruder from outer space. For the first time in earth's history, one of its species is capable of doing something about an approaching juggernaut, rather than simply taking it like a dinosaur. Ideas range from hitting an asteroid head-on with a mass of heavy metal to unfurling a ``solar sail'' that, over decades, could carry a threatening asteroid out of its earth-crossing orbit.

Not all of us needed the Jovian fireworks to remind us that the danger of an impact is more than just Hollywood fantasy. In the early 1990s, NASA convened a pair of workshops addressing the impact hazard - one on finding and another on destroying earth-crossing asteroids, or ECAs. The detection group, chaired by David Morrison of the NASA/Ames Research Center at Moffett Field, Calif., generated a report detailing the dimensions of the threat.

The earth lies at the center (to be terracentric about it) of a cosmic shooting gallery. Many millions of small worlds, remnants of the solar system's formationthat never got the chance to coalesce into planets, race through space at velocities relative to the earth of up to 75 times the speed of sound. Some are slabs of rock, dirt, or metal (asteroids); some are sooty balls of ice only just held together by gravity (comets). Astronomers say the smallest asteroids, those less than 10 meters across, continually intersect earth, burning up harmlessly in the upper atmosphere. The heftier ones - some as big as a mountain range - come far less frequently. But they do come.

Our own planet would be pockmarked like the moon were it not for erosion, plate tectonics, and other geological forces that reshape the surface. Indeed, geologists have discovered more than 130 weathered impact craters - the largest a 200-kilometer-wide site at the northern end of the Yucatan Peninsula that marks the spot of the asteroid impact believed to have done in the dinosaurs 65 million years ago. Morrison and his colleagues estimate that an asteroid hits earth with the force of 2,500 Hiroshimas about once a millennium and, once every million years or so, with the equivalent of a million megatons of TNT, or 50 million Hiroshimas.

Even though a million years is a long time, there's no reason to be complacent about the Big One, according to Thomas Gehrels, a University of Arizona astronomer who, since 1984, has headed up one of the few continuous efforts to find and define the trajectories of ECAs. ``In the first place,'' he says, ``the very small chance of it happening tomorrow is just as great as a million years from now. Second, the consequences are horrendous: modeling shows us that society as we know it would be destroyed.'' A large strike, researchers say, would throw so much dust and ash into the stratosphere that a nonnuclear ``winter'' could wipe out one or more growing seasons, causing epidemic disease and starvation worldwide. ``Instead of summer, fall, winter, spring,'' one physicist has remarked, ``what you get is summer, fall, winter, winter, winter, winter, winter . . .

Gehrels's effort, called Spacewatch, may soon have government-sponsored company. Given a wake-up call by Shoemaker-Levy 9, Congress last summer ordered NASA to appoint a committee to recommend how best to detect within a decade all ECAs bigger than one kilometer across. (One kilometer is believed to be the minimum size that would trigger the feared ``winter.'') Internationally, the asteroid threat is being taken no less seriously. Russia, Italy, and several other countries have recently hosted conferences on the hazard, and in April the United Nations will hold one of its own.

Nuke 'Em

First, what is the object made of? Near-earth objects (NEOs), of which a tiny fraction may be earth-crossing objects, come in all shapes and sizes, but they are believed to comprise three basic types. Asteroids are either metal, as was the nickel-iron slab that carved the kilometer-wide Meteor Crater near Winslow, Ariz., or rock, like the supposed 80-meter chondritic (round-granuled) slab that exploded over the Tunguska region of Siberia in 1908, flattening trees across an area the size of the Los Angeles Basin. The third type is comets. The Giotto spacecraft, which flew through Comet Halley's tail in 1986, gave astronomers their first taste of what comets are now believed to contain: roughly a third each of ice, clay, and organic matter, including hydrocarbons in the form of an oil-like tar. Dealing with a heavy-metal asteroid presents different challenges from dealing with a loose agglomeration of matter such as a comet (a dirty snowball, as some astronomers call it).

The amount of warning time is even more important than the object's composition. Astronomers say that if they were given the proper technology and funding, their chances of discovering earth-bound asteroids far enough in advance of an impact are very good. But comets pose a graver danger. While asteroids move at about 25 kilometers per second (roughly 50,000 m.p.h.), comets typically approach earth at more than twice that speed. A comet would strike with 10 times the energy of a comparably sized asteroid and give earthlings only half the time to intercept it in space. Perhaps most disturbing is the surprise factor of the so-called long-period comets. Because their orbits exceed 200 years, many of them have never been charted and can appear suddenly as if from nowhere. ``We don't tend to detect long-period comets more than a year in advance,'' says Clark Chapman, an astronomer at the Planetary Science Institute in Tucson, Ariz.

