My contribution to the post-Columbia discussion:

After we mourn the loss of seven good souls and an expensive but relatively ancient spacecraft, there are two main questions I have about our space program. Neither of them is whether we should have such a program.

No, my questions are: (1) How are we going to gain access to space in the future and (2) What are we going to do once we get there.

Rand Simberg is right. Economics is the key to both questions. The cost of space access must be brought down. If (when?) it is cheap to get into space, people will get into space. Once people are in space in large numbers, off-earth facilities will follow inevitably. But the cost of establishing those facilities must also be limited. There must be an eventual end to (or at least a vast reduction in) the support required to maintain the facility once it is up and running.

For the time being, access to space will continue to be via chemical fired rockets. The only other possiblity now on the horizon appears to be a space elevator. You don't need to go farther than Arthur Clarke to see how such a thing might be constructed and operated. There are two major problems, however. One, because the elevator would have a fixed base on earth, its other fixed point would have to be in geostationary orbit, about twenty four thousand miles above the equator. I don't think we have the materials or the technology to do it in anything like the near future. Two, it would present a big fat vulnerable target on earth for the crazies. And the crazies, like the poor, will always be with us. These are people who thought it was cool to bring down two 100 story towers (about 1000 feet each). How much cooler would they think it was to bring down a structure twenty four thousand miles high? Bringing down the WTC towers caused billions in damage to New York City. Bringing down a space elevator would cause worldwide damage on a scale hard to imagine, though someone already has. I think it was Fred Pohl (recently dead, alas) wrote a story about the collapse of a space elevator as a result of a terrorist attack. How would you defend the parts of the structure accessible to the crazies (say the parts within reach of your basic military aircraft, or about one hundred thousand feet)?

The second question is, to my mind, a no brainer. When we get off this planet (to wherever) on a large scale, we should stay and live there. I fervently believe that we have to get a significant number of us off this planet. That means we have to establish permanent, self supporting and nearly self sufficient communities elsewhere. The self supporting and nearly self sufficient parts of that are the most important. If those are achieved, permanent will take care of itself. For this to work, once the project is up and running, it should not require much time, effort or money to keep it running. Otherwise, the facility will never be built or will be abandoned once built. Again, economics is the key to the problem.

Where should this be done? The moon, Mars and space itself are the only three possiblities for the foreseeable future.

Sgt Stryker argues for a mission to Mars as a new overarching goal for the space program. Going to Mars and establishing an off-earth facility there has several advantages over the alternatives. There would be more resources available than would be the case at either a moon or a space based habitat. Mars has an actual an atmosphere, such as it is. It would be easier to build and maintain a facility there than it would be in space or on the moon. And presumably Mars can be mined in a manner similar to the moon or the asteroids. The primary disadvantage, as I see it is distance. Mars is far, far away, and hard to get to. How far away and how hard varies. Mars and the Earth do not march around the Sun in lockstep, so they are not always the same distance from each other. That means that it will be impossible to deal with emergencies on Mars from earth. With a moon or space based facility, help can arrive within days. Not so for Mars. Whoever is on Mars will largely be on their own. A secondary disadvantage is that Mars has its own gravity well. Assuming we get to Mars and establish a self sufficient, self supporting colony there means we escaped Earth's gravity well (paying a big price to do so) and climbed right back down into another one. That said, it would be easier to climb back out of Mars' gravity for two reasons: Mars' mass is substantially lower than that of Earth, so its gravity is not as strong and Mars' atmosphere presents much less of a barrier than does Earths to vehicles traveling at escape velocity. Mars also has a huge mountain that nearly pokes itself right out of the atmosphere.

My guess is that Mars would be an eventual goal because it can be assumed to be able to support many more people than either a space based or moon based facility could.

Space: Space habitats are a long, long way off. It's taken us decades to build a space station that requires constant resupply and can support a only few people at any one time. Indeed, because the destruction of Columbia has (temporarily?) grounded the shuttle fleet, the resupply of the International Space Station is a significant problem. How much longer would it take and how much more would it cost to build a nearly self sufficient station that can support thousands and is economically self supporting? If it is to be space based, it doesn't matter where it is. Whether you put the thing in high earth orbit, in orbit around the moon, or at a Lagrange Point is irrelevant. The problem is getting it built. Even assuming that such a thing is feasible using available technology, the cost would be prohibitive. And once you build it, what have you got? A place for thousands to live and do exactly what? What resources will be available to work with? Lots of solar energy and precious little more, unless you sent the thing out to the asteroid belt to do some mining. But in that case (and if you locate it at either of the two Lagrange points where orbital mechanics are stable), you have the same distance problem as you do with Mars.

The Moon: I think that its possible that a moon facility would be of more immediate practical use. Mines and mass accelerators on the moon can vastly reduce the amount of stuff we have to haul up from sea level to low earth orbit to continue operations in space. Escaping the earth's gravity well is a large chunk of the cost (and risk) of space operations. Who said "when you achieve LEO, you're halfway to anywhere", Heinlein? Pohl? Adding a source of supply outside that gravity well (and not in any other gravity well) may be the key to reducing that cost. It might make either or both of the other possiblities economically feasible. It would certainly change, but probably not reduce, the risks involved. And we already have most, if not all, of the technology to do it.

So my vote is for a permanent, self supporting and nearly self sufficient facility on the moon as a first step into space.

UPDATE. Paul Palubicki (a/k/a Sgt.Stryker) writes to inform me that the novel concerning the destruction of a space elevator was Red Mars by Kim Stanley Robinson. He also notes correctly that the Earth end of an elevator would have to be on the equator. He's not too keen on the governmental structures in place in most equatorial countries and suggests that if the US is to build an elevator, the treatment of Panama and Columbia during the construction of the Panama Canal might be instructive. Good point, although I could see creating an aritficial island to serve as the Earth end of things.