As I said at the parent thread, I am enthused and subscribed! And one reason for this is that I tried to spark interest in just such a WI when I was very new to the site. I am very keenly interested in the ATL question of just what this does to the space programs of the various nations.
As far as basic mechanics go, as others have already pointed out, reaching Venus is in fact within capacity of a Saturn V! And OTL Apollo Applications included a proposal, indeed a set of various proposals, for flyby missions (that would not attempt to achieve orbit around any of their proposed target planets) for a manned Apollo derived spacecraft. Essentially the idea is to replace the mass of the LM with a habitat extension for the Command Module, and use essentially unmodified CSM.
These missions considered simple flights to Venus, and also very complex missions that would take many years, flying by both Venus and Mars before returning to Earth. Although it only takes less than half a year to get to Venus, to then return to Earth on the same trajectory (factoring in how it could be changed passing by Venus) takes a lot longer, and bouncing around all three planets would take several years. The question of whether three astronauts in a tin can would go berserk cooped up like that rivals the question of how much radiation damage they'd pick up on the way to cast doubt on the feasibility of such voyages of course!
A translunar trajectory of the type Apollo actually used very nearly achieves escape velocity from Earth as it is. To go to Venus on a minimum energy Hohmann trajectory requires additional velocity, against Earth's orbital motion, to send it down to encounter Venus where it will have perihelion orbital speed in excess of Venus's. But the additional velocity change above a typical TLI required from a low Earth parking orbit is remarkably small as i recall, and compared to the 45+ tons the Saturn V third stage could throw at the Moon, some 30 or more are available for a simple Venus mission.
Now unfortunately part of the plan, for the safety of the astronauts, was to provide an abort mode whereby the CSM would dock with the habitation module, as Apollo Lunar missions had it dock with the LM, and then fire the third stage--this allowed the CSM the option of escaping a misfiring stage and braking hard immediately to return the crew to Earth. I don't know if the entire fuel supply needed in the Service Module for a Lunar mission would be needed for an abort of this kind--if so, there isn't any mass left over for the habitation!
And if there is no abort, the huge mass of propellant is mostly useless for the rest of the mission. Some midcourse corrections would be needed but basically once launched on a flyby trajectory, the craft will return to Earth of its own accord under the guidance of Sir Newton. So the less propellant reserved for this abort, the better; every ton freed up is a ton toward the actual mission.
The point being, sending a man or three to land on Venus is already an option once the Saturn V proves itself. Returning them to Earth is of course an entirely different question!
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Given this, what would a reasonable development of the US, Soviet and other space programs be, once Venus is known to be habitable?
Well, as I concluded my first post at the parent thread--even if it is known that Venus's atmosphere is breathable, and temperatures at suitable landing sites are within the range human beings are prepared to bear--what about biological threats? If Venus has a 10+ percent oxygen atmosphere, that very strongly suggests life exists there. Is this life essentially similar to Terran life? Even if it is, there is a good chance it is basically poisonous to us--that evolution, having randomly branched in different, equally plausible, directions, over billions of years has accumulated a suite of biochemical interactions fundamentally similar to ours, yet different enough that contact between them results in mutual poisoning. This would be the way to bet in advance of detailed knowledge--that Venus may be alive, but for humans to live there would require as much infrastructure and protection from an actively toxic environment as if we were to try to live on the airless Moon. Advantages over the Moon (lots of water, plenty of biomass at hand for us to burn up and convert to sterile ash for our own greenhouses, protection from radiation, moderate temperatures and a heat sink for active cooling, etc) would be offset by the pervasive and penetrating nature of Venusian biological "poison." Colonization of Venus would imply massive eradication of the existing ecosystem and if reserves are left in place, they and imported Terran life would mutually poison one another.
But suppose it doesn't turn out to be that way--suppose that on the whole Venus organisms and Terran can interact with each other without immediately poisoning each other. There still might be quite a bit of nastiness, such as allergies, but suppose these turn out to be manageable too. So that a hearty Terran explorer can indeed open their faceplate, take off their pressure suit, don skimpy tropical exploration clothing, and breathe deep the Venerian atmosphere, drink the water--even eat local plants and animals! How exciting!
Unfortunately we still aren't out of the biological woods yet. If we can eat Venerian life forms, they can eat us. Specifically we have the
War of the Worlds scenario. A Venerian microorganism, brought home to Earth, might get loose, and wreck many possible kinds of havoc, from nuisance level to the extermination of humanity and God knows how much more of the Terran ecosystem.
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In light of that potential threat, I would propose that, after the era of the Moon race, during which the space programs are much the same as OTL, except for somewhat more concentration of space probe activity on Venus, both the USA and (despite their liabilities of OTL) the Soviet Union will indeed sustain a high level of funding for Venus exploration. At first, despite the technical bare possibility of the Americans already being able to land an Apollo CM with three astronauts on the Moon, both nations will start with probes--probes with a very heavy emphasis on settling essential biological questions.
