After months of anticipation, SpaceX CEO Elon Musk has at last revealed his plan for making humans a multiplanetary species by sending crews to Mars and beyond.

The announcement, which envisions settlements of up to a million people on Mars by the 2060s, was delivered on September 27 to a packed crowd of aerospace experts at the International Astronautical Congress in Guadalajara, Mexico.

Excitement for the talk was buoyed in large part by Musk’s charisma and SpaceX’s impressive track record of backing up its lofty rhetoric with action—particularly with the company’s regular cargo missions to the International Space Station and its self-landing, satellite-delivering rockets.

Still, things have not been entirely smooth sailing for SpaceX, with a recent launch pad explosion potentially casting a shadow on their Mars ambitions. And keep in mind that when it comes to human missions to Mars, visions like Musk’s have been in the works for quite a while: the last 70 years straight.

The delay is at least in part technical. A trip to the red planet is like visiting an even more inhospitable Antarctica, and its unbreathable atmosphere is less than two percent of what you’d find at Everest’s summit. Never mind the fact that you have to fly at least a year, round-trip, to get there in the first place.

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MARS: Behind the Scenes Get a peek behind the scenes and meet some of the international cast and crew on the set of National Geographic's global event series, MARS, premiering this November.

And don’t even get started on the politics of such an undertaking.

“It’s a choice, not an imperative,” says John Logsdon, an emeritus professor at George Washington University’s Space Policy Institute. “Mars is far away, it’s hard to get there, and it costs a lot of money.”

But for decades, imaginative engineers and policymakers have dreamt of ways over these hurdles and toward the red planet. Some designs were aimed to inspire; others truly aimed to put boots on the Martian surface. But they all have one thing in common:

They’ve never left the drawing board.

Disney and the Germans (1947-1957)

The first plausible Mars plan came from an unexpected source: a terrible novel by a brilliant scientist formerly employed the Nazis. In the aftermath of World War II, German rocketeer Wernher von Braun—who’d later design the Apollo mission’s Saturn rockets—was essentially commandeered as war booty, kept in the New Mexico desert to perform tests of the German V-2 rocket for the U.S. Army.

In an effort to liven up his days, von Braun researched and wrote The Mars Project, a novel about a manned Mars expedition. “The main idea, I think, was to escape from where he was,” says David Portree, an archivist at the U.S. Geological Survey's Astrogeology Science Center and author of Humans to Mars. The novel’s detailed technical appendix described a physically plausible series of spacecraft, routes, and even launch dates.

Von Braun called for a mission to Mars in 1985, with ten 4,000-ton ships and 70 crewmates. After a months-long cruise, the fleet would have sent a landing party to the Martian ice caps on gliders equipped with skis. These astronauts then would hike 4,000 miles to build a landing strip near the Martian equator for the rest of the ships.

View Images Von Braun’s original Mars plan, depicted in the novel The Mars Project, called for ten 4,000-ton ships and a crew of 70, a quarter of whom were “commandos” sent to defend against native Martians. (It was a novel, after all.) Reproduction courtesy Bonestell LLC

Editors at Collier’s magazine soon became enamored with von Braun’s ideas, commissioning a lavishly illustrated article series about the future of space exploration. In 1957, von Braun and former V-2 colleague Ernst Stuhlinger teamed up with Walt Disney for several space-focused episodes of the TV show Disneyland, including one about humans on Mars.

Von Braun’s plans—and relentless popularization of them—helped soften up the American public to the idea of space travel. “They created in popular culture the notion that it was realistic,” says Logsdon.

NASA's First Plan: Nuclear Rockets (1959-1961)

Barely six months into NASA’s official existence, the agency was itching to plan a mission to Mars.

Its first official study served as the outline for NASA’s future plans and borrowed heavily from the “von Braun paradigm,” though it was much smaller and employed highly efficient nuclear-thermal rockets, which use fission reactors to heat hydrogen into a plasma exhaust.

Through the 1960s, the U.S. government conducted ground-based tests of these nuclear rockets, and ever since, they have remained popular among NASA mission designers. But sending nukes into space has proven politically unsettling: To get one into orbit, you’d have to launch huge amounts of uranium into space. They’ve never left the Earth’s surface.

Photos of Mars Get a Big Yawn (1965)

In 1966, NASA fought hard to send astronauts careening past Mars in 1976. The Joint Action Group (JAG) plan would have sent a four-person crew to Mars and back without landing, equipping them with a 40-inch telescope to scout the surface as they zoomed by.

View Images In 1965, NASA’s Mariner 4 probe snapped 22 pictures of Mars’ surface, the first close-up photos taken of another planet’s surface. The seemingly barren terrain quashed enthusiasm in the 1960s for a crewed Mars mission. Photograph courtesy NASA

But new, up-close images of Mars proved to be a buzzkill. The 1965 flyby of NASA’s Mariner 4 probe revealed that the planet's barren surface was pockmarked with craters, and that its atmosphere was much thinner than previously thought—ruling out cruising around on a Martian airplane.

Budget deficits, unrest over the Vietnam War, and a disastrous fire on the Apollo 1 launchpad didn’t help. Congress declined to fund the JAG program, ultimately scuttling piloted flyby plans by 1968. And the following year, similar tensions struck down a short-lived NASA bid for a “second Apollo” landing—the last Mars plan that von Braun designed.

Buzz Aldrin's Big Plan (1985-present)

In 1985, Apollo 11 astronaut Buzz Aldrin began working on an elaborate “cycler” Mars mission, which requires two motherships revolving around the sun to routinely intercept Earth’s and Mars’ orbits. At the mission’s peak, this interplanetary bus route would annually ferry batches of astronauts to and from permanent colonies on Mars and Phobos, one of Mars’s moons.

