ere a large asteroid to be discovered on impact trajectory with the Earth； presumably no one's hand would be stayed by the niceties of international diplomacy。 There is a danger； though； that relaxing prohibitions on such weapons in space might make us less attentive 。bout the positioning of warheads for offensive purposes in space。
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 ets 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 e 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。
* What should we call this world？ Naming it after the Greek Fates or Furies or Nemesis seems inappropriate； because whether it misses or hits the Earth is entirely in our hands。 If we leave it alone； it misses。 If we push it cleverly and precisely； it hits。 Maybe we should call it 〃Eight Ball。〃
Now imagine a time； a few decades hence； when all such near…Earth asteroids are inventoried and their orbits piled。 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 pletely 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； non…allergenic； 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 pleted。 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。
* There is of course a wide range of other problems brought on by the devastatingly powerful technology we've recently invented。 But in most cases they're not Camarinan disasters…damned if you do and damned if you don't。 Instead they're dilemmas of wisdom or timing…for example； the wrong refrigerant or refrigeration physics out of many possible alternatives。
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 acplish 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 b