This story appears in the January 2013 issue of National Geographic magazine.

On the edge of a parking lot at the Marshall Space Flight Center in Huntsville, Alabama, stands a relic from a time when our future as a spacefaring species looked all but inevitable, as clear and grand as a rocket ascending over Cape Canaveral.

“This is not a model,” NASA physicist Les Johnson says as we gaze at the 35-foot-tall assemblage of pipes, nozzles, and shielding. “This is an honest-to-goodness nuclear rocket engine.” Once upon a time, NASA proposed to send a dozen astronauts to Mars in two spaceships, each powered by three of these engines. Marshall director Wernher von Braun presented that plan in August 1969, just two weeks after his Saturn V rocket delivered the first astronauts to the moon. He suggested November 12, 1981, as a departure date for Mars. The nuclear engines had already passed every test on the ground. They were ready to fly.

Thirty years after the Mars landing that never was, on a humid June morning, Johnson looks wistfully at the 40,000-pound engine in front of us. He heads a small team that assesses the feasibility of “advanced concepts” in space technology—and NERVA, the old nuclear engine, just might qualify. “If we’re going to send people to Mars, this should be considered again,” Johnson says. “You would only need half the propellant of a conventional rocket.” NASA is now designing a conventional rocket to replace the Saturn V, which was retired in 1973, not long after the last manned moon landing. It hasn’t decided where the new rocket will go. The NERVA project ended in 1973 too, without a flight test. Since then, during the space shuttle era, humans haven’t ventured more than 400 miles from Earth.

All of which might seem to make the question Johnson and I have spent the morning discussing—will humans ever travel to the stars?—sound a little out of touch.

Why did it seem more reasonable half a century ago? “Of course we were crazy in a way,” says physicist Freeman Dyson of the Institute for Advanced Study in Princeton. In the late 1950s Dyson worked on Project Orion, which aimed to build a manned spacecraft that could go to Mars and the moons of Saturn. Instead of using nuclear reactors to spew superheated hydrogen, as NERVA did, the Orion spacecraft would have dropped small nuclear bombs out the back every quarter of a second or so and surfed on the fireballs. “It would have been enormously risky,” says Dyson, who planned to go to Saturn himself. “We were prepared for that. The mood then was totally different. The idea of a risk-free adventure just didn’t make sense.” A few years after Orion ended, Dyson outlined in Physics Today how a bomb-powered spacecraft might travel to a star.

These days it’s easier to outline why we’ll never go. Stars are too far away; we don’t have the money. The reasons why we might go anyway are less obvious—but they’re getting stronger. Astronomers have detected planets around many nearby stars; soon they’re bound to find one that’s Earthlike and in the sweet spot for life, and in that instant they’ll create a compelling destination. Our technology too is far more capable than it was in the 1960s; atom bombs aren’t cutting-edge anymore. In his office that morning, Les Johnson handed me what looked like a woven swatch of cobwebs. It was actually a carbon-fiber fabric sample for a giant spaceship sail—one that might carry a probe beyond Pluto on rays of sunlight or laser beams. “Be very careful with it,” Johnson said. “This is a material that might help us get there.”

To get to the stars, we’ll need many new materials and engines but also a few of the old intangibles. They haven’t vanished. In fact, they almost seem to be bursting forth again in the imaginative space vacated by the space shuttle, which in 2011 joined the Saturn V as a museum exhibit. In the conversation of certain dreamer-nerds, especially outside NASA, you can now hear echoes of the old aspiration and adventurousness—of the old craziness for space.

Last spring, three weeks before I met with Johnson, SpaceX, a private company based near Los Angeles, used one of its own rockets to launch an unmanned capsule that docked with the International Space Station. SpaceX leads several other companies in the race to replace the shuttle as the space station’s supply ship. A month before that, a company called Planetary Resources, backed by billionaire investors such as Google’s Larry Page and Eric Schmidt, announced plans to use robotic spacecraft to mine asteroids for precious metals. Working with Virgin Galactic, a company whose main business is space tourism, Planetary Resources expects within the next year or two to launch a lightweight telescope into low Earth orbit. “We hope by the end of the decade that we will have identified our initial targets and begun prospecting,” says Peter Diamandis, the firm’s co-founder.

“We’re going to look back at this decade as the dawn of the commercial space age,” says Mason Peck, NASA’s chief technologist. “It’s about companies large and small finding ways to make a market out of space. The energy we see now—the economic motivation to go into space—we haven’t seen that before.”

Economics has long spurred exploration on Earth. Medieval merchants risked the hazards of the Silk Road to reach the markets of China; Portuguese caravels in the 15th century sailed beyond the bounds of the known world, searching less for knowledge than for gold and spices. “Historically, the driver for opening frontiers has always been the search for resources,” says Diamandis. “Science and curiosity are weak drivers compared with wealth generation. The only way to really open up space is to create an economic engine, and that engine is resource extraction.”

One resource he and co-founder Eric Anderson have their eyes on is platinum, so rare on Earth that it currently fetches $1,600 an ounce. Sending robots a million miles or more to extract and refine ore on asteroids in near-zero gravity, or to tow an asteroid closer to Earth, will require technology that doesn’t yet exist. “There’s a significant probability that we may fail,” Anderson said at the press conference in April. “But we believe that attempting this and moving the needle for space is important. Of course we hope to make a lot of money.”

Elon Musk, the 41-year-old founder of PayPal, Tesla Motors, and SpaceX, has already made a lot of money, and he is devoting a sizable portion of that fortune to his own space program. The new rocket SpaceX is developing, the Falcon Heavy, will be capable of carrying twice the payload of the space shuttle, he says, for about one-fifth the price. His goal is to reduce launch costs by a further factor of 50 or 100, to $10 to $20 a pound, by developing the first fully reusable rockets. “This is extremely difficult, and most people think it’s impossible, but I do not,” Musk says. “If airplanes had to be thrown away after every flight, no one would fly.”

