In August, NASA used a series of precise and daring maneuvers to put a one-ton robotic rover named Curiosity on Mars. A capsule containing the rover parachuted through the Martian atmosphere and then unfurled a “sky crane” that lowered Curiosity safely into place. It was a thrilling moment: here were people communicating with a large and sophisticated piece of equipment 150 million miles away as it began to carry out experiments that should enhance our understanding of whether the planet has or has ever had life. So when I visited NASA’s Johnson Space Center in Houston a few days later, I expected to find people still basking in the afterglow. To be sure, the Houston center, where astronauts get directions from Mission Control, didn’t play the leading role in Curiosity. That project was centered at the Jet Propulsion Laboratory, which Caltech manages for NASA in Pasadena. Nonetheless, the landing had been a remarkable event for the entire U.S. space program. And yet I found that Mars wasn’t an entirely happy subject in Houston—especially among people who believe that humans, not only robots, should be exploring there.

In his long but narrow office in the main building of the sprawling Houston center, Bret Drake has compiled an outline explaining how six astronauts could be sent on six-month flights to Mars and what they would do there for a year and a half before their six-month flights home. Drake, 51, has been thinking about this since 1988, when he began working on what he calls the “exploration beyond low Earth orbit dream.” Back then, he expected that people would return to the moon in 2004 and be on the brink of traveling to Mars by now. That prospect soon got ruled out, but Drake pressed on: in the late 1990s he was crafting plans for human Mars missions that could take place around 2018. Today the official goal is for it to happen in the 2030s, but funding cuts have inhibited NASA’s ability to develop many of the technologies that would be required. In fact, progress was halted entirely in 2008 when Congress, in an effort to impose frugality on NASA, prohibited it from using any money to further the human exploration of Mars. “Mars was a four-letter dirty word,” laments Drake, who is deputy chief architect for NASA’s human spaceflight architecture team. Even though that rule was rescinded after a year, Drake knows NASA could perpetually remain 20 years away from a manned Mars mission.

If putting men on the moon signified the extraordinary things that technology made possible in the middle of the 20th century, sending humans to Mars would be the 21st-century version. The flight would be much more arduous and isolating for the astronauts: whereas the Apollo crews who went to the moon were never more than three days from home and could still make out its familiar features, a Mars crew would see Earth shrink into just one of billions of twinkles in space. Once they landed, the astronauts would have to survive in a freezing, windswept world with unbreathable air and 38 percent of Earth’s gravity. But if Drake is right, we can make this journey happen. He and other NASA engineers know what will be required, from a landing vehicle that could get humans through the Martian atmosphere to systems for feeding them, sheltering them, and shuttling them around once they’re there.

The problem facing Drake and other advocates for human exploration of Mars is that the benefits are mostly intangible. Some of the justifications that have been floated—including the idea that people should colonize the planet to improve humanity’s odds of survival—don’t stand up to an economic analysis. Until we have actually tried to keep people alive there, permanent human settlements on Mars will remain a figment of science fiction.

A better argument is that exploring Mars might have scientific benefits, because basic questions about the planet remain unanswered. “We know Mars was once wet and warm,” Drake says. “So did life ever arise there? If so, is it any different than life here on Earth? Where did it all go? What happened to Mars? Why did it become so cold and dry? How can we learn from that and what it may mean for Earth?” But right now Curiosity is exploring these very questions, firing lasers at rocks to determine their composition and hunting for signs of microbial life. Because of such robotic missions, our knowledge of Mars has improved so much in the past 15 years that it’s become harder to make the case for sending humans. People are far more adaptable and ingenious than robots and surely would find things drones can’t, but sending them would jack up the cost of a mission exponentially. “There’s just no real way to justify human exploration solely on the basis of science,” says Cynthia Phillips, a senior research scientist at the SETI Institute, which hunts for evidence of life elsewhere in the universe. “For the cost of sending one human to Mars, you could send an entire flotilla of robots.”

And yet human exploration of Mars has a powerful allure. No planet in our solar system is more like Earth. Our neighbor has rhythms we recognize as our own, with days slightly longer than 24 hours and polar ice caps that grow in the winter and shrink in the summer. Human explorers on Mars would profoundly expand the boundaries of human experience—providing, in the minds of many space advocates, an immeasurable benefit beyond science. “There have always been explorers in our society,” says Phillips. “If space exploration is only robots, you lose something, and you lose something really valuable.”

