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Show SCIENCE to the largest nuclear-weapons explosion ever detonated. Every 10,000 years, we’re hit by a 200-meter object that might induce serious regionalclimatic effects. Every million years, an impact by a body morethan 2 kilometers in diameter occurs, equivalent to nearly a million megatons of TNT —an explosion that would produce a global catastrophe, killing (unless unprecedented precautions had been taken) a significant portion of the human species. A million megatons of TNT is 100 times the explosiveyield ofall the nuclear weaponsonthe planet. ON kilometerin diameter, will come within 2.25 million kilometers of the Earth’s orbit (six times the distance to the Moon). To deflect 1991 OA soit hits the Earth, the equivalentof only about 60 megatons of TNT—a small number of currently available nuclear warheads —needs to be explodedin the nght way. Now,imagine a time, a few decades hence, whenall such near-Earthasteroids are inventoried and their orbits compiled. Then, it might take only a year to select a suitable object, alter its orbit and send it crashing into the PARADE gy, it may do usin. If we don’t, some asteroid or comet may do us in. The resolution of the dilemmahinges, I think, on the fact that the probable times we haveto wait for the two dan- gers to comeaboutare very different— & And in 100 million years or so, you can bet on something evenbigger,like the event that seemsto have killed off the dinosaurs. The destructive energy latent in a large near-Earth asteroid dwarfs anything the human species can nowgetits hands on. So somescientists have begun to ask: Shouldn’t we be monitoringall sizable near-Earth interplanetary objects, characterizing them and predicting which ones may be dangerous? And if we find one on a collision course withus, shouldn’t we be figuring out how to preventthe collision from happening? With current technology, the best way to deflect an asteroid away from the Earth is to explode nuclear weapons near it. A flurry of explosions, each giving a little push in the desired direction, could be enoughto herd away a big asteroid even with only a few weeks’ warning. Of course, we’d have to be ready. Some in government ae ¢ > weaponslaboratories have emREDS braced this prospect with open arms: Here’s a way, in the postCold Warera,to secure a seat for The Earth shortly after the impact of a 10nuclear weaponson the Save-the- kilometer asteroid or cometlike that which extinguished most species oflife 65 million Earth bandwagon. years ago: In sequence, there was a worldThe problem, Steven Ostro of immolating fire that burned to a crisp vegeNASA's Jet Propulsion Labora- tation all over the planet; a stratospheric dust that made it too dark for any surviving tory and I have suggested, is that cloud green plants to makea living from photoif you can deflect a threatening synthesis; global freezing temperatures; torworldlet so it does not collide with rential rains of caustic acids; massive dethe Earth, you also can deflect a pletion of the ozone layer; and—to top it all off—after the Earth healed itself from these harmless worldlet soit does col- other assaults, a prolonged greenhouse lide with the Earth. warming. It was not a single catastrophe S = é arn Suppose werestrict our atten- bility commensurate with the risk? There’s a terrible dilemmahere: If we develop and deploy this technolo- ities in the next few decades. Then a handful of nuclear warheads could be made to do more damagethanall our nuclear weaponsif simultaneously detonated on Earth. What kind of world will we then have made? Can we humansbetrusted with civilization-threatening technologies? If the chance is almost one in 1000 that muchof the human populationwill be killed by an impactin the next century, isn’t it more likely that asteroiddeflection technology will get into the wrong handsin another century—some but a parade of them, a concatenation of terrors, Organisms weakened by one dis- tion to the 2000 or so near-Earth aster were finished off by the next. Similar asteroids that are a kilometer disasters will be worked by future impacts. across or bigger—thatis, the ones mostlikely to cause a global catastro- Earth with cataclysmic effect. The technology required—large opphe. Today, with only about 100of these bodies catalogued, it would take tical telescopes, sensitive detectors, about a century or so to catch oneat fast computers, rocket-propulsion systhe point in its orbit whereit’s easily tems able tolift a few tons of payload and make a precise rendezvous in neardeflected to Earth. by space, and thermonuclear weapons as-yet an one: Wethink we’ ve found 199] as —exists today. If we’re not careful, only denoted unnamedasteroid many nations may have these capabila about worldlet, OA. In 2070, this es + Sa short for the former, long for thelatter. Theexistenceof interplanetary collision hazards, when widely understood, works to bring our species together. When facing a commondanger, we humans sometimes have reached heights thought impossible in setting aside our differences—at least until the danger passed. Butthis danger never passes. The asteroids, gravitationally churning,are slowly altering their orbits. Without warning, new comets comecareening toward us from the trans-Plutonian darkness. Since the danger of misusing deflection technology seems so muchgreater than the dangerof an imminent impact, we can afford to wait, take precautions and rebuild political institutions—for decadescertainly, probably for centuries. If we play our cardsright, we can pace what wedoup there by what progress we're making downhere. But the asteroid hazard forces our hand. Eventually, we mustestablish a formidable human presence throughout the inner Solar System. Onthis matter, I don’t think we’d be content with robots.To do this safely, we must changeourpolitical and international systems. In the long term, even if we were not motivated by exploratory passions, some of us would still have to leave the Earth simply to ensure the survivalof the rest of us. Since hazards from asteroids and comets must apply to inhabited planets (if there are such) all over the Universe, intelligent beings everywhere will have to unify their home planets and move small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction. This might be something to think about when, in coming weeks, sociopathicleaderlike a Hitler or a Stalin eager to kill everybody, a megalomaniac lusting after “greatness” and “glory,” a victim of ethnic violence bent onrevenge, somereligious fanatic hastening the Day of Judgment, or technicians incompetentor insufficiently vigilant in handling the controls and safeguards? Such people exist. The we hear about Comet Shoemaker-Levy risks seem far worse than the potential or when we contemplate why,in the long benefits, the cure worse than the disease. run, we have a space program. th Tracking asteroids and cometsis prudent, it’s good science, and it Carl Saganis the David Duncan Prodoesn’t cost much. It would provide fessor of Astronomyand Space Sciearly warning about collisions. But, encesand director of the Laboratory knowing our human weaknesses, why for Planetary Studies at Cornell Uniwould we even consider now devel- versity. An experimenter on the Galileo oping the technology to deflect small and Voyagermissionstothe planets, he worlds? If such technology were in serves as president of the Planetary hand, can any international safeguards Society, the world’s largest space innowbeenvisionedthat have a relia- terest group. nae PAGE 10 JUNE 5, 1994 » PARADE MAGAZINE |