THE MARSH OF CAMARINA -Pale Blue Dot_ A Vision of the Human Future in Space
It’s too late to make any improvements now. The universe is finished; the copestone is on, and the chips were carted off a million years ago.
HERMAN MELVILLE, MOBY DICK, CHAPTER 2 (1851)
Camarina was a city in southern Sicily, founded by colonists from Syracuse in 598 B.C. A generation or two later, it was threatened by a pestilence festering, some said, in the adjacent marsh. (While the germ theory of disease was certainly not widely accepted in the ancient world, there were hints-for example, Marcus Varro in the first century B. C. advised explicitly against building cities near swamps “because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious disease.”) The danger to Camarina was great. Plans were drawn to drain the marsh. When the oracle was consulted, though, it forbade such a course of action, counseling patience instead. But lives were at stake, the oracle was ignored, and the marsh was drained. The pestilence was promptly halted. Too late, it was recognized that the marsh had protected the city from its enemies among whom there had now to be counted their cousins the Syracusans. As in America 2,300 years later, the colonists had quarreled with the mother country. In 552 B.C., a Syracusan force crossed over the dry land where the marsh had been, slaughtered every man, woman, and child, and razed the city. The marsh of Camarina became proverbial for eliminating a danger in such a way as to usher in another, much worse.
THE CRETACEOUS-TERTIARY COLLISION (or collisions there may have been more than one) illuminates the peril from asteroids and comets. In sequence, a world-immolating fire burned vegetation to a crisp all over the planet; a stratospheric dust cloud so darkened the sky that surviving plants had trouble making a living from photosynthesis; there were worldwide freezing temperatures, torrential rains of caustic acids, massive depletion of the ozone layer, and, to top it off, after the Earth healed itself from these assaults, a prolonged greenhouse warming (because the main impact seems to have volatilized a deep layer of sedimentary carbonates, pouring huge amounts of carbon dioxide into the air). It was not a single catastrophe, but a parade of them, a concatenation of terrors. Organisms weakened by one disaster were finished off by the next. It is quite uncertain whether our civilization would survive even a considerably less energetic collision.
Since there are many more small asteroids than large ones, run-of-the-mill collisions with the Earth will be made by the little guys. But the longer you’re prepared to wait, the more devastating the impact you can expect. On average, once every few hundred years the Earth is hit by an object about 70 meters in diameter; the resulting energy released is equivalent to the largest nuclear weapons explosion ever detonated. Every 10,000 years, we’re hit by a 200-meter object that might induce serious regional climatic effects. Every million years, an impact by a body over 2 kilometers in diameter occurs, equivalent to nearly a million megatons of TNT an explosion that would work a global catastrophe, killing (unless unprecedented precautions were taken) a significant fraction of the human species. A million megatons of TNT is 100 times the explosive yield of all the nuclear weapons on the planet, if simultaneously blown up. Dwarfing even this, in a hundred million years or so, you can bet on something like the Cretaceous-Tertiary event, the impact of a world 10 kilometers across or bigger. The destructive energy latent in a large near-Earth asteroid dwarfs anything else the human species can get its hands on.
As first shown by the American planetary scientist Christopher Chyba and his colleagues, little asteroids or comets, a few tens of meters across, break and burn up on entering our atmosphere. They arrive comparatively often but do no significant harm. Some idea of how frequently they enter the Earth’s atmosphere has been revealed by declassified Department of Defense data obtained from special satellites monitoring the Earth for clandestine nuclear explosions. There seem to have been hundreds of small worldlets (and at least one larger body) impacting in the last 20 years. They did no harm. But, we need to be very sure we can distinguish a small colliding comet or asteroid from an atmospheric nuclear explosion.
Civilization-threatening impacts require bodies several hundred meters across, or more. (A meter is about a yard; 100 meters is roughly the length of a football field.) They arrive something like once every 200,000 years. Our civilization is only about 10,000 years old, so we should have no institutional memory of the last such impact. Nor do we.
Comet Shoemaker-Levy 9, in its succession of fiery explosions on Jupiter in July 1994, reminds us that such impacts really do occur in our time and that the impact of a body a few kilometers across can spread debris over an area as big as the Earth. It was a kind of portent.
