Artur Debat / Jackal Pan / Getty / The Atlantic

Last week, NASA put out a call for applications for its next class of astronauts. In recent years, when the agency has asked for résumés, the job was shuttling back and forth between Earth and the International Space Station (ISS). But for the first time in nearly 50 years, some of the aspiring space travelers might be training for a mission to the moon.

President Donald Trump wants NASA to fly American astronauts to the lunar surface and beyond. Congress, the president said during his recent State of the Union address, should fund his administration’s new program “to ensure that the next man and the first woman on the moon will be American astronauts.” This effort, he said, would serve as “a launching pad to ensure that America is the first nation to plant its flag on Mars.”  

But what if NASA didn’t stop at the moon first? What if, instead of pouring time, effort, and money into reaching the lunar surface again, the space agency embarked on an unprecedented journey straight to the red planet?

The moon-versus-Mars question is older than the Trump administration’s space ambitions. It has been debated since the last men departed from the moon in the 1970s, when NASA, having achieved a seemingly impossible feat, started pondering what to try next. The conditions that fueled the space program half a century ago—when less than a decade passed between John F. Kennedy’s vow to send a man to the moon and the Apollo moon landing—no longer exist (and unless you wanted to risk war, it wouldn’t be wise to replicate them). Neither does the budget that made it happen. So the country’s next chapter in exploring other worlds may be more open-ended than ever.

Scientists have their own arguments for the most compelling celestial destinations and what humankind could learn from them. These arguments are, of course, subject to the whims of presidents, who have their own ideas about the nation’s space policy, a circumstance that has ended up swinging NASA priorities from one part of the solar system to another every eight years. In the most recent administrations, George W. Bush wanted to go back to the moon, Barack Obama didn’t, and Trump very much does. Presidents’ dreams are limited by others’ whims, too: Congress ultimately decides how much money NASA gets and for what.

When Jim Bridenstine, NASA’s administrator, talks about the agency’s Artemis program for lunar exploration—named for Apollo’s sister in Greek mythology—he describes the moon as a “proving ground” for Mars. If something went wrong there, astronauts could make it home in a matter of days instead of months. And while NASA’s newest moon rocket is currently behind schedule and over budget, at least the space agency knows how to build one already.

“We need to crawl before we walk, much less run,” says James Rice, a senior scientist at the Planetary Science Institute who spent 15 years working on Mars rover missions. “Say you’ve got a bunch of new camping equipment. If you’re smart, you’re going to learn how to use that in your backyard. You’re not going to go, say, out to Mount Everest and try to figure out how to work this stuff.”

Supporters of a Mars-direct mission argue that, in fact, the moon is terrible practice for Mars. On the moon, astronauts don’t have to worry about surviving a plunge through an atmosphere; on Mars, they will. The tug of gravity is weaker on the lunar surface than on Mars. The moon is extremely cold, and a single day there lasts nearly an Earth month. By contrast, a Mars day is nearly equal in length to an Earth day, and a summer day on the red planet, near the equator, could feel as pleasant as a spring day on our own.

Even the dust on the two planets is different. Lunar dust is a fine powder of tiny particles with jagged edges, and, as the Apollo astronauts learned, nearly impossible to brush off spacesuits and equipment. Martian dust is more like that found on Earth, and less likely to shred human lungs if it’s accidentally breathed in. “If we had done lunar sample return prior to sending Neil Armstrong and Buzz Aldrin to the surface, I’m convinced it would have been decades before we sent people to the moon,” says John Grunsfeld, a retired NASA astronaut and a former associate administrator of the agency’s science division. “The medical community would have gotten those samples back, looked at them in the microscope, and gone, ‘Oh my gosh, it’s broken glass at the micron scale. The astronauts will inhale it, hemorrhage, and die on the moon.’”

One of the most promising discoveries on both Mars and the moon is water, but how that water is situated varies between the two. Spacecraft and samples from the Apollo missions have shown that the moon has potentially massive amounts of water frozen as hard as granite in deposits deep at its poles, which future astronauts could mine for their life-support systems. But the icy water on Mars may be more evenly distributed just beneath the surface. The technology that future astronauts would use to extract water from ice on the moon could be overkill on the red planet.

