HOUSTON—Possessed of slight build and modest height, John Grunsfeld is not an imposing man. But this appearance belies a brilliant mind and a considerable resume. A quarter-century ago, Grunsfeld worked as a physicist at the California Institute of Technology, studying X-ray and gamma ray astronomy, before becoming an astronaut. He flew in space five times and made three trips to the Hubble Space Telescope where he performed eight spacewalks to repair and upgrade the iconic instrument.

In 2012, Grunsfeld assumed responsibility for all of NASA’s science missions. He oversaw the dramatic landing of Curiosity on the surface of Mars, and he was in charge during the exploration of Pluto. He wrangled the James Webb Space Telescope toward the launch pad, and he plotted a pair of ambitious flights to Jupiter’s moon Europa.

On a gray day in early December, I met Grunsfeld—who left NASA earlier this year—at a Starbucks across the street from Johnson Space Center. In the shadow of the buildings where he spent nearly 20 years training to fly in space, we spoke for two hours about all areas of spaceflight, from the intriguing oceans of Europa to human destinations.

Still, the conversation kept circling back to Mars. The Red Planet captivates Grunsfeld, and, from inside NASA, he attempted to steer the massive US spaceflight enterprise toward landing humans there by the 2030s. Although he had some success, Grunsfeld realizes that NASA's fragile, tentative plan for a human mission to Mars may soon unravel.

“I think all anybody has right now is faith that we’ll reach Mars,” Grunsfeld acknowledged. “Right now, it is just a dream.”

A few days later, and a few miles down the perpetually congested NASA Parkway that skirts the southern flank of Johnson Space Center, I met another eminent scientist for lunch. An academic through and through, Paul Spudis lacks the adventurer-astronaut credentials of Grunsfeld, but he has a formidable scientific mind. And Spudis, inspired to specialize in lunar geology while watching the Apollo 15 astronauts explore the lunar surface, is also a formidable advocate for human exploration—of the Moon.

Over a hamburger and dark beer at Boondoggles Pub, Spudis spoke about why he believes humans must return to the Moon before going deeper into the Solar System. As Spudis has outlined in a new book, The Value of the Moon, the Moon is nearby, scientifically interesting, and useful. Moreover, he argues, it is precisely because NASA has chased after ephemeral Mars dreams for the better part of the last half century that humans have remained confined to low-Earth orbit since the end of Apollo.

Both men agree on one point: with NASA’s limited funds, even before possible cuts under a Trump administration, the space agency can’t do both. Sending astronauts to the Moon and establishing a colony would push human exploration of Mars into the second half of this century. Alternatively, making a direct push toward Mars would preclude any meaningful human exploration of the Moon.

A choice must be made. For the last six years, NASA has continued developing a deep space capsule, Orion, as well as begun construction on a large new rocket, the Space Launch System, as the foundation of an exploration program. NASA has promoted a “Journey to Mars,” but in reality the space agency has taken no definitive steps to preclude either a Moon or Mars pathway. That decision will have to be made soon. Within the next four years or so, the space agency must start designing and building specific hardware, for landing and living on either the Moon or Mars.

Which should it be?

The case for Mars

The aerospace community rarely agrees on anything, but pretty much everyone accepts that the next “mountain” for human explorers to climb involves astronauts to the fourth planet in the Solar System. Although it is a poor substitute for Earth—conditions on Mars are less hospitable than at Earth's South Pole during winter—Mars is the closest, best alternative humanity has to our world.

There are essentially three potential rationales for sending humans to Mars now. One is geopolitical. During the Cold War, the United States landed on the Moon to demonstrate technological superiority over the Soviet Union. A Mars landing would send such a statement with regard to Russia or China in the coming decades. However, unlike the 1960s and the race to the Moon, no other country today aspires to land on Mars. In fact, no other agency besides NASA has even successfully landed a rover on Mars.

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A second rationale is colonization. Elon Musk and SpaceX espouse this idea, as do many in the “new space” community. Earth, they say, needs a backup plan, and Mars offers the most suitable location in the Solar System for humans to eke out a living. Over time, with its limited atmosphere and water resources, Mars could become marginally habitable. However, NASA has expressed no formal interest in colonization. Any plan NASA supports will bring astronauts home.

Science offers the final reason to go to Mars. This explains why NASA has increasingly talked about “finding life” elsewhere in the Solar System, or on exoplanets, in the next two decades. Robots can do interesting science on Mars, Grunsfeld said, but they cannot replace the field work done by astrobiologists or other specialists working in situ.

“My passionate interest is in where we came from, where we’re going, and are we alone,” Grunsfeld said. “Mars, 3.8 billion years ago, was a happening place. It was in an orbit where it was cold at the mountaintops, with snowy peaks, and river deltas running into large lakes. Mars was habitable at the same time life began on Earth. All the right ingredients were there. As life took hold here, did the same thing happen on Mars? And, if so, have any of those microbes survived today?”

Curiosity’s exploration of Gale Crater during the last four years has raised a number of questions begging for follow-up exploration, Grunsfeld said. For example, while driving over sediments and rocks last summer, the rover measured a huge rise in methane. That could be of geophysical origin or produced by microbes below ground.

Most importantly, advocates say, Mars represents the next big leap for human civilization. If our evolutionary goal over the next 1,000 years is to find another Earth-like world in the neighborhood of Earth’s Solar System and develop the technologies and skills to send humans there, Mars represents the first step toward such a grand journey.

“Technically, if we decided that we, as a nation, wanted to go to Mars, we could go in a decade,” Grunsfeld said. “I also think that we could do a Mars orbit mission well within our current budget. Mostly, it’s not a budget problem. It’s a will problem.”