The Apollo Hangover

Mars was proposed as a place to explore even before the space program existed. In the 1950s, scientists such as Wernher von Braun (who had developed Nazi Germany’s combat rockets and later oversaw work on missiles and rockets for the United States) argued in magazines and on TV that as space became mankind’s next frontier, Mars would be an obvious point of interest. “Will man ever go to Mars?” von Braun wrote in Collier’s magazine in 1954. “I am sure he will—but it will be a century or more before he’s ready.”

Von Braun and other space architects saw Mars as an end point in a stepwise approach to human space exploration—a formulation that influenced NASA’s long-range plan in 1959, shortly after the agency was created. Under this framework, humans would first reach low Earth orbit. Then they would develop ships that could reliably go to and from orbit. A space station would follow. Next, sometime after 1970, people would land on the moon and, eventually, at an unspecified future date, on Mars. All the while, unmanned probes would explore the solar system as well. The underlying idea—that each step would provide expertise useful for the ones that followed—is “one of the great memes in the history of spaceflight,” says Roger Launius, a former chief historian for NASA who is now senior curator in spaceflight at the Smithsonian Institution. “A whole lot of people bought into it.”

The plan might have held, except that in 1961, when only the first step had been accomplished, President John F. Kennedy jumped over the next two and vowed to reach the moon by the end of the decade.

Kennedy’s objective was not to further science or even, really, to further space exploration. Going to the moon was a proxy for a nuclear strike on the Soviet Union, a psychological tactic aimed at asserting American superiority. And it turned out to be a suboptimal way to build a space program for the long haul. An unsustainable level of resources went into reaching the moon—at its peak in the mid-1960s, NASA got $5 billion a year, more than 4 percent of the U.S. budget. (It gets about 0.5 percent now.) Even before Neil Armstrong and Buzz Aldrin bounced across the lunar surface in 1969, NASA’s budgets and workforce were being slashed. “By casting Apollo as a race, there was no reason to continue once we won the race,” says John Logsdon, founder of the Space Policy Institute at George Washington University.

NASA leadership suggested human exploration of Mars after the moon, but the Nixon administration scuttled the idea as too expensive. The president’s economic advisor cited a poll, published in Newsweek magazine about two months after the moon landing, in which 56 percent of respondents said the government should reduce funding for space exploration. (In 1979, another poll would find that half of Americans felt landing men on the moon had not been worth it.) NASA continued its ambitious and successful program of exploring Mars and other planets with unmanned probes such as Viking, Mariner, and Voyager. But human exploration retreated to step two in NASA’s original framework: the space shuttle would fly 135 times from 1981 through 2011. Next came step three, the International Space Station. By the time Bret Drake got his aerospace-engineering degree and began working as a contractor on shuttle missions in the mid-1980s, the idea of sending humans to Mars had essentially fallen off the radar.

Then in 1986, the Challenger exploded shortly after launch, killing all seven astro­nauts aboard. NASA suspended shuttle flights for two and a half years and was forced into a wrenching reassessment of its purpose. Commissions were ­appointed; manned exploration of the moon and Mars were once again suggested as long-term goals. And on July 20, 1989, the 20th anniversary of the first moon landing, President George H.W. Bush said the United States should strive for both ­places. “Like Columbus, we dream of distant shores we’ve not yet seen,” he said. “Why the moon? Why Mars? Because it is humanity’s destiny to strive, to seek, to find.”

What Do You Eat?

Congress killed Bush’s plan, at least partly because NASA estimated it would cost about $500 billion over 30 years. Bush’s son, President George W. Bush, and his successor, Barack Obama, have since held up human exploration of Mars as a goal for NASA, but the funding necessary to make it happen hasn’t followed during either administration.

All the while, Drake and his colleagues have plugged away, in fits and starts, at the long-term work required to keep the possibility open. Befitting a lifelong NASA employee, Drake lacks the wild-eyed bluster of space-colonization zealots. He is plainspoken and reserved, even when he acknowledges his frustration over the endless studies his group has had to perform in the absence of an actual Mars mission. “We know what the challenges are,” he says. “We know what technologies, we know what systems we need.”