In the very week of the Shoemaker-Levy impact, the Science and Space Committee of the U.S. House of Representatives drafted legislation that requires NASA “in coordination with the Department of Defense and the space agencies of other countries” to identify and determine the orbital characteristics of all Earth-approaching “comets and asteroids that are greater than 1 kilometer in diameter.” The work is to be completed by the year 2005. Such a search program had been advocated by many planetary scientists. But it took the death throes of a comet to move it toward practical implementation.
Spread out over the waiting time, the dangers of asteroid collision do not seem very worrisome. But if a big impact happens, it would be an unprecedented human catastrophe. There’s something like one chance in two thousand that such a collision will happen in the lifetime of a newborn baby. Most of us would not fly in an airplane if the chance of crashing were one in two thousand. (In fact for commercial flights the chance is one in two million. Even so, many people consider this large enough to worry about, or even to take out insurance for.) When our lives are at stake, we often change our behavior to arrange more favorable odds. Those who don’t tend to be no longer with us.
Perhaps we should practice getting to these worldlets and diverting their orbits, should the hour of need ever arise. Melville notwithstanding, some of the chips of creation are still left, and improvements evidently need to be made. Along parallel and only weakly interacting tracks, the planetary science community and the U.S. and Russian nuclear weapons laboratories, aware of the foregoing scenarios, have been pursuing these questions: how to monitor all sizable near-Earth interplanetary objects, how to characterize their physical and chemical nature, how to predict which ones may be on a future collision trajectory with Earth, and, finally, how to prevent a collision from happening.
The Russian spaceflight pioneer Konstantin Tsiolkovsky argued a century ago that there must be bodies intermediate ill size between the observed large asteroids and those asteroidal fragments, the meteorites, that occasionally fall to Earth. He wrote about living on small asteroids in interplanetary space. He did not have military applications in mind. In the early 1980s, though, some in the U.S. weapons establishment argued that the Soviets might use near-Earth asteroids as first-strike weapons; the alleged plan was called “Ivan’s Hammer.” Countermeasures were needed. But, at the same time, it was suggested, maybe it wasn’t a bad idea for the United States to learn how to use small worlds as weapons of its own. The Defense Department’s Ballistic Missile Defense Organization, the successor to the Star Wars office of the 1980s, launched an innovative spacecraft called Clementine to orbit the Moon and fly by the near-Earth asteroid Geographos. (After completing a remarkable reconnaissance of the Moon in May 1994, the spacecraft failed before it could reach Geographos.)
In principle, you could use big rocket engines, or projectile impact, or equip tile asteroid with giant reflective panels and shove it with sunlight or powerful Earth-based lasers. But with technology that exists right now, there are only two ways. First, one or more high-yield nuclear weapons might blast the asteroid or comet into fragments that would disintegrate and atomize on entering the Earth’s atmosphere. If the offending worldlet is only weakly held together, perhaps only hundreds of megatons would suffice. Since there is no theoretical upper limit to the explosive yield of a thermonuclear weapon, there seem to be those in the weapons laboratories who consider making bigger bombs not only as a stirring challenge, but also as a way to mute pesky environmentalists by securing a seat for nuclear weapons on the save-the-Earth bandwagon.
Another approach under more serious discussion is less dramatic but still an effective way of maintaining the weapons establishment a plan to alter the orbit of any errant worldlet by exploding nuclear weapons nearby. The explosions (generally near the asteroid’s closest point to the Sun) are arranged to deflect it away from the Earth. A flurry of lowyield nuclear weapons, each giving a little push in the desired direction, is enough to deflect a medium-sized asteroid with only a few weeks’ warning. The method also offers, it is hoped, a way to deal with a suddenly detected long-period comet on imminent collision trajectory with the Earth: The comet would be intercepted with a small asteroid. (Needless to say, this game of celestial billiards is even more difficult and uncertain and therefore even less practical in the near future than the herding of an asteroid on a known, well-behaved orbit with months or years at our disposal.)