The third critical factor in any interception scenario is the intruder's size. Objects smaller than about 100 meters in diameter could be stopped by simply smashing something big into them: Boeing's Lightweight Exoatmospheric Projectile, an impactor device developed under the Strategic Defense Initiative, would work well, researchers say. But any object significantly larger - especially one beyond the ``winter''-causing 1,000-meter diameter - would require draconian measures.

Physicists agree that the only way to generate enough energy to deal with a large object on short notice would be with a nuclear device. ``A nuclear weapon has the highest energy per unit mass, and we're limited right now by the amount of mass we can put in space,'' says Gregory Canavan, a physicist at Los Alamos National Laboratory who coedited the proceedings of the 1992 NASA interception workshop. Either a U.S. Titan missile or a Proton, which the Russians routinely use to launch military payloads into low-earth orbit, could be used as a booster rocket, he says. To ensure that the object or objects were destroyed and not merely fragmented, with pieces still raining down on earth, Edward Teller, the developer of the hydrogen bomb, says simply that he would send up enough explosives to make sure the job was done right. ``In other words,'' he says, ``we are very sick, I have a cure, and my only concern is to achieve overkill.''

If overkill did not succeed and large chunks still came at earth, researchers say the danger could actually be greater than if the original object were left alone. Recent studies show that a host of fair-sized pieces could have more devastating global consequences than a single ``winter''-causing object, by igniting many separate conflagrations that merge into a global firestorm. For this reason, the Planetary Science Institute's Chapman says that if the lead time were very short, he would prefer to mount efforts to ride an impact out, such as evacuating the region expected to become ground zero and stockpiling food, rather than risk worldwide incineration.

To avoid the dangers of fracturing an object, many researchers say the preferred method would be to nudge it slightly off course so that it bypasses earth, just as a hockey goalie deflects a speeding puck. The amount an ECA needs to be pushed out of a lethal orbit depends on the lead time: long-period comets first detected on the home stretch would need to be shoved sideways at speeds of a few meters to a few hundred meters per second to miss earth (velocities that, in the frictionless environment of space, would be maintained). ECAs with trajectories that can be calculated years or decades ahead of time - that is, billions of kilometers from impact - might call for only a few-centimeters-per-second nudge.

Johndale Solem, a mathematical physicist at Los Alamos whose papers on deflection scenarios could fill a fat three-ring binder, says the most efficient way to nudge an ECA is by detonating a neutron bomb - a device capable of producing a relatively gentle ``blanket'' of force instead of a violent explosion - or a series of such bombs. Solem's studies show that a so-called standoff burst, detonated about half of the ECA's radius above its surface, would offer optimal nudging with the least chance of splintering. While bringing two speeding bodies together with such pinpoint accuracy might seem daunting, Canavan, who chaired the interception workshop's session on homing technologies, believes it could be done with adaptations of existing ``smart-bomb'' devices.

Solem calculates that the most a standoff burst could move a kilometer-sized object without fragmenting it would be half a meter per second using a 4-megaton nuclear explosion. Given that the earth's radius is 6,370 kilometers, he notes, a rocket would have to hit the asteroid at least five months ahead of time for it to miss earth. With more lead time, less explosive power could be used. In a review article in Nature (December 3, 1992), Thomas Ahrens of Caltech and Alan Harris of the Jet Propulsion Laboratory in Pasadena, Calif., estimate that a 100-kiloton standoff burst would suffice to change the velocity - and hence the orbit - of a one-kilometer-wide object by one centimeter per second.

A series of one-two punches would further lessen the chance of fragmentation. ``The first wave of bombs flashes, blisters the surface, and raises a great cloud of dust,'' says Anthony Zuppero, a physicist at the Idaho National Engineering Laboratory in Idaho Falls. ``The second wave of bombs, milliseconds later, explodes and takes that dust cloud and blows it up and makes it push the object.''

With World Enough and Time

The next step would be actual landings. While dramatically more expensive, soft landings by humans or robots could verify and fine-tune the information gleaned from a flyby, physicists say. Such data include an object's size, shape, volume, mass, gravity field, and spin state, as well as surface properties such as mineral composition and texture. But even hard landings could be informative. If an approaching spacecraft were to fire off a seismograph to the surface before crashing, says Canavan, the instrument could record the acoustic waves from the craft's impact and radio this information back to earth, allowing scientists to calculate the object's interior structure and strength.