The challenges are mind-boggling. That’s abundantly clear from the mission outline, officially called a “design reference architecture,” that he completed in 2009. Going into space for more than two years would subject the astronauts to an unprecedented degree of isolation and extended weightlessness; the longest stay in space so far has been 14 months. Poten­tially deadly cosmic rays, which are blocked by Earth’s magnetic field and atmosphere, would hit the spacecraft in flight and threaten the astronauts on Mars. NASA could reduce exposure to the normal background radiation in space by building shielding into the spacecraft and the Martian habitats. But it probably needs a better method of predicting occasional solar flares that spew higher doses of radiation, so that astronauts could be sure to retreat to special “storm shelters.”

Another unsolved problem is that the Martian atmosphere is thick enough for a landing vehicle to need thermal protection against the friction it would generate on entry, but it’s also too thin to substantially slow such a craft down. That means a novel descent vehicle would be required: the sky crane used to land Curiosity wouldn’t work for landing humans, whose craft could weigh 30 times more. Although NASA is building a heavy-lift vehicle that could take humans to Mars—essentially a bigger version of the rockets that flew to the moon—a lander is not yet in the works. Drake says development and testing of technologies for a lander must start in the next few years if a mission in the mid-2030s is to be possible.

All that is daunting, but the rocketry is at the heart of what NASA has done before. A much bigger challenge would come from having to do something entirely new: protect and feed humans on another planet over a long stretch. Astronauts spending extended periods on Mars would need to take off their space suits and helmets and breathe inside an enclosed structure. There are reasons to be optimistic; the space station has offered important lessons on building and maintaining “closed loop” life-support systems in which water and air are recycled. It’s also possible to extract oxygen from the carbon dioxide that makes up 95 percent of the Martian air.

But basic problems remain, such as figuring out what the travelers would eat. A Mars habitat might have a greenhouse, but it’s unlikely that astronauts could grow enough to meet all their caloric needs. And NASA food scientist Michele Perchonok doesn’t believe the dehydrated food that astronauts inject with water on the space station can retain adequate nutrients for five years—which is how long it will have to last if some is sent ahead of the first crew, as envisioned. Many solutions are nonstarters because a rocket to Mars would be able to carry only so much weight in food and cooking equipment. Pressure-treating the food is probably the answer, but perfecting that method is hard. When I mentioned to Perchonok that a Mars mission was surely more than 20 years away, she laughed. “I hope so,” she said. “Because we’ve got a lot of work to do.”

In another building, Bruce Sauser—who has been at NASA as long as Drake—is overseeing several projects that could be used on Mars. One of them is a habitat that could withstand the vagaries of a planet where temperatures range from −140 °C to 25 °C and windstorms pound the landscape with dust. Sauser (who has a classic title: manager of the systems architecture and integration office of the engineering directorate) is trying to master a mix of materials for the habitat. Some layers would provide insulation and radiation shielding, for example, while others would be tough enough to stave off punctures. In turn, these layers would fit over bladders to hold in air. One idea is to make the habitat inflatable, so it could be packed tightly en route to Mars.

Sauser estimates it could take 10 to 15 years to be sure of this habitat’s reliability. His team has experimented with materials such as Kevlar and Nextel, an insulator used on the shuttles. But he says that funding cuts and freezes have made it nearly impossible to make more than incremental progress.

“The dollars are spread across so many things, you don’t have enough in any one bucket to take that thing to the next level,” he says. “We don’t have a mission. We don’t have an end goal. If I don’t have that real need, funding, or deadline, I’m going to keep putzing around with it. Well, putzing around can take 30 years.”

The Right Stuff

Solving such problems, Drake says, wouldn’t require as much money as you might think. He says a ­panel appointed by the Obama administration, the Augustine Commission, got it about right when it determined in 2009 that NASA could sustain programs for having both humans and robots explore Mars if its annual budget were boosted by about $3 billion over its 2010 level of $19 billion.

