ould 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 munity 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 mensurate 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 ets； 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 bination 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 municating 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 et 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 positions。 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 fortable with the means of world destruction in the hands of some dedicated （or even potential） enemy nation； whether or not our nation had parable powers？ The existence of interplanetary collision hazards； when widely understood； works to bring our species together。 When facing a mon 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 ets e careening toward us from the transplutonian 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 munications 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 ets of different positions 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 et 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 ets 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 worldl