But Global Aerospace claims to have worked out the complex timing for each segment of travel and the logistics of intercepting the always-moving Astrotels. The company has also engineered the complex mechanics of the taxi, which straps onto the Astrotel and then performs the aerocapture maneuver at Mars.

But can it all work?

It is a grand plan, to be sure. But any plan to put people permanently in space must be grand. With that in mind, some space settlement enthusiasts fear that if the task of getting to Mars is left entirely to NASA, the result might be a one-time-only scheme.

Bruce Mackenzie, who is on the board of directors of the National Space Society and a proponent of Mars settlements, said Nock's plan could work, but feels that Astrotels are not necessarily the best way to begin a human-to-Mars program.

Mackenzie would build up a Mars crew over time. He favors sending two crews of four people each to Mars over a three-year period and leaving them there. Only when the next four arrive, seven years after the first crew left Earth, do two of the original crew members return, "to go around and shake hands with their funders" and rouse public support for continued settlement of Mars.

About a decade into the program, Mackenzie figures enough infrastructure would be in place to begin employing something like an Astrotel.

Mackenzie thinks his plan is cheaper and safer. Among the greatest costs of initial Mars missions would be carrying or creating the fuel and spacecraft needed to lift off from the Red Planet.

"About half the total cost of a 500-day Mars surface stay is the return spacecraft, fuel processing equipment, and supplies for the return leg," Mackenzie said. The return flight also adds to the risk. "It would be ironic if a fatal accident during a return trip caused us to cancel a program to send permanent settlers to Mars."

But Kerry Nock sees it differently. History tells him that getting to Mars should not be just the goal, but the result of having built a transit system.

"I don't want to see that happen again," he said. "If we go to Mars, I'd like to see us go permanently and create the infrastructure along the way to make it happen."