If the plan sounds enormous, it is: Aldrin fervently believes that if humans are going to go to Mars, they might as well go big.

“What did we get out of [the Apollo missions’ funding]? We landed two people for a day and then brought them back,” he says.

“Why do we think that [less] would establish a credible, inspiring mission sequence to Mars?”

View Images A mission to Mars would require crew to do everything in zero- or low-gravity, including showers. In this 1988 test, Sue Eley tries out an early NASA shower aboard a KC-135 Stratotanker, also known as a “Vomit Comet,” to simulate weightlessness. Photograph by Roger Ressmeyer, Corbis

Over the years, he has fleshed out his plan in numerous books. In April 2015, Purdue University engineering students completed a detailed technical analysis of Aldrin’s plan. And Aldrin recently opened a research institute at the Florida Institute of Technology with his cyclers in mind.

But for the foreseeable future, Aldrin’s wings are clipped by politics. NASA has a more austere plan, called Journey to Mars, but the details haven't been announced, in large part because such a massive, long-term spending project would require the unlikely support of several successive U.S. presidents.

USSR Down, Mars to Go (1989-1991)

On the twentieth anniversary of the Apollo 11 moon landings, President George H.W. Bush announced his Space Exploration Initiative (SEI), a lofty realignment of NASA’s priorities that culminated with boots on the Martian ground by 2019, the 50-year anniversary of Apollo 11.

Though fond of space, Bush never seemed personally invested in the plan. In the months before the announcement, he essentially delegated the White House's space policy to Vice President Dan Quayle and the White House’s space advisers, including National Space Council head Mark Albrecht, who fought hard for it.

Yet from the beginning, the plan showed cracks: Disagreements between NASA and the White House totally soured the effort. “There was a fundamental disconnect,” says Albrecht. “NASA assumed a blank check but didn’t get one,” Portree affirms.

By the time the SEI reached Congress, its price tag—conservatively ballparked at an eye-popping $450 billion—alarmed key members of Congress, who spiked the initiative altogether.

Humans to Mars—In 1999! (1990-present)

After Bush's initiative tanked, Mars advocates looked for a cleaner, simpler plan. In other words, why not go to Mars directly?

So they called it Mars Direct. Designed by a pair of aerospace engineers, the plan called for a robotic advance mission to supply the crew’s living quarters and vehicles using derivatives of the Martian soil and atmosphere. Humans would then follow, staying some 500 days on the Martian surface before returning home.

View Images Colonies on Mars are a sci-fi staple, but even today some Mars mission advocates, such as the Mars Society's Robert Zubrin, envision self-sustaining, enclosed colonies with ample room for growing food. Reproduction courtesy Bonestell LLC

As president of the Mars Society advocacy group, engineer Robert Zubrin has championed his mission for the last 25 years, claiming that the only impediment is NASA itself. An early version of the plan declared that the agency could put “Humans on Mars in 1999!” if it had so chosen.

Though NASA didn’t take up Zubrin’s timetable, its in-house Mars mission takes cues from Mars Direct’s lean, “live-off-the-land” approach. NASA’s upcoming Mars 2020 rover also will carry experiments designed to make fuel and oxygen from the Martian atmosphere.

Private Money, Public Problems (2010-present)

Without a firm NASA commitment, private entities such as Dennis Tito’s Inspiration Mars Foundation, the Planetary Society, and Lockheed Martin have stepped into the fray, laying out Mars missions of their own—with mixed results.

Would You Go to Mars If You Could Never Come Back to Earth? The five would-be astronauts in this film contemplate a possible one-way trip to Mars. A private foundation called Mars One plans to send a crew of four to colonize the red planet by 2025.

The non-profit Mars One, perhaps the most prominent effort, has enlisted dozens of Mars enthusiasts to mount a one-way colonization of the red planet by the 2030s.

But many have derided Mars One as a fool’s errand—and perhaps a scam. One analysis suggests that the colonists likely would starve, and questions about the organization’s finances continue to undermine its credibility.

Journey to Mars (2014-present)

As SpaceX and other private companies jostle for a decades-long Martian payday, NASA continues to tout its agency-wide “Journey to Mars” initiative.

The plan, detailed in late 2015, breaks down the technologies and stepping-stones needed to get humans safely to Mars and back again—amounting to a flexible, and somewhat vague, “Evolvable Mars Campaign.”

And NASA is steadily chipping away at the problems Mars poses: The agency and a bevy of private contractors are busy developing the tech for a Martian trek, such as the Orion crew capsule and the Space Launch System rocket. In fact, NASA baked the red planet into its recent astronaut recruitment, saying that the job postings were “in preparation for the agency’s journey to Mars.”

But today’s bright-eyed astronaut applicants should probably check their expectations for a voyage to the red planet.

For one, even at its most aggressive, the Journey to Mars won’t culminate with boots on the ground until the 2030s—a timetable recognized by NASA and Congress, which is poised to legally bind the agency to send humans to Mars. The most ambitious near-term venture on the books? An oft-criticized plan that would send astronauts into lunar orbit to study a boulder robotically plucked from an asteroid.

And despite successful design reviews, the Space Launch System and Orion still face an uphill climb: The U.S. Government Accountability Office recently criticized the two programs for cost overruns and potential delays.

Most importantly, it’s an open question if a decades-long Mars project could maintain broad political support and funding in the U.S., even if it relied on international partners or private collaborators such as SpaceX.

“When Kennedy said ‘go to the moon,’ he’d already been told that it was possible by 1967—and he fully expected to be president then,” says Logsdon. “One presidential administration is different than doing [a Mars mission] over five or six.”

But that won't stop would-be astronauts from dreaming big.

After all, says Logsdon, “it’s what space people do.”