However, NASA’s budget is going in the opposite direction (it’s now under $18 billion), and the cuts are not just a reflection of Washington’s financial woes. They also reflect the ambivalence the public feels about what NASA achieves. On the most practical level, the best scientific research from human missions, from the 1970s Skylab space station through the shuttle and the International Space Station, has had to do with the bone loss, blurry vision, and other problems astronauts suffer when removed from Earth’s gravity. This is crucial research if our species is to have a future in space. But it has the ring of circular logic: we must keep sending people into space so we understand what happens to people in space. “People who are not part of NASA or not part of the space community—maybe they’re well informed, but they don’t see practical applications in their daily lives,” says Launius, the former NASA historian. “And they just have to scratch their heads and wonder: why are we doing this at the costs that we are?”

The question then becomes how much we should value the intrinsic benefits of spaceflight as an expression of our desire to explore our world in the most ambitious way possible.

Stan Love has a PhD in astronomy and has worked on dozens of projects for NASA, but the one-word title on his business card sums him up best: “astronaut.” He flew to the space station on the shuttle Atlantis in 2008 and did two spacewalks. He says the argument for human space missions should be simple: “Exploring is one of the best things people do. Explorations that aren’t easy inspire us. We learn new things. Often the things we learn seem to be worthless at the time.”

Why not do it solely with robots? “We like it, as people, when people do things. If all you’re after is science data—sure, send robots. But we as human beings feel an attachment when humans go and do things like this.”

Love adds that even in the absence of an obvious economic reason to do it, the cost of an inspiring exploration would be worth bearing. The price would be small, he says, compared with the money and effort that get spent in the service of “greed and our ancestral urge to beat the crap out of each other.”

One easy answer might be to say that if people want to go to Mars, then they should plan it and pay for it themselves. Indeed, several companies are proving they can do things in space relatively efficiently. One of them is Space Exploration Technologies, or SpaceX, a private company that is sending rockets to the space station for NASA. SpaceX’s founder, Elon Musk, dreams of going to Mars. He says he wants to put people there in 12 to 15 years.

However, that goal seems to dramatically underestimate the technological obstacles. SpaceX is working on a heavy-lift rocket that could take robotic payloads to Mars. But humans require a bigger and more expensive rocket, and Musk has not revealed plans to build one. Even more important, he has not said how he would carry out the other difficult tasks, like nourishing the travelers (through a spokeswoman, he declined to comment). And even if such a private effort were able to invent and develop the needed technology, the logistics of Mars travel are so difficult that the costs would surely be too high to be covered solely by a new market in space tourism. In other words, it would demand the involvement of large institutions with financial and technical wherewithal—such as a space agency or a coalition of several.

The problem then remains: convincing the public that it is a worthwhile goal. Even within NASA there is no consensus. Brent Sherwood, who formulates solar-system missions for NASA at the Jet Propulsion Laboratory, contends that given the agency’s funding constraints, the resources used to get people to Mars would be much better spent on other human space endeavors. Among other things, he advocates colonizing the moon, stimulating space tourism, and harvesting solar energy from geosynchronous Earth orbit. “I’m a space architect. I’d like to see us do amazing things in space,” Sherwood says. “I just don’t believe that the only measure of amazingness is half a dozen civil servants visiting Mars in 40 years.”

Sherwood, 54, got into the spaceflight business in 1988, when he worked for Boeing on planning human Mars missions. He considers Drake an old friend; he points out that like many people their age, both were inspired by Apollo to join the space program. But he thinks the Apollo legend looms too large, leading people to suggest Mars as the natural successor to the moon when, in fact, the moon landings were anomalies of the Cold War. “Bret has been at it for all those years. I’ve watched us grow old,” Sherwood marvels. “I think it’s particularly elegiac that the generation that was motivated to get into this business because of Apollo is trapped in a limited and limiting and sort of quaint vision of what we thought we’re here to do.”

What is NASA really here to do? It’s a question that hasn’t been adequately answered in 40 years—not by politicians, space experts, or the agency itself. In the meantime, Bret Drake and his colleagues do what they can to keep a flame alive, just in case our society decides that we really want to do something fundamental and magnificent simply because we can.

Brian Bergstein is the deputy editor of MIT Technology Review.