We don’t know what a standoff nuclear explosion would do to an asteroid. The answer may vary from asteroid to asteroid. Some small worlds might be strongly held together; others might be little more than self-gravitating gravel heaps. If an explosion breaks, let’s say, a 10kilometer asteroid up into hundreds of 1-kilometer fragments, the likelihood that at least one of them impacts the Earth is probably increased, and the apocalyptic character of the consequences may not be much reduced. On the other hand, if the explosion disrupts the asteroid into a swarm of objects a hundred meters in diameter or smaller, all of them might ablate away like giant meteors on entering the Earth’s atmosphere. In this case little impact damage would be caused. Even if the asteroid were wholly pulverized into fine powder, though, the resulting high-altitude dust layer might be so opaque as to block the sunlight and change the climate. We do not yet know.
A vision of dozens or hundreds of nuclear-armed missiles on ready standby to deal with threatening asteroids or comets has been offered. However premature in this particular application, it seems very familiar; only the enemy has been changed. It also seems very dangerous.
The problem, Steven Ostro of JPL and I have suggested, is that if you can reliably deflect a threatening worldlet so it does not collide with the Earth, you can also reliably deflect a harmless worldlet so it does collide with the Earth. Suppose you had a full inventory, with orbits, of the estimated 300,000 near-Earth asteroids larger than 100 meters each of them large enough, on impacting the Earth, to have serious consequences. Then, it turns out, you also have a list of huge numbers of inoffensive asteroids whose orbits could be altered with nuclear warheads so they quickly collide with the Earth.
Suppose we restrict our attention to the 2,000 or so near-Earth asteroids that are a kilometer across or bigger that is, the ones most likely to cause a global catastrophe. Today, with only about 100 of these objects catalogued, it would take about a century to catch one when it’s easily deflectable to Earth and alter its orbit. We think we’ve found one, an as-yet-unnamed asteroid so far denoted only as 1991OA. In 2070, this world, about 1 kilometer in diameter, will come within 4.5 million kilometers of the Earth’s orbit only fifteen times the distance to the Moon. To deflect 1991OA so it hits the Earth, only about 60 megatons of TNT equivalent needs to be exploded in the right way the equivalent of a small number of currently available nuclear warheads.
Now imagine a time, a few decades hence, when all such near-Earth asteroids are inventoried and their orbits compiled. Then, as Alan Harris of JPL, Greg Canavan of the Los Alamos National Laboratory, Ostro, and I have shown, it might take only a year to select a suitable object, alter its orbit, and send it crashing into the Earth with cataclysmic effect.
The technology required large optical telescopes, sensitive detectors, rocket propulsion systems able to lift a few tons of payload and make precise rendezvous in nearby space, and thermonuclear weapons all exist today. Improvements in all but perhaps the last can be confidently expected. If we’re not careful, many nations may have these capabilities in the next few decades. What kind of world will we then have made?
We have a tendency to minimize the dangers of new technologies. A year before the Chernobyl disaster, a Soviet nuclear power industry deputy minister was asked about the safety of Soviet reactors, and chose Chernobyl as a particularly safe site. The average waiting time to disaster, he confidently estimated, was a hundred thousand years. Less than a year later . . . devastation. Similar reassurances were provided by NASA contractors the year before the Challenger disaster: You would have to wait ten thousand years, they estimated, for a catastrophic failure of the shuttle. One year later . . . heartbreak.
Chlorofluorocarbons (CFCs) were developed specifically as a completely safe refrigerant to replace ammonia and other refrigerants that, on leaking out, had caused illness and some deaths. Chemically inert, nontoxic (in ordinary concentrations), odorless, tasteless, nonallergenic, nonflammable, CFCs represent a brilliant technical solution to a well-defined practical problem. They found uses in many other industries besides refrigeration and air conditioning. But, as I described above, the chemists who developed CFCs overlooked one essential fact that the molecules’ very inertness guarantees that they are circulated to stratospheric altitudes and there cracked open by sunlight, releasing chlorine atoms which then attack the protective ozone layer. Due to the work of a few scientists, the dangers may have been recognized and averted in time. We humans have now almost stopped producing CFCs. We won’t actually know if we’ve avoided real harm for about a century; that’s how long it takes for all the CFC damage to be completed. Like the ancient Camarinans, we make mistakes. Not only do we often ignore the warnings of the Oracles; characteristically we do not even consult them.
