The dazzling sunlight that flooded the lake-front restaurant where I sat down with Chris Kraft in 2014 was nothing compared to the brightness in his eyes. He'd just turned 90 and was frustrated that NASA hadn't flown any humans beyond low-Earth orbit since he was the agency's first flight director, leading the Mercury and Gemini programs. As much as anyone else, Kraft had built NASA and put men on the moon. You would think he'd want to see humans on Mars soon. Instead, he spent the next 90 minutes eating pasta and explaining that Mars, for now, is best left to robots.

NASA’s justification for sending humans to Mars has something to do with jump-starting the search for life while furthering research and exploration on the red planet. However, even under the space agency’s most wildly optimistic plans, humans will not reach the surface of Mars until the late 2030s. During his lifetime, Kraft has watched the increasing sophistication of robots and artificial intelligence. He imagines that this progress will continue apace or even accelerate. With these trends, the robots and rovers of the 2030s will certainly have some impressive capabilities. If so, why should NASA spend 20 to 40 times as much to send humans to Mars when robots could be almost as able, at a fraction of the cost?

The human rationale

It’s a question perhaps best answered by one of the space agency’s foremost modern explorers, John Grunsfeld. Not only was Grunsfeld a five-time flier on the space shuttle and chief repairman of the Hubble Space Telescope, he also served as the agency’s chief scientist. I had a chance to put the question to Grunsfeld before he left NASA this spring.

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“There’s been a myth that there’s some things you can do with robots and some only with people," Grunsfeld replied. “All exploration is human research. Even when we use robotic spacecraft, it’s still human research. The question is how close are the people to the action? And it’s also about the pace of discovery. When you have people on the scene, especially putting planetary scientists, geologists, astrobiologists on Mars, it’s really going to accelerate the pace at which we can make discoveries.”

Rovers like Spirit, Opportunity, and Curiosity are great for doing initial, exploratory research, Grunsfeld said. But robots are not so good with surprises because they’re optimized for a certain range of science. If there’s something Earth-based engineers and scientists didn’t anticipate, the rover may completely miss the discovery because it doesn’t have the instrumentation or programming to respond to the situation.

“But when we have senior scientists with proper instrumentation, they’re going to be able to do a much broader range of science and be able to discover things much faster,” Grunsfeld said. “And so it’s really this convergence of wanting to have humans explore beyond low-Earth orbit and the heightened ability of scientists to make progress working with robotic spacecraft that’s going to be the real power.”

Grunsfeld noted that Mars’ “recurring slope lineae” (or RSL features) were recently confirmed to be briny water that flows during the planet’s relatively warm summer months. If any life exists today on Mars, he said, it’s likely to occur at the ice-mineral interface near the planet’s surface.

“These are places we’re going to have to go very carefully if we don’t want to contaminate Mars,” he said. “We may bring some small robot that is highly sterilized that we can deploy and then operate within visual range. This would allow us to access these sites that are very special regions rather than send someone in a spacesuit that is relatively dirty.”

What about colonization?

There are other reasons to send humans to Mars aside from science, of course. SpaceX wants to colonize the red planet. As the company seeks to make humanity a multi-planet species, it makes no sense to send robots in the place of people.

Compared to SpaceX, NASA's general desire to expand human exploration beyond low-Earth orbit seems nebulous. Perhaps the most politically expedient goal the agency could adopt would resemble an Apollo-like push to Mars. Over a decade, NASA could send four or six crews to stay for 30 days at a time. Such a “flags and footprints” effort would of course rank among the grandest human achievements of all time.

Yet that is precisely the scenario Kraft and others worry most about. They have lived through the more than four decades of halting exploration NASA has conducted since the final Apollo flight in 1972. NASA went so far, so fast, that when it pulled back from the moon there was no program left to execute. Space historian John Logsdon has chronicled some of these struggles in his book After Apollo.

For his part, Kraft would like to see a more sustainable and practical program. Start out at Earth’s nearby moon, he says. Tap into its water ice and other resources to build a lunar colony with other interested nations. Humans might construct a large radio telescope on the far side of the moon, or we could use the lunar regolith and its abundant silicates to build a large solar farm to beam power back to the Earth. Let robots explore the solar system's dangerous frontier while humans build a more permanent presence in space closer to home.

After lunch Kraft and I drove back to his home, which overlooks a golf course a stone’s throw from Johnson Space Center. As we shook hands in his driveway, he reiterated his closing argument to me: “Oh yes, I’ve heard the argument that we’ve been there before. I know that more than most. But we have unfinished business on the moon.”