me； 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）； think what we would have lost。 Who knows what wonders； what practical knowledge they contain？ We would also obliterate much of Venus' gorgeous surface geology—which we've just begun to understand； and which may teach us much about the Earth。 This is an example of bruteforce terraforming。 I suggest we want to steer entirely clear of such methods； even if someday we'll be able to afford them （which I very much doubt）。 We want something more elegant； more subtle； more respectful of the environments of other worlds。 A microbial approach has some of those virtues； but does not do the trick； as we've just seen。
We can imagine pulverizing a dark asteroid and spreading the powder through the upper atmosphere of Venus； or carrying such dust up from the surface。 This would be the physical equivalent of nuclear winter or the Cretaceous…Tertiary post…impact climate。 If the sunlight reaching the ground is sufficiently attenuated； the surface temperature must fall。 But by its very nature； this option plunges Venus into deep gloom； with daytime light levels perhaps only as bright as on a moonlit night on Earth。 The oppressive； crushing 90…bar atmosphere would remain untouched。 Since the emplaced dust would sediment out every few years； the layer would have to be replenished in the same period of time。 Perhaps such an approach would be acceptable for short exploratory missions； but the environment generated seems very stark for a self…sustaining human munity on Venus。
We could use a giant artificial sunshade in orbit around Venus to cool the surface； but it would be enormously expensive； as well as having many of the deficiencies of the dust layer。 However； if the temperatures could be lowered sufficiently； the CO2 in the atmosphere would rain out。 There would be a transitional time of CO2 oceans on Venus。 If those oceans could be covered over to prevent re…evaporation—for example； with water oceans made by melting a large； icy moon transported from the outer Solar System—then the CO2 might conceivably be sequestered away； and Venus converted into a water （or low…fizz seltzer） planet。 Ways have also been suggested to convert the CO2 into carbonate rock。
Thus all proposals for terraforming Venus are still brute…force； inelegant； and absurdly expensive。 The desired planetary metamorphosis may be beyond our reach for a very long time； even if we thought it was desirable and responsible。 The Asian colonization of Venus that Jack Williamson imagined may have to be redirected somewhere else。
MARS： For Mars we have just the opposite problem。 There's not enough greenhouse effect。 The planet is a frozen desert。 But the fact that Mars seems to have had abundant rivers； lakes； and perhaps even oceans 4 billion years ago—at a time when the Sun was less bright than it is today—makes you wonder if there's solve natural instability in the Martian climate； something on hair trigger that once released would all by itself return the planet to its ancient clement state。 （Let's note from the start that doing so would destroy Martian landforms that hold key data on the past—especially the laminated polar terrain。）
As we know very well from Earth and Venus； carbon dioxide is a greenhouse gas。 There are carbonate minerals found on Mars； and dry ice in one of the polar caps。 They could be converted into CO2 gas。 But to make enough of a greenhouse effect to generate fortable temperatures on Mars would require the entire surface of the planet to be plowed up and processed to a depth of kilometers。 Apart from the daunting obstacles in practical engineering that this represents—fusion power or no fusion power—and the inconvenience to whatever self…contained； closed ecological systems humans had already established on the planet it would also constitute the irresponsible destruction of a unique scientific resource and database； the Martian surface。
What about other greenhouse gases？ Alternatively； we might take chlorofluorocarbons （CFCs or HCFCs） to Mars after manufacturing them on Earth。 These are artificial substances that； so far as we know； are found nowhere else in the Solar System。 We can certainly imagine manufacturing enough CFCs on Earth to warm Mars； because by accident in a few decades with present technology on Earth we've managed to synthesize enough to contribute to global warming on our planet。 Transportation to Mars would be expensive； though： Even using Saturn V… or Energiya…class boosters； it would require at least a launch a day for a century。 But perhaps they could be manufactured from fluorine…containing minerals on Mars。
There is； in addition； a serious drawback： On Mars as on Earth； abundant CFCs would prevent formation of an ozone layer。 CFCs might bring Martian temperatures into a clement range； but guarantee that the solar ultraviolet hazard would remain extremely serious。 Perhaps the solar ultraviolet light could be absorbed by an atmospheric layer of pulverized asteroidal or surface debris injected in carefully titrated amounts above the CFCs。 But now we're in the troubling circumstance of having to deal with propa