They extracted propulsion energy from the space-time around them so it wasn't necessary to carry fuel mass.

We cannot conquer the solar system with chemical rockets any more than you can row cargo vessels across oceans. Its that simple. The next forseeable technological steps, fission power, ion drives, and solar sails will allow us to explore, but not exploit industrially, the outer solar system. Permanent settlements and self-sustaining colonies, real space industry, will require another order-of-magnitude advance in technology; perhaps one we haven't even foreseen.

We may get lucky and be able to leapfrog that gap in a few decades, like the jump from sail to steam. But we cannot guarantee that will happen. We have already picked all the low-hanging fruit on the technological tree. There may be other, better fruit higher in the branches, but we can't see them and they will require long and expensive ladders.

The best thing we can do now is to spend our limited resources as effectively as possible, a massive program of discovery and exploration using the cheapest robotic techniques possible. We must maximize our chances of stumbling across that one stash of Aztec Gold that will release the massive R and D investment needed to develop the long-shot technologies we need to bring it home.

This is pretty much the plan we've been following up to now, and it has yielded immense scientific results at a very modest cost. "Modest cost" is important, or people (and investors) will quickly lose interest. There have been a few deviations that have allowed us to develop techniques and capabilities we'll need later for manned spaceflight (such as Mercury, Gemini, Apollo, the Shuttle and the Space Station). These projects have been more experimental and expensive, and the results haven't been quite as successful, but we need to do them in order to be ready when the breakthrough comes.

Our current efforts in manned space travel will not lead to the breakthrough we will need to become a true spacefaring species. That will come in an industrial or university laboratory right here on earth, and it will be financed and inspired by the entrepreneurial response to data acquired by a robotic probe.

Its going to take time. Technology moves a lot faster today than it did in the 16th century, but the volume of space we need to explore and the energy required to do so have expanded too. But fear not. Neolithic cultures on resource-poor islands conquered the Pacific Basin. Surely we can do better.