If Venus life turns out, as one might reasonably guess, to be poisonous, then the prospects for manned exploration are severely impeded. Better than OTL to be sure and I think the effort will be made. If the author rules that this is indeed the case, interesting stories still can be told and will be. But it would be a project comparable in difficulty to exploring Mars of OTL, in some ways easier--in others, much harder.
What if it turns out though that by and large, the ecosystems are broadly compatible? (This might be explained by means of extensive natural panspermia between the two worlds during the past billion years, more or less "synchronizing" basic biochemical options). Such a situation would greatly ease Venus exploration; a craft might well come down with its crew planning to stay for many many years, eating a mix of Terran crops and local food, freed of the need for creating or even filtering breathable air--able to roam the planet stripped naked of all Terran goods if necessary.
But in this Eden lies a serpent--if the crew don't keel over from some common microorganism in just a few days, Venus may be colonizable--but even if they don't contract some dread disease for years after landing, that doesn't prove there isn't some bug that can devastate Earth one way or another lying in wait on the next island. Odds are good, if anything on Venus threatens Terran life, then it will be the human explorers who are the guinea pigs, the canaries in the coal mine who will get sick and perhaps die to give the warning. But just maybe, something that poses a long-term threat, say after it has mutated, can take up residence in humans or the samples, geological or biological, they may wish to return to Earth, and get loose too late to stop it.
Quarantine then seems to be the order of the day. The missions to send human astronauts to investigate Venus can be greatly simplified if there is no provision whatsoever to take them back home. If Venus is explored by volunteers who know that they probably will never be given a chance to come home to Earth, and if they do it will be decades later when sufficient contact assures that there probably isn't a planet-killer bug for them to stumble upon, then the task of simply getting them and the equipment and supplies they need to Venus is much simpler than the job of landing them there along with a spacecraft that can take them back to orbit to another one that can then take them back to Earth.
So--first a decade or so of careful (and very extensive) unmanned probes. Long before the question of sending humans to Venus comes up, the planet will be examined by probes attempting to determine if those human explorers can survive there or not.
I'm guessing the author's answer is, yes they can.
Then--the call for volunteers, to be sent to Venus and expect to spend the rest of their lives there, be it short or long, goes out.
There is also the prospect of some sort of serious international cooperation in the program, not so much out of friendliness but rather its opposite. Venus exploration can provide a treasure trove of useful new information--including knowledge (or samples of Venus organisms) that can be used as weapons. Given ongoing rivalry and suspicion between the US and Soviet blocs, and rising ambitions of nations like the PRC, paradoxically the outcome might be that the two, three or more blocs involved agree to a shared exploration program, or anyway interlinked ones, so that the rival powers have some assurance that what is learned on Venus will be shared among all the powers.
Alternatively, one can imagine a hard-driving competition where separate parties have jealously guarded separate agendas; such a situation would probably drive a harder competition and secure higher budgets--but it would also be threatening, and I think international diplomacy would tend to defuse some of this.
I'm romantically inclined to favor a unified effort nominally under the aegis of the United Nations--there would really be separate American and Soviet programs, and possibly an independent European one and maybe even a Chinese one too, but all except possibly the latter would agree to some openness and cross-bloc crewing, and signatory to a charter that forswears keeping dangerous secrets and favors cooperation.
Realistically such a dream might kill the program--serving as a cartel to suppress the programs since nations cannot hope to secure secret advantages to themselves, or subversive intentions to do so despite the treaty torpedoes the cooperation. I like to think better but I'm a romantic after all.
And of course the thing might go through phases--born in competition, with reconciliations in thaw periods, followed by renewed rivalry.
Programs of sending people to Venus with no immediate provision of return incur moral obligations on the nations doing it to commit to support their people indefinitely even if no return materializes--therefore they might hesitate to take such a course. But the expense of providing for a round trip combined with the global interest not to risk contaminating Earth could make the one-way option look affordable and proper anyway.
I believe the draw of another planet with life on it, especially one so relatively easy to reach as Venus, will pretty much compel action on someone or other, and if one bloc is going ahead with sending people to Venus, others who might be poorer or more reluctant to make such sacrifices will fear being left behind and follow through themselves. The Venus Race will be a thing, even if international tensions are relaxed.
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Given these considerations, I envision:
1) That the USA will spend a little bit more in the 1960s, sustaining Apollo much as OTL and also adding a few unmanned missions to Venus, including massive probes in the 20-30 ton range launched on Saturn V's.
2) While some of the opposition to spending on Apollo and space launches in general will emerge, some of the parties who OTL backed away from supporting the space program, or anyway were content to see it downsized, will not do so here, because of the looming Venus Race. It will seem more apparent and inevitable to more people that Venus is the next target, and Apollo and its Applications will bend more toward preparation for an eventual (and reasonably soon) manned mission to Venus. Rather--manned/womanned--the first Americans to land on Venus will be prepared for the prospect that something might kill them right off, but if not, they will be living on Venus for decades to come and common sense would suggest they be a married couple, the US Adam and Eve. Therefore the pressure to accept women into the astronaut corps will be there already in the 1960s, or at any rate once early probes confirm that Terran and Venerian life are compatible.