If they had still more time on their hands - at least a century - to respond to an incoming ECA, some physicists cite a number of futuristic technologies that might be harnessed to help stop a doomsday asteroid or comet. ``Spark guns,'' for example - such as those that Sandia National Laboratories developed for Star Wars - would be landed on the threatening object and would generate brief, intense electric sparks to send chunks of the ECA's own surface material into space many times a second, propelling the object in the opposite direction.

Anthony Zuppero, the Idaho physicist, proposes using a ``steam rocket'' to nudge a threatening object by pushing against it like a tugboat against a tanker. An on-board nuclear reactor would boil water and direct the steam out the rocket's nozzles. Since it would be impractical to launch enough water from earth, Zuppero proposes mining it from ``nearby'' comets. (Remember, these are futuristic scenarios.)

Perhaps the most elegant means proposed so far to nudge an ECA is ``solar sails.'' The concept is simple: unfurl a giant reflector from the object's surface and let the radiation streaming out from the sun ease the asteroid into a new orbit. No nuclear devices required, only plenty of time.

While physicists delight in discussing such advanced interception concepts, all agree that the obvious solution today or even decades from now is nuclear. ``If you have a huge amount of time, you can talk about solar sails or putting engines on the thing,'' Solem says. ``But when you get down to the question of what is cost-effective, a real hardheaded look always comes back with a nuclear device.''

What Next?

A Congress-approved survey would add to those already under way. Since 1984, Gehrels's Spacewatch program has searched the skies continuously, using equipment that is less powerful than that proposed for Spaceguard - the University of Arizona's old 36-inch telescope. Spacewatch espies hundreds of objects each night, most in the asteroid belt; one in a thousand is an ECA. A radar astronomer, Steven Ostro of the Jet Propulsion Laboratory, also ranges and images NEOs with radars at the Goldstone tracking station in California's Mojave Desert and the Arecibo Observatory in Puerto Rico. The Air Force Space Command may soon add one or more of its array of GEODSS telescopes, currently used to track earth-orbiting satellites, to the search.

While the United States arguably lies at the forefront of this field, it has plenty of company. Canavan, the Los Alamos physicist, recalls visiting with astronomers in Russia in 1991 during the very week the Soviet Union fell apart. ``They wanted to talk about asteroid defenses,'' he says. ``It was kind of charming.'' All the nuclear nations and dozens of nonnuclear ones have sent representatives to various recent conferences on the hazard, and all have called for improved detection.

Astronomers and physicists also unanimously call for more theoretical work to gain a better understanding of NEOs and impact hazards. ``We don't really know the composition or strength of these objects very well,'' notes Canavan. ``How hard can you push on them, for example, before they break up into a bunch of fragments? That's precisely the kind of physics that needs to be measured during direct experiments in space.''

Given the remoteness of the threat, some experts wonder whether actual space tests are worth the cost. Technological advances in coming decades, some scientists argue, may make obsolete any experiments today. Space tests would occur at the expense of other space research, not to mention more pressing earthly issues such as overpopulation and environmental degradation. Since space missions typically run about $200 million, the Planetary Science Institute's Chapman says, a single deflection experiment would be at least that and probably more. ``At some point,'' he says, ``you cross the line where, given the improbabilities of such a disaster, it's difficult to justify spending billions of dollars on it.'' Canavan disagrees. He calculates that, even with the slim chances, the expected losses from the impact of a large ECA - say, two kilometers across - would amount to $400 million a year over the course of 1 million years. In a chapter for the forthcoming book Hazards Due to Comets and Asteroids, Canavan arrives at that figure by multiplying the world's total gross product of $20 trillion a year by the presumed 20 years it might take to get back to normal after such an impact, then dividing by the estimated frequency of such an impact (once every million years). Compared with the cost of a disaster, a few hundred million dollars devoted to experiments in space would be a drop in the bucket. And without such experiments, says Canavan, scientists may never learn how to nudge a comet or asteroid without fragmenting it.

Space missions would yield scientific spin-offs as well, proponents argue. NEOs have ``more beauty than danger,'' says the University of Arizona's Gehrels, and through reconnaissance missions such as NEAR they can provide insights into the birth of the solar system. Astronomers believe near-earth asteroids in particular bear clues to the nature of the building blocks (known as ``planetesimals'') that formed the inner planets, including earth. Such missions could also set the stage for mining materials for use on earth, according to Zuppero. Certain NEOs are believed to contain industrially valuable metals such as platinum, palladium, iridium, and nickel, while other NEOs are thought to hold vast stores of hydrocarbons. ``Something 16 kilometers across, like Comet Halley, has 1,000 OPEC-years of hydrocarbons,'' Zuppero says. For his part, Edward Teller feels the greatest spinoffs would be in creating research jobs and sowing international cooperation in dealing with a threat that concerns all humanity.

