That may turn out to be the case, but why try to pick a winning method before anyone has tried anything?

It probably will be easier to mine the moon's pole for volatiles for bases or settlements on the moon. But what about for deep space missions, or LEO or L2 space stations? The moon's gravity well, while much smaller than the earth's, is still much bigger than any asteroid we could capture, so there would be a non-trivial delta-v requirement to get material from the moon's surface to those other outposts.

Asteroids also have some other intriguing possibilities. The NASA study mentioned using just raw asteroid material for radiation shielding. Why not go a step further and just turn an asteroid into a spaceship? It would take a larger body than the one the study considered capturing, but why not, eventually?

For Mars enthusiasts, living, working, and utilizing asteroids would be darn good practice for setting up bases on Phobos and Deimos, which themselves are quite likely captured asteroids. There's a lot of potential in working with asteroids, and I hope that whatever this venture turns out to be fulfills some of that.

They're not going to have the 20% gold problem, anyways. If you had bothered to read the study, you would have known that the asteroids targeted would be C-type, which are full of useful volatiles and organics that can be turned into handy things like water, and hydrogen, and oxygen (which also happen to be pretty good rocket fuels). Any asteroid mining isn't going to be returning stuff to earth. It's going to be using it for other purposes IN ORBIT. That's where the profit comes in: you don't have to launch 500 tons into lunar orbit at today's launch prices.

Plus, that 2.6 billion cost estimate was for a "Prime contractor design, test & build based on NASA-provided specs" with NASA insight/oversight. I'd be willing to bet that a wholly private effort could do a similar mission at a cost quite a bit less than that. (I would also point you to the NASA study that stated the cost difference between SpaceX's Falcon 9 and a NASA developed Falcon 9 was more than half.)

The study wasn't talking about mining the asteroid to return the material to Earth! The asteroid mass would be used to generate water, hydrogen, and oxygen (primarily) for use IN ORBIT, where it is far more valuable than returning x amount of minerals back to earth. It would also be used as a test bed for advancing mining tech, becoming more efficient, and driving down the cost of the next operation.
However, long term, it could very well end up being economical to return materials to earth. If any initial effort at mining of materials that are useful in orbit succeeds, then there will be an existing industrial base for mining asteroids, and the incremental cost of the next one will be less. As mining methods are refined and become more efficient and the industrial capacity in orbit expands, it becomes possible to create more and more of what you need in orbit instead of launching it from earth (which is where much of the expense comes from). Then, when all you have to do is turn the less valuable parts of an asteroid into shipping containers, load it with the more valuable stuff, add an electric propulsion system, then it might be worth returning stuff to earth.
But the bottom line is that mining asteroids is going to be most useful for getting lots of useful material in orbit (be it lunar or Lagrange points or whatnot) without having to go through the process of getting out of earth's gravity well.

Unfortunately this story is now down the page, so this probably won't be read much, but I'm going to correct the false assumption here that seems to have played a major part in this thread.

NASA does not launch military spacecraft. That job, today, falls to the United Launch Alliance (primarily, smaller payloads can go on other US commercial providers), a wholly separate organization from NASA. (ULA does occasionally launch NASA spacecraft, but at that point, NASA is simply a customer who is buying a ride to orbit.) The last time NASA itself launched a military payload was STS-53 in 1992. Since then, all payloads have gone up on unmanned Air-force or commercial launch vehicles. (Why is this? Challenger. The military did not want to be grounded for another two years if another shuttle had an accident.)

So no, we do not need NASA for national security, and have not since 1992.

Back to the point of the main article, I find it interesting that congress appears to be perfectly happy to send hundreds of millions of dollars to Russia for rides to orbit, but have to be dragged kicking a screaming to let NASA pay some American companies to develop the same capability, possibly for even cheaper (i.e. SpaceX's goal of 20-30 million per seat to the ISS)

Though a vehicle may be designed to work in 0.38 earth gravity, that doesn't mean it will collapse or otherwise not work in standard earth conditions. Most often the structural driver for spacecraft, rovers, etc is the launch vehicle environment. Curiosity will be going up on an Atlas V, which will subject the rover to 5-6 G and a strenuous acoustic, shock, and vibration environment. In addition to the launch loads, it also has to survive the sky-crane landing on the surface of Mars. So it really isn't too surprising that it can support its own weight on earth.

