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 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 monly arise on planets throughout the Galaxy； few of them will be both long…lived and non…technological。 Since hazards from asteroids and ets 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。
CHAPTER 19 REMAKING THE PLANETS
Who could deny that man could somehow also make the heavens；
could he only obtain the instruments and the heavenly material？
—MARSILIO FICINO； 〃THE SOUL OF MAN〃 （CA。 1474）
In the midst of the Second World War； a young American writer named Jack Williamson envisioned a populated Solar System。 In the twenty…second century； he imagined； Venus would be settled by China；* Japan； and Indonesia； Mars by Germany； and the moons of Jupiter by Russia。 Those who spoke English； the language in which Williamson was writing； were confined to the asteroids…and of course the Earth。
* In the real world； Chinese space officials are proposing to send a two…person astronaut capsule into orbit by the turn of the century。 It would be propelled by a modified Long March 2E rocket and be launched from the Gobi Desert。 If the Chinese economy exhibits even moderate continuing growth—much less the exponential growth that marked it in the early to mid…1990s—China may be one of the world's leading space powers by the middle of the twenty…first century。 Or earlier。
The story； published in Astounding Science Fiction in July 1942； was called 〃Collision Orbit〃 and written under the pseudonym Will Stewart。 Its plot hinged on the imminent collision of an uninhabited asteroid with a colonized one； and the search for a means of altering the trajectories of small worlds。 Although no one on Earth was endangered； this may have been the first appearance； apart from newspaper ic strips； of asteroid collisions as a threat to humans。 （ets impacting the Earth had been a staple peril。）
The environments of Mars and Venus were poorly understood in the early 1940s； it was conceivable that humans could live there without elaborate life…support systems。 But the asteroids were another matter。 It was well known； even then； that asteroids were small； dry； airless worlds。 If they were to be inhabited； especially by large numbers of people； these little worlds would somehow have to be fixed。
In 〃Collision Orbit；〃 Williamson portrays a group of 〃spatial engineers；〃 able to render such barren outposts clement。 Coining a word； Williamson called the process of metamorphosis into an Earth…like world 〃terraforming。〃 He knew that the low gravity on an asteroid means that any atmosphere generated or transported there would quickly escape to space。 So his key terraforming technology was 〃paragravity；〃 an artificial gravity that would hold a dense atmosphere。
As nearly as we can tell today； paragravity is a physical impossibility。 But we can imagine domed； transparent habitats on the surfaces of asteroids； as suggested by Konstantin Tsiolkovsky； or munities established in the insides of asteroids； as outlined in the 1920s by the British scientist J。 D。 Bernal。 Because asteroids are small and their gravities low； even massive subsurface construction might be paratively easy。 If a tunnel were dug clean through； you could jump in at one end and emerge some 45 minutes later at the other； oscillating up and down along the toll diameter of this world indefinitely。 Inside the right kind of asteroid； a carbonaceous one； you can find materials for manufacturing stone； metal； and plastic construction and plentiful water—all you might need to build a subsurface closed ecological system； an underground garden。 Implementation would require a significant step beyond what we have today； but—un