Indeed, some see thorough study, including experiments, as the moral obligation of our species. Notwithstanding our responsibility to protect other species on the planet, Solem says, our first concern is for Homo sapiens. ``It's fairly conspicuous that if there is other intelligent life in the galaxy, it's not talking to us very loudly,'' he says. ``If intelligence is that rare, then it's precious and there's a strong cosmic need to maintain it.'' While few would dispute the importance of protecting ourselves, some feel the costs of reducing the impact risk must be weighed against other potential global disasters. Asteroids represent a hazard like any number of other perils that confront civilization, says the Jet Propulsion Laboratory's Ostro, and we must be ``strategically intelligent'' in reducing risks from all sources. In a recent article in Issues in Science and Technology he coauthored with Carl Sagan, Ostro offers examples of other low-probability but potentially catastrophic risks, including the rise of drug-resistant lethal organisms or the chance that AIDS, through a mutation or transgenic exchange, could become as infectious as the common cold. Ostro and Sagan note that even annual deaths from cigarette smoking or diarrhea - millions each worldwide - arguably deserve more of our attention than potential cosmic impacts. ``A critic might contend,'' they write, ``that with limited global resources, the human species would benefit much more from global anti-smoking and oral-rehydration campaigns.''

Weighing the economic cost of testing nuclear charges in space is one thing; weighing the political cost is quite another. For starters, two agreements - the 1963 Limited Test Ban Treaty and the 1967 Outer Space Treaty - forbid the use of nuclear weapons in space. Even if authorities deemed the threat serious enough to relax the rules, harnessing nuclear explosives as instruments of salvation, even on a trial basis, might be a hard sell to a public still reeling from decades of nuclear anxiety.

Costs, both economic and political, arguably would be an order of magnitude or two higher if the next logical step were taken - namely, developing and deploying actual defensive systems. In their Issues article, Ostro and Sagan argue strongly against preparing any such system now. The threat, in their minds, is too remote to justify the costs, much less the danger that such a system could fall into the hands of a Hitler or a Stalin. The two astronomers also suggest that if researchers can devise ways to divert a threatening asteroid away from earth, then at some point over the next few hundred thousand years they may also be able to divert an otherwise harmless asteroid into earth.

Most researchers in the field agree that developing a defensive system before the full dimensions of the threat are known would be premature. Solem proposes a graduated approach to dealing with the hazard. Get Spaceguard under way, continue theoretical studies, perhaps undertake an experiment or two in space to learn more about NEOs. At least a decade of such work will be required, Solem estimates, before results indicate whether we should build a defensive system.

If the benefits were deemed to outweigh the risks, what would such a system look like? Solem imagines half a dozen unarmed boosters under the control of the United Nations. The boosters could be modified Titan IVs - ``we seem to have more of them than anything else,'' he notes - or even something smaller. (The Spartan missile, for example, was designed to carry a four-megaton warhead, the same size needed to deflect a one-kilometer object five months ahead of time.) In Solem's hypothetical system, the nuclear payload would be held separately by a consortium of nuclear nations. When and if a threat developed, the boosters could be married with the explosives, transported to a launch pad, and dispatched. The cost to maintain such a system would run about $100 million a year, Solem figures. ``That's reasonably conservative but also reasonably reasonable,'' he says. ``If it was decided that you would need to be able to pull something together in a week,'' he adds, ``then you'd have to have the boosters on standby status.''

Which, ironically, puts it just in the range of the response depicted in the movie Meteor. Though as Hollywood as they come, the film was actually inspired by a 1967 engineering class at MIT in which students, presented with a hypothetical asteroid strike, came up with a nuclear missile-based solution. In fact, while Meteor may have seemed to viewers little more than science fiction when it appeared, 16 years later it seems remarkably prescient. Last July's fusillade of fragments on Jupiter echoed those depicted in the film. The movie's overnight thaw of Cold War animosities - embodied in actor Brian Keith's too-quick-to-grin Soviet physicist Dubov - surely must have rung false with 1979 viewers, but in today's post-glasnost climate, it seems quite plausible. Finally, NASA's nuclear solution in the film, in which the joint Russian-American rocket force successfully pulverizes the incoming asteroid, is precisely the one we would use now under similar conditions, though perhaps minus the thrilling crescendo of martial music.

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