The notion of moving asteroids into Earth orbit has proved attractive to some space scientists and long-range planners. They foresee mining the minerals and precious metals of these worlds or providing resources for the construction of space infrastructure without having to fight the Earth’s gravity to get them up there. Articles have been published on how to accomplish this end and what the benefits will be. In modern discussions, the asteroid is inserted into orbit around the Earth by first making it pass through and be braked by the Earth’s atmosphere, a maneuver with very little margin for error. For the near future we can, I think, recognize this whole endeavor as unusually dangerous and foolhardy, especially for metal worldlets larger than tens of meters across. This is the one activity where errors in navigation or propulsion or mission design can have the most sweeping and catastrophic consequences.
The foregoing are examples of inadvertence. But there’s another kind of peril: We are sometimes told that this or that invention would of course not be misused. No sane person would be so reckless. This is the “only a madman” argument. Whenever I hear it (and it’s often trotted out in such debates), I remind myself that madmen really exist. Sometimes they achieve the highest levels of political power in modern industrial nations. This is the century of Hitler and Stalin, tyrants who posed the gravest dangers not just to the rest of the human family, but to their own people as well. In the winter and spring of 1945, Hitler ordered Germany to be destroyed even “what the people need for elementary survival” because the surviving Germans had “betrayed” him, and at any rate were “inferior” to those who had already died. If Hitler had had nuclear weapons, the threat of a counterstrike by Allied nuclear weapons, had there been any, is unlikely to have dissuaded him. It might have encouraged him.
Can we humans be trusted with civilization-threatening technologies? If the chance is almost one in a thousand that much of the human population will be killed by an impact in the next century, isn’t it more likely that asteroid deflection technology will get into the wrong hands in another century some misanthropic sociopath like a Hitler or a Stalin eager to kill everybody, a megalomaniac lusting after “greatness” and “glory,” a victim of ethnic violence bent on revenge, someone in the grip of unusually severe testosterone poisoning, some religious fanatic hastening the Day of judgment, or just technicians incompetent or insufficiently vigilant in handling the controls and safeguards? Such people exist. The risks seem far worse than the benefits, the cure worse than the disease. The cloud of near-Earth asteroids through which the Earth plows may constitute a modern Camarine marsh.
It’s easy to think that all of this must be very unlikely, mere anxious fantasy. Surely sober heads would prevail. Think of how many people would be involved in preparing and launching warheads, in space navigation, in detonating warheads, in checking what orbital perturbation each nuclear explosion has made, in herding the asteroid so it is on an impact trajectory with Earth, and so on. Isn’t it noteworthy that although Hitler gave orders for the retreating Nazi troops to burn Paris and to lay waste to Germany itself, his orders were not carried out? Surely someone essential to the success of the deflection mission will recognize the danger. Even assurances that the project is designed to destroy some vile enemy nation would probably be disbelieved, because the effects of collision are planet-wide (and anyway it’s very hard to make sure your asteroid excavates its monster crater in a particularly deserving nation).
But now imagine a totalitarian state, not overrun by enemy troops, but one thriving and self-confident. Imagine a tradition in which orders are obeyed without question. Imagine that those involved in the operation are supplied a cover story: The asteroid is about to impact the Earth, and it is their job to deflect it but in order not to worry people needlessly, the operation must be performed in secret. In a military setting with a command hierarchy firmly in place, compartmentalization of knowledge, general secrecy, and a cover story, can we be confident that even apocalyptic orders would be disobeyed? Are we really sure that in the next decades and centuries and millennia, nothing like this might happen? How sure are we?
It’s no use saying that all technologies can be used for good or for ill. That is certainly true, but when the “ill” achieves a sufficiently apocalyptic scale, we may have to set limits on which technologies may be developed. (In a way we do this all the time, because we can’t afford to develop all technologies. Some are favored and some are not.) Or constraints may have to be levied by the community of nations on madmen and autarchs and fanaticism.