For Musk, it’s all part of a much grander plan: establishing a permanent human colony on Mars. NASA has had enormous success on Mars with unmanned rovers, most recently Curiosity, but has repeatedly pushed back a manned mission. Musk thinks SpaceX could land astronauts on Mars within 20 years—and then keep landing them for decades after that.

“The real thing that’s needed is not to send one little mission to Mars,” he says. “It’s ultimately to take millions of people and millions of tons of equipment to Mars to make it a self-sustaining civilization. It will be the hardest thing humanity has ever done, and it’s far from certain that it will occur.

“I should emphasize this is not about escaping Earth. It’s about making life multiplanetary. It’s about getting out there and exploring the stars.”

The fastest spacecraft ever built—the Helios 2 probe, launched in 1976 to monitor the sun—attained a top speed of 157,000 miles an hour. At that rate, a spacecraft headed to Proxima Centauri, the nearest star, would take more than 17,000 years to make the 24-trillion-mile journey, a temporal span equal to the one that separates us from Cro-Magnon cave painters. Those inescapable facts lead even some of the staunchest advocates of human spaceflight to conclude that interstellar travel, aside from robotic probes, will remain forever in the realm of science fiction. “It’s Mars or nowhere,” says Louis Friedman, an astronautics engineer and one of the founders of the Planetary Society, a space-exploration advocacy group.

Some scientists, however, find the prospect of eternal confinement to two small planets in a vast galaxy just too depressing to contemplate. “If we start now, and we have started, I believe we can achieve some form of interstellar exploration within a hundred years,” says Andreas Tziolas. A physicist and former NASA researcher, Tziolas is a leader of Icarus Interstellar, a nonprofit organization that aims, as its mission statement says, “to realize interstellar flight before the year 2100.” It is now collaborating with former shuttle astronaut Mae Jemison. In early 2012 the Defense Advanced Research Projects Agency (DARPA) awarded her $500,000 for something called the 100 Year Starship project. “Our task is not to launch a starship but to make sure the technologies and abilities exist within the next hundred years to do that,” Jemison says.

Tziolas thinks we could develop a starship engine that harnesses nuclear fusion, the energy source of stars and hydrogen bombs. When the nuclei of small atoms such as hydrogen fuse, they release enormous energy—much more than is released by the nuclear fission of large atoms such as uranium, the energy source of nuclear power plants and of the old NERVA. While physicists have built fusion reactors, they haven’t yet found a way to make one that yields more energy than it consumes. “I have faith in our ingenuity,” Tziolas says. Only seven decades elapsed between the discovery of subatomic particles and NERVA, he points out; by 2100, he thinks, we should be able to create a fusion engine that could propel a starship to a top speed of 15 to 20 percent of the speed of light.

That would allow it to reach the nearest star in another few decades—if its machinery could last that long. “Twenty years is getting near the upper limit for how long you can design a spacecraft to reliably operate,” says Les Johnson. NASA asked Johnson to look into a 20-year mission, not to a star but to the edge of interstellar space—to the region known as the heliopause, several times as far as Pluto, where the sun’s influence is balanced by that of other stars. “The thought was, you don’t want to immediately start talking about going to the nearest star,” says Johnson. “It’s over four light-years away. It’s just ... daunting, unfathomable.” Johnson’s task was to plan a realistic mission with a technology that’s at least close to existing—a first small step toward the stars.

Right now, fusion engines aren’t close to existing; a nuclear engine like NERVA would be too expensive; chemical rockets might reach the heliopause but could never carry enough fuel to reach a star in a reasonable amount of time. (The Voyager spacecraft, were it headed the right way, would drift by Proxima Centauri in 74,000 years.) In the end Johnson’s team settled on the most evocative technology: a solar sail. Sunlight, like all light, consists of particles called photons, which exert pressure on everything they touch. At Earth’s distance from the sun, the pressure is only about a tenth of an ounce spread over a football field. But a large, thin sheet of reflective fabric, unfurled in the vacuum of space, will feel this gentle force and will slowly accelerate.

NASA launched a 110-square-foot light sail in 2010 that survived for several months in low Earth orbit. It hopes to launch a sail in 2014 that measures a bit under a third of an acre and weighs just 70 pounds. Movable vanes on the corners will allow ground control to maneuver the Sunjammer, which on its yearlong mission will tack some two million miles upwind toward the sun. A 16-billion-mile mission to the heliopause would require a disk-shaped sail 1,500 feet in diameter. After a year or two of sailing, the spacecraft would exceed 100,000 miles an hour.

Proxima Centauri lies 1,500 times farther still. “To sail to another star,” Johnson says, “we’ll need a sail the size of Alabama and Mississippi combined. We don’t know how to build that yet.” What’s more, sunlight alone couldn’t push the sail to the star within a human lifetime, or even many lifetimes; you’d need powerful space-based lasers. “If you take the total energy output of humanity and put it in a laser on a satellite,” says Johnson, “then you could get trip times of a few decades to Proxima Centauri.” And that’s to send a robot the size of Johnson’s desk.

What about humans, with their need for 24/7 life support? Johnson throws up his hands. “When you start thinking about what it takes to supply people,” he says, “and how big the spacecraft would have to be and how much energy it would have to have, you enter the realm of science fiction.”

To build a starship, you first have to build a future that converts fiction into fact, and that takes a lot more than rocket science. The task isn’t figuring out right now how to design a starship; it’s continuing to build the civilization that will one day build a starship. Framed like that, more expansively, it begins to seem less impossible. But it’s a 100-year project or maybe a 500-year project, depending on your craziness level. Johnson’s level is lowish.