After half a dozen landings that supplied hundreds of pounds of lunar samples that have been carefully studied for decades, the moon may feel pretty familiar to us. But it still has its mysteries, and scientists are eager to probe them. Mars has its mysteries, too, including one that might answer one of humankind’s most existential questions. While the moon is generally understood to be lifeless, on Mars “you can actually go look for signs of existing life on the surface,” says Briony Horgan, a planetary-science professor at Purdue University who works on NASA’s Mars missions, including a rover that is scheduled to launch toward the planet in July. “We can do that with robots, but it’s hard.” (Take it from the little Mars spacecraft that landed last year, ready to drill into the soil to measure seismic activity, and became stuck almost immediately.)

Grunsfeld, the former astronaut, believes that NASA has already shown it doesn’t need the moon to go to Mars. The agency has proved it can keep astronauts healthy for long stretches in space; Christina Koch just came home after nearly a year on the International Space Station, a record for a woman. A journey to Mars would expose astronauts to more cosmic radiation than they experience on the ISS, though, which could increase their lifetime risk of getting cancer. When I asked Grunsfeld about that, he launched into a list of dangers so harrowing, it would be difficult to fault aspiring astronauts for quietly rescinding their applications: “How does that compare to the risk of blowing up on the launchpad or on ascent; getting hit by a meteor, asteroid, debris, some kind of space junk on the way there; burning up in the Mars atmosphere; burning up in the Earth’s atmosphere on the way back; or missing the Earth? You add up all those risks, and the [risk of radiation exposure] is kind of just another one.”

Even Apollo astronauts think it’s time to shoot for Mars. Michael Collins has said that he sees “more moon missions as delaying Mars, which is a much more interesting place to go.” Buzz Aldrin has been writing op-eds for a decade urging the nation to focus on Mars. Outside NASA, Elon Musk’s SpaceX is currently developing a spaceship and rocket powerful enough to head right to Mars. The pro-Mars camp has even included, at least for a time, Trump himself, when, in an uncomfortable rebuke of his own administration’s policy, he tweeted last summer, “For all of the money we are spending, NASA should NOT be talking about going to the Moon. We did that 50 years ago.”

Money, however, is decidedly one of the limiting factors. “If we went all in—if we had an Apollo-like budget—we could probably get to Mars without going to the moon within 10 years,” says Chris Carberry, the CEO of Explore Mars, an industry group that advocates for sending people to Mars by the 2030s. (With that kind of budget, Carberry says, NASA might not even have to choose between the two worlds.)

A Mars journey would still be difficult, of course. There are some engineering problems that money alone can’t solve. NASA has a better success rate of landing on Mars than any other space agency in the world, but the missions that have touched down were small rovers, not spaceships full of astronauts. And no agency has ever launched anything back off Mars, an important detail if you want any of those astronauts to come home.

While Carberry believes that NASA technically could mount a Mars-direct mission, a pit stop on the moon would help. But it would have to be short: An extended stay on the lunar surface, or even outpost-construction projects, could leave less room (and cash) for a journey to Mars, he says.

For now, the Trump administration is moving ahead with its goal to land the next man and the first woman at the lunar south pole in 2024. Last week, the White House presented Congress with its budget request for NASA for the next fiscal year, which seeks a 12 percent increase over current funding. NASA’s Mars goal remains the same as it has been for the past decade: Astronauts would make a few orbits around the planet and back in 2033, followed by a second mission that would touch down.

NASA has presented these journeys as inevitable, but they grow more nebulous with time. For some perspective: The world is now closer to 2033 than it is to Y2K. And if Musk gets to Mars first, the federal government might have trouble convincing the public that it needs taxpayer money to fund something a rich guy is already doing on his company’s dime. And how, in the coming years, might the government convince Americans that exploring another planet is worth it as climate change transforms their own?

Rice, the moon fan, says that if and when NASA goes to Mars, whether or not it stops on the moon along the way, astronauts should land rather than just loop around. Otherwise, he says, the trip would be infuriatingly anticlimactic: “It’s akin to flying across the Atlantic, going to Paris, pressing your nose up against a French bakery, looking at the pastry, and then coming home.”

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