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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 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 etary 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 ets 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 w