3) The Soviets will take the Moon race more seriously and commit to it more deeply, since they won't want to concede Venus to the capitalists. OTL of course Venus was more a Soviet than Western specialty and their early probes will probably inspire great excitement in Russia, even absent worries about capitalist competition. Therefore they will not see the race to the Moon as merely an optional competition proposed by John Kennedy, but as preparation for the longer term goal of Soviet presence on Venus. I expect they might attempt to scoop the Americans by sending something manned around the Moon first (though not achieving orbit there) and develop the N-1 rocket earlier and more carefully, so that even if they can't beat Americans to the Lunar surface first, they still follow through, perhaps with some delay to make a better lander than the LK (using a two-launch strategy instead of one) and continue to refine the N-1, developing space stations as preparation for assembling a Venus landing mission--all the while sending space probes, including big ones launched on N-1, to Venus of course.
The stronger Soviet space presence will probably not bankrupt the USSR; if it is not on any better course than OTL I suppose it will collapse in 1990, and not sooner despite even heavier Soviet space expenditures. Conceivably Soviet space exploration will shake down to be more efficient and cost no more than OTL despite achieving more.
And it will tend to confirm the Western pro-space lobbies and sustain higher American and possibly European space budgets.
4) The Chinese were talking about their own manned program in the early 70s OTL; perhaps fearing to let the Soviets and capitalists monopolize the planet between them, they will follow through with funding and start developing something long before they committed to it OTL. We can expect them to stay well behind, at least unless and until there is the massive development of industry along the lines of the Deng chairmanship of OTL, which will be delayed at least until the 1980s unless some technocrats hold a coup against Mao (and they get away with it) which might give results by the late 1970s.
5) The drive to Venus in the Western bloc might initially be a wholly American project, but once the first American couple has landed there and survived a year or so, I think the industrialized US allies, in Europe, Japan, and Australia will want "in" and might pony up significant funds in return for a recognized share of the American program, and perhaps reciprocal US adoption of some European and other technology. If the Americans are haughty enough, these parties might resolve to develop their own "people to Venus" program. (It might even be the Europeans who develop the first round-trip program, expecting it to mature a decade or more after the first human landings when it might be safe enough to offer the pioneers a ride home. Surely though the Americans and Soviets will have their own contingency plans along those lines in development).
6) I expect the first human couples to be launched to Venus before 1980; that they'd be Americans on an upgraded Saturn/Apollo suite is a good bet but the Soviets might cut some corners to get the jump on an earlier launch window.
I don't see any point in wasting a launch window on flyby or still more problematic, Venus orbiting, manned missions. The probes will relay their information well enough without any humans in the loop; the half year exposure to radiation involved in simply getting to Venus will be quite enough risk without doubling it or more on a return trajectory.
7) before launching such missions, NASA and probably Soviet space authorities will have contingencies for theoretically returning people from Venus back to Earth, but they will be massively expensive, involving many times the launched mass of simply sending people with a couple year's supplies there. A vehicle to get two people from the surface to low Venus orbit would have to be comparable at least to the Titan II used for Gemini missions. And it couldn't use the hypergolic fuels that rocket used; propellant would have to be produced in situ on Venus, and would probably have to be cryogenic, meaning a mission or three to ship the necessary equipment down to the stranded astronauts. Rockets typically need a lot of ground support; this one would have to be designed to operate with a minimum of that. Upon reaching Venus orbit the crew has accomplished nothing unless another spacecraft, one with habitable space and supplie allowing for a year or so of living, is there waiting for them, with plenty of propellant to achieve the trajectory back to Earth. This could have been gradually shipped up from Venus, in penny packet lots, if the surface to orbit rocket is reusable or Earth sends hundreds of them. Or it could have been shipped from Earth, with the whole fueled return craft being aerobraked to low Venus orbit--that would require a really huge launch from low Earth orbit of course. As would sending a large supply of one-way Venus to Venus orbit rockets, so unless these latter are reusable I'd suppose the return craft was indeed sent fully fueled.
Realistically then NASA and others will have plans for being able to do this, but not actually do it for decades. They might put off developing any return capability in reality until the 1990s, by which time Venus explorers will have been landed for fifteen years or more, and number (assuming later vehicles are much larger than the original Apollo derivatives) in the hundreds. Such a large contingent of Venus pioneers would be much more suitable for operating systems like in situ fuel production plants of course, or serving as launcher crews to send some of their number back home.
Prior to anyone coming back, then, I'd expect communities of American, Soviet, and perhaps even Chinese colonists to exist, and with shipments of equipment sent from Earth, these communities would be raising their own food and even developing the beginnings of industry.
8) More people will go to Venus than ever come back from it. By the 2010s, the human population of Venus might number in the thousands, perhaps in the tens of thousands, and this will include people born there.
