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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—unlike　〃paragravity〃—nothing　in　such　a　scheme　seems　impossible。　All　the　elements　can　be　found　in　contemporary　technology。　If　there　were　sufficient　reason；　a　fair　number　of　us　could　be　living　on　（or　in　asteroids　by　the　twenty…second　century。

They　would　of　course　need　a　source　of　power；　not　just　to　sustain　themselves；　but；　as　Bernal　suggested；　to　move　their　asteroidal　homes　around。　（It　does　not　seem　so　big　a　step　from　explosive　alteration　of　asteroid　orbits　to　a　more　gentle　means　of　propulsion　a　century　or　two　later。）　If　an　oxygen　atmosphere　were　generated　from　chemically　bound　water；　then　organics　could　be　burned　to　generate　power；　just　as　fossil　fuels　are　burned　on　the　Earth　today。　Solar　power　could　be　considered；　although　for　the　main…belt　asteroids　the　intensity　of　sunlight　is　only　about　10　percent　what　it　is　on　Earth。　Still；　we　could　imagine　vast　fields　of　solar　panels　covering　the　surfaces　of　inhabited　asteroids　and　converting　sunlight　into　electricity。　Photovoltaic　technology　is　routinely　used　in　Earth…orbiting　spacecraft；　and　is　in　increasing　use　on　the　surface　of　the　Earth　today。　But　while　that　might　be　enough　to　warm　and　light　the　homes　of　these　descendants；　it　does　not　seem　adequate　to　change　asteroid　orbits。

For　that；　Williamson　proposed　using　anti…matter。　Antimatter　is　just　like　ordinary　matter；　with　one　significant　difference。　Consider　hydrogen：　An　ordinary　hydrogen　atom　consists　of　a　positively　charged　proton　on　the　inside　and　a　negatively　charged　electron　on　the　outside。　An　atom　of　anti…hydrogen　consists　of　a　negatively　charged　proton　on　the　inside　and　a　positively　charged　electron　（also　called　a　positron）　on　the　outside。　The　protons；　whatever　the　sign　of　their　charges；　have　the　same　mass；　and　the　electrons；　whatever　the　sign　of　their　charges；　have　the　same　mass。　Particles　with　opposite　charges　attract。　A　hydrogen　atom　and　an　anti…hydrogen　atom　are　both　stable；　because　in　both　cases　the　positive　and　negative　electrical　charges　precisely　balance。

Anti…matter　is　not　some　hypothetical　construct　from　the　perfervid　musings　of　science　fiction　writers　or　theoretical　physicists。　Anti…matter　exists。　Physicists　make　it　in　nuclear　accelerators；　it　can　be　found　in　high…energy　cosmic　rays。　So　why　don't　we　hear　more　about　it？　Why　has　no　one　held　up　a　lump　of　antimatter　for　our　inspection？　Because　matter　and　anti…matter；　when　brought　into　contact；　violently　annihilate　each　other；　disappearing　in　an　intense　burst　of　gamma　rays。　We　cannot　tell　whether　something　is　made　of　matter　or　anti…matter　just　by　looking　at　it。　The　spectroscopic　properties　of；　for　example；　hydrogen　and　anti…hydrogen　are　identical。

Albert　Einstein's　answer　to　the　question　of　why　we　see　only　matter　and　not　anti…matter　was；　〃Matter　won〃—by　which　he　meant　that　in　our　sector　of　the　Universe　at　least；　after　almost　all　the　matter　and　anti…matter　interacted　and　annihilated　each　other　long　ago；　there　was　some　of　what　we　call　ordinary　matter　left　over。*　As　far　as　we　can　tell　today；　from　gamma　ray　astronomy　and　other　means；　the　Universe　is　made　almost　entirely　of　matter。　The　reason　for　this　engages　the　deepest　cosmological　issues；　which　need　not　detain　us　here。　But　if　there　was　only　a　one…particle…in…a…billion　difference　in　the　preponderance　of　matter　over　anti…matter　at　the　beginning；　even　this　would　be　enough　to　explain　the　Universe　we　see　today。

*　If　it　had　been　the　other　way；　then　we　and　everything　else　in　this　part　of　the　Universe　would　be　made　of　anti…matter。　We　would；　of　course；　call　it　matter—and　the　idea　of　worlds　and　life　made　of　that　other　kind　of　material。　the　stuff　with　the　electrical　charges　reversed；　we'd　consider　wildly　speculative。

