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n　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　the　Earth。　A　world　10　kilometers　across　could　be　transported　from　Saturn；　say；　to　Mars　through　nuclear　burning　of　the　hydrogen　in　an　icy　et　a　kilometer　across。　（Again；　I'm　assuming　a　time　of　much　greater　political　stability　and　safety。）



PUT　ASIDE　FOR　THE　MOMENT　any　qualms　you　might　have　about　the　ethics　of　rearranging　worlds；　or　our　ability　to　do　so　without　catastrophic　consequences。　Digging　out　the　insides　of　worldlets；　reconfiguring　them　for　human　habitation；　and　moving　them　from　one　place　in　the　Solar　System　to　another　seems　to　be　within　our　grasp　in　another　century　or　two。　Perhaps　by　then　we　will　have　adequate　international　safeguards　as　well。　But　what　about　transforming　the　surface　environments　not　of　asteroids　or　ets；　but　of　planets？　Could　we　live　on　Mars？

If　we　wanted　to　set　up　housekeeping　on　Mars；　it's　easy　to　see　that；　in　principle　at　least；　we　could　do　it：　There's　abundant　sunlight。　There's　plentiful　water　in　the　rocks　and　in　underground　and　polar　ice。　The　atmosphere　is　mostly　carbon　dioxide。　It　seems　likely　that　in　self…contained　habitats…perhaps　domed　enclosures—we　could　grow　crops；　manufacture　oxygen　from　water；　recycle　wastes。

At　first　we'd　be　dependent　on　modities　resupplied　from　Earth；　but　in　time　we'd　manufacture　more　and　more　of　them　ourselves。　We'd　bee　increasingly　self…sufficient。　The　domed　enclosures；　even　if　made　of　ordinary　glass；　would　let　in　the　visible　sunlight　and　screen　out　the　Sun's　ultraviolet　rays。　With　oxygen　masks　and　protective　garments—but　nothing　as　bulky　and　cumbersome　as　a　spacesuit—we　could　leave　these　enclosures　to　go　exploring；　or　to　build　another　domed　village　and　farms。

It　seems　very　evocative　of　the　American　pioneering　experience；　but　with　at　least　one　major　difference：　In　the　early　stages；　large　subsidies　are　essential。　The　technology　required　is　too　expensive　for　some　poor　family；　like　my　grandparents　a　century　ago；　to　pay　their　own　passage　to　Mars。　The　early　Martian　pioneers　will　be　sent　by　governments　and　will　have　highly　specialized　skills。　But　in　a　generation　or　two；　when　children　and　grandchildren　are　born　there—　and　especially　when　self…sufficiency　is　within　reach—that　will　begin　to　change。　Youngsters　born　on　Mars　will　be　given　specialized　training　in　the　technology　essential　for　survival　in　this　new　environment。　The　settlers　will　bee　less　heroic　and　less　exceptional。　The　full　range　of　human　strengths　and　deficiencies　will　begin　to　assert　themselves。　Gradually；　precisely　because　of　the　difficulty　of　getting　from　Earth　to　Mars；　a　unique　Martian　culture　will　begin　to　emerge—distinct　aspirations　and　fears　tied　to　the　environment　they　live　in；　distinct　technologies；　distinct　social　problems；　distinct　solutions—and；　as　has　occurred　in　every　similar　circumstance　throughout　human　history；　a　gradual　sense　of　cultural　and　political　estrangement　from　the　mother　world。

Great　ships　will　arrive　carrying　essential　technology　from　Earth；　new　families　of　settlers；　scarce　resources。　It　is　hard　to　know；　on　the　basis　of　our　limited　knowledge　of　Mars；　whether　they　will　go　home　empty—or　whether　they　will　carry　with　them　something　found　only　on　Mars；　something　considered　very　valuable　on　Earth。　Initially　much　of　the　scientific　investigation　of　samples　of　the　Martian　surface　will　be　done　on　Earth。　But　in　time　the　scientific　study　of　Mars　（and　its　moons　Phobos　and　Deimos）　will　be　done　from　Mars。

Eventually—as　has　happened　with　virtually　every　other　form　of　human　transportation—interplanetary　travel　will　bee　accessible　to　people　of　ordinary　means：　to　scientists　pursuing　their　own　research　projects；　to　settlers　fed　up　with　Earth；　even　to　venturesome　tourists。　And　of　course　there　will　be　explorers。

If　the　time　ever　came　when　it　was　possible　to　make　the　Martian　environment　much　more　Earth…like—so　protective　garments；　oxygen　masks；　and　domed　farmlands　and　cities　could　be　dispensed　with—the　attraction　and　accessibility　of　Mars　would　be　increased　many…fold。　The　same；　of　course；　would　be　true　for　any　other　world　which　could　be　engineered　so　that　humans　could　live　there　without　elaborate　contrivances　to　keep　the　planetary　environment　out。　We　would　feel　much　more　fortable　in　our　adopted　home　if　an　intact　dome　or　spacesuit　weren't　all　that　stood　between　us　and　death。　（But　perhaps　I　exaggerate　the　dangers。　People　who　live　in　the　Netherlands　seem　at　least　as　well　adjusted　and　carefree　as　other　inhabitants　of　Northern　Europe；　vet　their　dikes　are　all　that　stand　between　them　and　the　sea。

Recognizing　the　speculative　nature　of　the　question　and　the　limitations　in　our　knowledge；　is　it　nevertheless　possible　to　envision　terraforming　the　planets？

We　need　look　no　further　than　our　own　world　to　see　that　humans　are　now　able　to　alter　planetary　environments　in　a　profound　way。　Depletion　of　the　ozone　layer；　global　warming　from　an　increased　greenhouse　effect；　and　global　cooling　from　nuclear　war　are　all　ways　in　which　present　technology　can　significantly　alter　the　environment　of　our　world—and　in　each　case　as　an　inadvertent　consequence　of　doing　something　else。　If　we　had　intended　to　alter　our　planetary　environment；　we　would　be　fully　able　to　generate　still　greater　change。　As　our　technology　bees　more　powerful；　we　will　be　able　to　work　still　more　profound　changes。

But　just　as　（in　parallel　parking）　it's　easier　to　get　out　of　a　parking　place　than　into　one；　it's　easier　to　destroy　a　planetary　environment　than　to　move　it　into　a　narrowly　prescribed　range　of　temperatures；　pressures；　positions；　and　so　on。　We　already　know　of　a　multitude　of　desolate　and　uninhabitable　worlds；　and—with　very　narrow　margins—only　one　green　and　clement　one。　This　is　a　major　conclusion　from　early　in　the　era　of　spacecraft　exploration　of　the　Solar　System。　In　altering　the　Earth；　or　any　world　with　an　atmosphere；　we　must　be　very　careful　about　positive　feedbacks；　where　we　nudge　an　environment　a　little　bit　and　it　takes　of　on　its　own—a　little　cooling　leading　to　runaway　glaciation；　as　may　have　happened　on　Mars；　or　a　little　warming　to　a　runaway　greenhouse　effect；　as　happened　on　Venus。　It　is　not　at　all　clear　that　our　knowledge　is　suffi