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　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　sufficient　to　this　purpose。

As　far　as　I　know；　the　first　suggestion　in　the　scientific　literature　about　terraforming　the　planets　was　made　in　a　1961　article　I　wrote　about　Venus。　I　was　pretty　sure　then　that　Venus　had　a　surface　temperature　well　above　the　normal　boiling　point　of　water；　produced　by　a　carbon　dioxide/water　vapor　greenhouse　effect。　I　imagined　seeding　its　high　clouds　with　genetically　engineered　microorganisms　that　would　take　CO2；　N2；　and　H2O　out　of　the　atmosphere　and　convert　them　into　organic　molecules。　The　more　CO2　removed；　the　smaller　the　greenhouse　effect　and　the　cooler　the　surface。　The　microbes　would　be　carried　down　through　the　atmosphere　toward　the　ground；　where　they　would　be　fried；　so　water　vapor　would　be　returned　to　the　atmosphere；　but　the　carbon　from　the　CO2　would　be　converted　irreversibly　by　the　high　temperatures　into　graphite　or　some　other　involatile　form　of　carbon。　Eventually；　the　temperatures　would　fall　below　the　boiling　point　and　the　surface　of　Venus　would　bee　habitable；　dotted　with　pools　and　lakes　of　warm　water。

The　idea　was　soon　taken　up　by　a　number　of　science　fiction　authors　in　the　continuing　dance　between　science　and　science　fiction—in　which　the　science　stimulates　the　fiction；　and　the　fiction　stimulates　a　new　generation　of　scientists；　a　process　benefiting　both　genres。　But　as　the　next　step　in　the　dance；　it　is　now　clear　that　seeding　Venus　with　special　photosynthetic　microorganisms　will　not　work。　Since　1961　we've　discovered　that　the　clouds　of　Venus　are　a　concentrated　solution　of　sulfuric　acid；　which　makes　the　genetic　engineering　rather　more　challenging。　But　that　in　itself　is　not　a　fatal　flaw。　（There　are　microorganisms　that　live　out　their　lives　in　concentrated　solutions　of　sulfuric　acid。）　Here's　the　fatal　flaw：　In　1961　I　thought　the　atmospheric　pressure　at　the　surface　of　Venus　N…as　a　few　〃bars；〃　a　few　times　the　surface　pressure　on　Earth。　We　now　know　it　to　be　90　bars；　so　that　if　the　scheme　worked；　the　result　would　be　a　surface　buried　in　hundreds　of　meters　of　fine　graphite；　and　an　atmosphere　made　of　65　bars　of　almost　pure　molecular　oxygen。　Whether　we　would　first　implode　under　the　atmospheric　pressure　or　spontaneously　burst　into　flames　in　all　that　oxygen　is　an　open　question。　However；　long　before　so　much　oxygen　could　build　up；　the　graphite　would　spontaneously　burn　back　into　CO2；　short…circuiting　the　process。　At　best；　such　a　scheme　can　carry　the　terraforming　of　Venus　only　partway。

Let's　assume　that　by　the　early　twenty…second　century　we　have　paratively　inexpensive　heavy…lift　vehicles；　so　we　can　carry　large　payloads　to　other　worlds；　abundant　and　powerful　fusion　reactors；　and　well…developed　genetic　engineering。　All　three　assumptions　are　likely；　given　current　trends。　Could　we　terraform　the　planets？*　James　Pollack　of　NASA's　Ames　Research　Center　and　I　surveyed　this　problem。　Here's　a　summary　of　what　we　found：

*　Williamson；　Professor　Emeritus　of　English　at　Eastern　New　Mexico　University　at　age　85　wrote　to　me　that　he　was　〃amazed　to　see　how　far　actual　science　has　e〃　since　he　first　suggested　terraforming　other　worlds。　We　are　accumulating　the　technology　that　will　one　day　permit　terraforming；　but　at　present　all　'V…a　have　are　suggestions　by　and　large　less　ground　breaking　than　Williamson's　original　ideas。

VENUS：　Clearly　the　problem　with　Venus　is　its　massive　greenhouse　effect。　If　we　could　reduce　the　greenhouse　effect　almost　to　zero；　the　climate　might　be　balmy。　But　a　90…bar　CO2　atmosphere　is　oppressively　thick。　Over　every　postage　stamp…sized　square　inch　of　surface；　the　air　weighs　as　much　as　six　professional　football　players；　piled　one　on　top　of　another。　Making　all　that　go　away　will　take　some　doing。

Imagine　bombarding　Venus　with　asteroids　and　ets。　Each　impact　would　blow　away　some　of　the　atmosphere。　To　blow　away　almost　all　of　it；　though；　would　require　using　up　more　big　asteroids　and　ets　than　there　are—at　least　in　the　planetary　part　of　the　Solar　System。　Even　if　that　many　potential　impactors　existed；　even　if　we　could　make　them　all　collide　with　Venus　（this　is　the　overkill　approach　to　the　impact　hazard　problem）；　th