I have to admit that I’m rather impressed and surprised with the amount of response my earlier posting on that topic received. Due to some misunderstandings, I realized though that I probably need to add and clarify a bit.
First and foremost, the list I came up with wasn’t supposed to be a wish-list of all the technologies it’d be wonderful to have at some point in the future. It was an attempt (albeit a bit feeble) at trying to list which technologies we would need to master before we could become a truly spacefaring society. An important corallary to that is that though we need to eventually master all of those technologies and techniques, the transition to becoming a spacefaring society is likely going to be a gradual, evolutionary process. I’ll come back to this in a second.
The second point I’d like to make is that there are a lot of cool and interesting technologies that I really don’t consider neccessary for becoming a spacefaring society. A lot of commenters discussed tethers or space elevators. While I think they’re interesting, I can easily envision a future where it turns out that we just couldn’t get CNTs strong enough for a space elevator, but tens of thousands or millions of people live off-planet anyway. Even if tethers work perfectly, you’re still going to have to do lots of normal rocket propulsion too, and if we haven’t figured out how to store or transfer propellants–we will not really be a true spacefaring society even if we have a dozen space elevators.
The third point goes back to the corallary of the first point. In transitioning from being a space-visiting society to becoming a spacefaring society, some of those technologies are more neccessary than others, because they enable others. Take water recylcing or space tugs. It’s possible to have a space tug at ISS, and have complete water recycling capabilities while still not becoming a spacefaring society. If we’re still launching all of our payloads on expendable boosters, it’s only going to make a marginal difference. A moonbase that is being built by single-use transportation modules that can’t be refueled and have to be launched on one or two big launchers will never be affordable regardless of how good your Extraterrestrial navigation skills are, or how good your closed-loop life support system is.
If I had to pick three of the technologies that I think are the most critical, the soonest, I’d have to say reusable launch vehicles, on-orbit propellant storage, and on-orbit propellant transfer. The reason why I feel these are the most important is that all of these strike at what I think is the key piece of the puzzle–transportation. When transportation is cheap, frequent, reliable, and flexible, everything else becomes easier. Here’s an example of what I mean:
Let’s take aerobraking for instance. Right now, almost all “aerobraking” that is done is either in the form of aerocapture where you only bleed off enough energy to enter an elliptical orbit, and then over several weeks or months you use propulsion and successive passes to bleed more and more energy off. What you really want to do, especially if you have people, or time-sensitive cargo, is to be able to take as much energy off as possible in a single pass, so that a small propulsive burn is all that is needed to leave you in your final orbit. The problem is that the drag medium (the atmosphere) isn’t constant. Its density changes quite a bit over time, and if you go too deep, you’ll burn up or reenter, but if you go too slow, you’ll bounce off and have to take several passes. We need to learn how to accurately measure the densities in the atmosphere (preferrably from sensors mounted on-board the aerobraking body), so that we can safely navigate through the atmosphere at the right angle and trajectory, in order to maximize the delta-V savings while simultaneously remaing safe. In order to do that (and in order to get enough practice and experience with doing that, so that new pilots can be trained, procedures developed, etc) you really need to actually fly. Research, CFD, wind tunnels–they’re all great, but utterly insufficient. You really want to build, test, and fly lots of small, cheap prototypes. Throw dozens or hundreds of aerobrakers at the problem until you’re so utterly familiar with the right way of doing it that it can becomes natural, until you’ve gotten it to the point where the same aerobraker can fly the mission succesfully ten or twenty times in a row without damage, need for refurbishment, or requiring excessive propellant use to reach its destination. The problem is that that will not be possible until you have regular, low-cost access to orbit. Which really requires reusable vehicles. If the aerobrakers can be designed to be reusable, and if you can recover and refuel them and try again, the total cost of learning how to do that goes way, way down.
I could give other examples, but that’s one that’s been floating around in the back of my mind long enough that I had to let it out.