Tracking asteroids and comets is prudent, it’s good science, and it doesn’t cost much. But, knowing our weaknesses, why would we even consider now developing the technology to deflect small worlds? For safety, shall we imagine this technology in the hands of many nations, each providing checks and balances against misuse by another? This is nothing like the old nuclear balance of terror. It hardly inhibits some madman intent on global catastrophe to know that if he does not hurry, a rival may beat him to it. How confident can we be that the community of nations will be able to detect a cleverly designed, clandestine asteroid deflection in time to do something about it? If such a technology were developed, can any international safeguards be envisioned that have a reliability commensurate with the risk?
Even if we restrict ourselves merely to surveillance, there’s a risk. Imagine that in a generation we characterize the orbits of 30,000 objects of 100-meter diameter or more, and that this information is publicized, as of course it should be. Maps will be published showing near-Earth space black with the orbits of asteroids and comets, 30,000 swords of Damocles hanging over our heads-ten times more than the number of stars visible to the naked eye under conditions of optimum atmospheric clarity. Public anxiety might be much greater in such a time of knowledge than in our current age of ignorance. There might be irresistible public pressure to develop means to mitigate even nonexistent threats, which would then feed the danger that deflection technology would be misused. For this reason, asteroid discovery and surveillance may not be a mere neutral tool of future policy, but rather a kind of booby trap. To me, the only foreseeable solution is a combination of accurate orbit estimation, realistic threat assessment, and effective public education so that in democracies at least, the citizens can make their own, informed decisions. This is a job for NASA.
Near-Earth asteroids, and means of altering their orbits, are being looked at seriously. There is some sign that officials in the Department of Defense and the weapons laboratories are beginning to understand that there may be real dangers in planning to push asteroids around. Civilian and military scientists have met to discuss the subject. On first hearing about the asteroid hazard, many people think of it as a kind of Chicken Little fable; Goosey-Lucy, newly arrived and in great excitement, is communicating the urgent news that the sky is falling. The tendency to dismiss the prospect of any catastrophe that we have not personally witnessed is in the long run very foolish. But in this case it may be an ally of prudence.
MEANWHILE WE MUST STILL FACE the deflection dilemma. If we develop and deploy this technology, it may do us in. If we don’t, some asteroid or comet may do us in. The resolution of the dilemma hinges, I think, on the fact that the likely timescales of the two dangers are very different short for the former, long for the latter.
I like to think that our future involvement with near-Earth asteroids will go something like this: From ground-based observatories, we discover all the big ones, plot and monitor their orbits, determine rotation rates and compositions. Scientists are diligent in explaining the dangers neither exaggerating nor muting the prospects. We send robotic spacecraft to fly by a few selected bodies, orbit them, land on them, and return surface samples to laboratories on Earth. Eventually we send humans. (Because of the low gravities, they will be able to make standing broad jumps of ten kilometers or more into the sky, and lob a baseball into orbit around the asteroid.) Fully aware of the dangers, we make no attempts to alter trajectories until the potential for misuse of world-altering technologies is much less. That might take a while.
If we’re too quick in developing the technology to move worlds around, we may destroy ourselves; if we’re too slow, we will surely destroy ourselves. The reliability of world political organizations and the confidence they inspire will have to make significant strides before they can be trusted to deal with a problem of this seriousness. At the same time, there seems to be no acceptable national solution. Who would feel comfortable with the means of world destruction in the hands of some dedicated (or even potential) enemy nation, whether or not our nation had comparable powers? The existence of interplanetary collision hazards, when widely understood, works to bring our species together. When facing a common danger, we humans have sometimes reached heights widely thought impossible; we have set aside our differences at least until the danger passed.
But this danger never passes. The asteroids, gravitationally churning, are slowly altering their orbits; without warning, new comets come careening toward us from the trans plutonian darkness. There will always be a need to deal with them in a way that does not endanger us. By posing two different classes of peril one natural, the other human- made the small near-Earth worlds provide a new and potent motivation to create effective transnational institutions and to unify the human species. It’s hard to see any satisfactory alternative.
In our usual jittery, two-steps-forward-one-step-back mode, We are moving toward unification anyway. There are powerful influences deriving from transportation and communications technologies, the interdependent world economy, and the global environmental crisis. The impact hazard merely hastens the pace.