Williamson　imagined　that　humans　in　the　twenty…second　century　would　move　asteroids　around　by　the　controlled　mutual　annihilation　of　matter　and　anti…matter。　All　the　resulting　gamma　rays；　if　collimated；　would　make　a　potent　rocket　exhaust。　The　anti…matter　would　be　available　in　the　main　asteroid　belt　（between　the　orbits　of　Mars　and　Jupiter）；　because　this　was　his　explanation　for　the　existence　of　the　asteroid　belt。　In　the　remote　past；　he　proposed；　an　intruder　anti…matter　worldlet　arrived　in　the　Solar　System　from　the　depths　of　space；　impacted；　and　annihilated　what　was　then　an　Earthlike　planet；　fifth　from　the　Sun。　The　fragments　of　this　mighty　collision　are　the　asteroids；　and　some　of　them　are　still　made　of　anti…matter。　Harness　an　anti…asteroid—Williamson　recognized　that　this　might　be　tricky—and　you　can　move　worlds　around　at　will。

At　the　time；　Williamson's　ideas　were　futuristic；　but　far　from　foolish。　Some　of　〃Collision　Orbit〃　can　be　considered　visionary。　Today；　however；　we　have　good　reason　to　believe　that　there　are　no　significant　amounts　of　anti…matter　in　the　Solar　System；　and　that　the　asteroid　belt；　far　from　being　a　fragmented　terrestrial　planet；　is　an　enormous　array　of　small　bodies　prevented　（by　the　gravitational　tides　of　Jupiter）　from　forming　an　Earthlike　world。

However；　we　do　generate　（very）　small　amounts　of　antimatter　in　nuclear　accelerators　today；　and　we　will　probably　be　able　to　manufacture　much　larger　amounts　by　the　twenty…second　century。　Because　it　is　so　efficient—converting　all　of　the　matter　into　energy；　E　=　MC2；　with　100　percent　efficiency—perhaps　anti…matter　engines　will　be　a　practical　technology　by　then；　vindicating　Williamson　Failing　that；　what　energy　sources　can　we　realistically　expect　to　be　available；　to　reconfigure　asteroids；　to　light　them　warm　them；　and　move　them　around？

The　Sun　shines　by　jamming　protons　together　and　turning　them　into　helium　nuclei。　Energy　is　released　in　the　process；　although　with　less　than　1　percent　the　efficiency　of　the　annihilation　of　matter　and　anti…matter。　But　even　proton…proton　reactions　are　far　beyond　anything　we　can　realistically　imagine　for　ourselves　in　the　near　future。　The　required　temperatures　are　much　too　high。　Instead）　of　jamming　protons　together；　though；　we　might　use　heavier　kinds　of　hydrogen。　We　already　do　so　in　thermonuclear　weapons。　Deuterium　is　a　proton　bound　by　nuclear　forces　to　a　neutron；　tritium　is　a　proton　bound　by　nuclear　forces　to　two　neutrons。　It　seems　likely　that　in　another　century　we　will　have　practical　power　schemes　that　involve　the　controlled　fusion　of　deuterium　and　tritium；　and　of　deuterium　and　helium。　Deuterium　and　tritium　are　present　as　minor　constituents　in　water　（on　Earth　and　other　worlds）。　The　kind　of　helium　needed　for　fusion；　3He　（two　protons　and　a　neutron　make　up　its　nucleus）；　has　been　implanted　over　billions　of　years　by　the　solar　wind　in　the　surfaces　of　the　asteroids。　These　processes　are　not　nearly　as　efficient　as　the　proton…proton　reactions　in　the　Sun；　but　they　could　provide　enough　power　to　run　a　small　city　for　a　year　from　a　lode　of　ice　only　a　few　meters　in　size。

Fusion　reactors　seem　to　be　ing　along　too　slowly　to　play　a　major　role　in　solving；　or　even　significantly　mitigating；　global　warming。　But　by　the　twenty…second　century；　they　ought　to　be　widely　available。　With　fusion　rocket　engines；　it　will　be　possible　to　more　asteroids　and　ets　around　the　inner　Solar　System　taking　a　main…belt　asteroid；　for　example；　and　inserting　it　into　orbit　around