“It [will] cost the world — and the U.S. — billions and billions of dollars to put these people there, and you’re going to bring them back?” Aldrin said during a panel discussion at the Massachusetts Institute of Technology this week. “What are you going to do when you bring them back here that can possibly compare [to] the value that they would be if they stayed there and Mars wasn’t empty? And then, they helped to work with the next group and it builds up a cadre of people. When we’ve got 100 — or whatever it is — then we start bringing people back.”

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This isn’t the first time Aldrin has posited the idea of having semi-permanent Mars colonizers. Earlier this year, in an interview with The Washington Post’s Joel Achenbach, Aldrin worried that the high cost of a Mars mission might deter public funders from sending enough people to Mars to make a difference scientifically.

“If we go and come back, and go and come back, I’m sure Congress will say, ‘Oh, we know how to do that, let’s spend the money somewhere else.’ And everything we will have invested will be sloughed aside,” he said.

Perhaps he was speaking from experience: Since the last manned Apollo mission to the moon, nearly 42 years ago, NASA hasn’t sent any astronauts back.

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In 2005, NASA announced “a new era in space exploration” with the Constellation program. The proposal sought to expand the human presence deeper into space, first by going back to the moon by 2020 and eventually landing on Mars.

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It’s no surprise that in the subsequent years, the program’s goal became increasingly distilled down to a simple objective: do more for less money than the last time.

“We want to do it cheaper, and we want to do it safer,” Jeff Hanley, NASA’s Constellation program manager told SPACE.com in 2009. “That’s a pretty tough prescription for NASA to meet.”

With the U.S. economy in the gutter, the program was eliminated the following year:

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“It means that essentially the U.S. has decided that they’re not going to be a significant player in human space flight for the foreseeable future. The path that they’re on with this budget is a path that can’t work,” [Former NASA administrator Michael Griffin] said, anticipating the Monday announcement. He said that, although he pushed for seed money for commercial cargo flights to space, he doesn’t believe that the commercial firms, such as SpaceX and Dulles-based Orbital Sciences, are ready to take over the risky and difficult job of ferrying human beings to orbit.

Private donors have since demonstrated that they’re serious about commercial space flight. But Aldrin said recently that he believes space tourism conflicts with his firm belief in the need for a semi-permanent human settlement on Mars. And it isn’t just about the money; Aldrin is similarly concerned about the need to produce long-term scientific research in the most cost-effective way.

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“I have considered whether a landing on Mars could be done by the private sector,” Aldrin commented in a Reddit AMA in July. “It conflicts with my very strong idea, concept, conviction, that the first human beings to land on Mars should not come back to Earth. They should be the beginning of a build-up of a colony/settlement, I call it a ‘permanence.'”

But while a return ticket from Mars may be expensive, so is figuring out how to keep humans alive in its inhospitable atmosphere for long periods of time.

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Earlier this year, MIT researchers evaluated a proposal for long-term Mars colonization — not unlike what Aldrin is proposing — called the Mars One plan. While they didn’t find it to be impossible, they concluded that the task would be very difficult and might require technology that doesn’t yet exist.

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For example, sustaining life long-term on Mars would require having enough food for the duration of the mission — which could potentially be indefinite. Carrying food up with passengers could be prohibitively burdensome and expensive. But growing food could also present its own problems, the researchers found:

For example, if all food is obtained from locally grown crops, as Mars One envisions, the vegetation would produce unsafe levels of oxygen, which would set off a series of events that would eventually cause human inhabitants to suffocate. To avoid this scenario, a system to remove excess oxygen would have to be implemented — a technology that has not yet been developed for use in space. Olivier de Weck, an MIT professor of aeronautics and astronautics and engineering systems, says the prospect of building a human settlement on Mars is an exciting one. To make this goal a reality, however, will require innovations in a number of technologies and a rigorous systems perspective, he says. “We’re not saying, black and white, Mars One is infeasible,” de Weck says. “But we do think it’s not really feasible under the assumptions they’ve made. We’re pointing to technologies that could be helpful to invest in with high priority, to move them along the feasibility path.”

And that’s just one of the potential complications.

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NASA scientists and other sharp minds are undoubtedly thinking hard about the answers to these very questions — not just for Mars space travel, but for virtually all future extraterrestrial endeavors. Reducing the energy requirements for space travel and increasing rocket capabilities will be essential to making space travel worth the investment.

Earlier this week, author Kate Greene wrote about her experience as a participant in the University of Hawaii’s HI-SEAS Mars mission simulation. Her take-away, controversial though it may be, was that women would be best suited for Mars missions precisely because they generally weigh less, eat less, and use less energy. It’s a difference that Greene suggest could make an enormous difference, at least in the strictly mathematical feasibility of such a mission:

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Week in and week out, the three female crew members expended less than half the calories of the three male crew members. Less than half! We were all exercising roughly the same amount—at least 45 minutes a day for five consecutive days a week—but our metabolic furnaces were calibrated in radically different ways…. The data certainly fit with my other observations. At mealtime, the women took smaller portions than the men, who often went back for seconds. One crew member complained how hard it was to maintain his weight, despite all the calories he was taking in. The calorie requirements of an astronaut matter significantly when planning a mission. The more food a person needs to maintain her weight on a long space journey, the more food should launch with her. The more food launched, the heavier the payload. The heavier the payload, the more fuel required to blast it into orbit and beyond. The more fuel required, the heavier the rocket becomes, which it in turn requires more fuel to launch.

That seems to be a compelling rationale. Though there are clearly other important reasons to desire more gender diversity — namely team dynamics, skills and fairness.

And there are also other reasons why indefinite trips to Mars might make recruiting any candidates a tad bit more difficult. How many would sign up for potentially never seeing their loved ones and friends on Earth again?