Eventually, cautiously, scrupulously careful to attempt nothing with asteroids that could inadvertently cause a catastrophe on Earth, I imagine we will begin to learn how to change the orbits of little nonmetallic worlds, smaller than 100 meters across. We begin with smaller explosions and slowly work our way up. We gain experience in changing the orbits of various asteroids and comets of different compositions and strengths. We try to determine which ones can be pushed around and which cannot. By the twenty-second century, perhaps, we move small worlds around the Solar System, using (see next chapter) not nuclear explosions but nuclear fusion engines or their equivalents. We insert small asteroids made of precious and industrial metals into Earth orbit. Gradually develop a defensive technology to deflect a large asteroid or comet that might in the foreseeable future hit the Earth, while, with meticulous care, we build layers of safeguards against misuse.
Since the danger of misusing deflection technology seems so much greater than the danger of an imminent impact, we can afford to wait, take precautions, rebuild political institutions for decades certainly, probably centuries. If we play our cards right and are not unlucky, we can pace what we do up there by what progress we’re making down here. The two are in any case deeply connected.
The asteroid hazard forces our hand. Eventually, we must establish a formidable human presence throughout the inner Solar System. On an issue of this importance I do not think we will be content with purely robotic means of mitigation. To do so safely we must make changes in our political and international systems. While much about our future is cloudy, this conclusion seems a little more robust, and independent of the vagaries of human institutions.
In the long term, even if we were not the descendants of professional wanderers, even if we were not inspired by exploratory passions, some of us would still have to leave the Earth simply to ensure the survival of all of us. And once we’re out there, we’ll need bases, infrastructures. It would not be very long before some of us were living in artificial habitats and on other worlds. This is the first of two mussing arguments, omitted in our discussion of missions to Mars, for a permanent human presence in space.
OTHER PLANETARY SYSTEMS must face their own impact hazards because small primordial worlds, of which asteroids and comets are remnants, are the stuff out of which planets form there as well. After the planets are made, many of these planetesimals are left over. The average time between civilization-threatening impacts on Earth is perhaps 200,000 years, twenty times the age of our civilization. Very different waiting times may pertain to extraterrestrial civilizations, if they exist, depending on such factors as the physical and chemical characteristics of the planet and its biosphere, the biological and social nature of the civilization, and of course the collision rate itself. Planets with higher atmospheric pressures will be protected against somewhat larger 1mpactors, although the pressure cannot be much greater before greenhouse warming and other consequences make life improbable. If the gravity is much less than on Earth, impactors will make less energetic collisions and the hazard will be reduced although it cannot be reduced very much before the atmosphere escapes to space.
The impact rate in other planetary systems is uncertain. Our system contains two major populations of small bodies that feed potential impactors into Earth-crossing orbits. Both the existence of the source populations and the mechanisms that maintain the collision rate depend on how worlds are distributed. For example, our Oort Cloud seems to have been populated by gravitational ejections of icy worldlets from the vicinity of Uranus and Neptune. If there are no planets that play the role of Uranus and Neptune in systems otherwise like our own, their Oort Clouds may be much more thinly populated. Stars in open and globular stellar clusters, stars in double or multiple systems, stars closer to the center of the Galaxy, stars experiencing more frequent encounters with Giant Molecular Clouds in interstellar space, may all experience higher impact fluxes at their terrestrial planets. The cometary flux might be
hundreds or thousands of times more at the Earth had the planet Jupiter never formed according to a calculation by George Wetherill of the Carnegie Institution of Washington. In systems without Jupiter-like planets, the gravitational shield against comets is down, and civilization threatening impacts much more frequent.
To a certain extent, increased fluxes of interplanetary objects might increase the rate of evolution, as the mammals that flourished and diversified after the Cretaceous-Tertiary collision wiped out the dinosaurs. But there must be a point of diminishing returns: Clearly, some flux is too high for the continuance of any civilization.
One consequence of this train of argument is that, even if civilizations commonly arise on planets throughout the Galaxy, few of them will be both long-lived and non-technological. Since hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, if there are such, intelligent beings everywhere will have to unify their home worlds politically, leave their planets, and move small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction.
THE MARSH OF CAMARINA -Pale Blue Dot_ A Vision of the Human Future in Space