You are correct that fuel for attitude control is generally the limiting factor for spacecraft (useable) lifetime. Using solar sails for attitude control would be possible, I think, for spacecraft operating far enough away from a planetary atmosphere. Otherwise, drag from the sail would certainly overwhelm solar pressure. So, though it may be possible, I'm not sure how economical it would be to use for stationkeeping. I would be interested in seeing a trade study between electric propulsion (another low thrust over long duration type system) and solar sails. Solar sails would probably mass more and take up more volume than an equivalent EP system, but would not require nearly as much electrical power, which would reduce solar panel size and save some mass and volume there. Bottom line, though, is that with the state of solar sail tech right now, it'll be a while before anyone tries using solar sails in such a manner. Most solar sail applications I think you'll see is as the main propulsion system getting craft between planets. That's where the greatest benefit of low thrust, but long duration burns that don't require propellant or little to no electric power will be.

Aside from propellant, the big limit is degradation of solar panel generation capacity (from atomic oxygen, radiation, etc) and battery capacity that also degrades over time. Moving parts (like momentum or reaction wheels that control spacecraft pointing) can also wear out. Space is hard on lubricants. Then there are things that can ruin your day like computer errors from radiation effects (like the Galaxy 15 spacecraft) and impacts from space debris. So while eliminating the need for attitude control propellant will improve spacecraft life and is a worthwhile endeavor, don't count on getting unlimited spacecraft life any time soon.

One thing I haven't seen discussed is that once this rocket is built, what the hell is going to fly on it? I don't see any mention of funding for building a 75mT payload that will then be ready to fly when the rocket is. Right now, all I see happening is that this giant rocket will be built and then will sit around for years waiting for something large enough to fly on it to be built.
Which raises another question: Do we even need it? What's the point in having a giant rocket without giant payloads to fly? What I would like to see is a study comparing the cost and timescale of doing the following two options:
1) Build this huge rocket, utilizing the expensive legacy shuttle hardware. Then design and build a huge payload as justification for your huge rocket. (Sound familiar, sort of like the ISS-Space shuttle relationship?) Fly this huge rocket a couple times per year, making you unable to amortize fixed costs (launch pads, support personnel, etc) over more launches. If the launcher fails, you've lost 75 mT of payload. Ouch. So you'll also need to spend more money making extra-sure that it'll succeed. Oh, and if you fail, your mission is completely grounded until the vehicle is fixed and re-tested and certified for flight again, because there are no other 75mT capacity launchers in existence.
2) Start designing and building payloads that will fit on existing (or near future) commercial launchers. Start a market for even more launches. Let some economies of scale come into the picture and reduce launch costs for you. Let commercial companies compete and bid for your business. If the launcher fails, you've lost a lot less payload than in option 1. Inconvenient, but not as devastating as losing option one's super-launcher. Also, if one of your commercial launchers does fail, you have some more to choose from to launch things in the meantime while the failed rocket is investigated, fixed, re-tested and re-certified. Your entire program does not have to grind to a halt while the launcher is fixed.

Now, which of those options would result in more activity in space? Personally, I think option 2 would be the way to go. More opportunity for cost saving. No single point of failure to get your stuff into orbit. You can start designing payloads to go up right away, instead of waiting for the funding to become available after your shuttle-derived super launcher is ready.

But, of course, what I wrote above does not matter to congress. All they care about is: Does this program pay back my donors enough for them to keep supporting me?

What trade studies were done that decided a 75mT payload capacity was needed as opposed to a 50 or 60mT? Is there a linear increase in cost vs. payload capacity? Is 75mT some sort of optimum, balancing cost vs. development time vs. existing hardware capabilities?

Or is it a number pulled out of someone's ass?

Arguing requirement vs. design is mostly semantics at this point. What matters is where the number came from and what sort of analysis went into it (if any).

Likely the FCC because the vast majority of commercial spacecraft are communications spacecraft. That means the FCC already regulates power/frequency allocation for ground and inter-spacecraft communication for those spacecraft.

The FCC calls for all US-registered spacecraft to be disposed of at the end of its useful life. This means either decay into the atmosphere within a specific amount of time (25 years, I think) or placement into a "disposal" orbit. For geosynchronous spacecraft, that disposal orbit is one slightly higher, getting it out of the way of operational spacecraft.