ON JULY 20th 1969, when Neil Armstrong and Buzz Aldrin set foot on the Moon, it capped half a century of extraordinary progress in aviation. In the six and a half decades since the Wright brothers’ Flyer had staggered into the air near Kitty Hawk, North Carolina, aeroplanes had shrunk the world, revolutionised warfare and created the modern travel industry. Technical records for altitude, speed and endurance had fallen helter-skelter.

For that reason, many of those watching the two astronauts on their black-and-white televisions could have been forgiven for thinking that going to the Moon was simply the first step in a human expansion into the solar system. Indeed, that had long been the dream of the space buffs who made the Moon missions possible. Wernher von Braun, the genius who designed the Saturn V Moon rockets—and who had been planning Mars expeditions since the publication, in 1948, of his book “Das Marsprojekt”—pitched a crewed Mars mission to then-President Richard Nixon soon after Armstrong and Dr Aldrin landed.

But it was not to be. The Apollo Moon programme was shut down early, and the world’s astronauts have spent the past 41 years pootling around in low-Earth orbit. Now, though, spurred by the rise of the buccaneering, private-sector “New Space” industry, which is offering access to the cosmos at prices far lower than government-backed rockets can manage, the old dream is enjoying a resurgence. Elon Musk, whose rocket firm SpaceX already flies cargo to the International Space Station (ISS), makes no secret of his Martian ambitions. Two privately run organisations in particular—Inspiration Mars, brainchild of Dennis Tito, an American tycoon who became the world’s first space tourist in 2001, and Mars One, run by Bas Lansdorp, a Dutch entrepreneur—have announced plans to send people to Mars without relying on the resources of a state.

Mr Lansdorp admits that, on hearing about his plans, people’s first response is that he must be crazy. But both he and Mr Tito (who started his career as an engineer at the Jet Propulsion Laboratory, JPL, in Pasadena, which runs NASA’s unmanned Mars missions) insist they are serious. Technical studies have been done, astronaut applications are being processed and deals are being signed with the firms that will build the spacecraft.

Both men are motivated by frustration with government efforts, which have gone around in circles—and not orbital ones—for decades. It is the stated policy of Barack Obama’s government to send a crewed mission to Mars in the 2030s. But given the recent history of NASA as a political football (the George Bushes senior and junior both proposed similar missions that went nowhere), and given looming cuts to its budget, few think such a mission will actually happen. As Mr Tito put it when he announced Inspiration Mars: “the way we’re going, we’ll never get started.”

Can-do talk aside, there are good reasons for scepticism. Sending people to Mars will be extremely difficult; far harder than sending them to the Moon. For one thing, Mars is much farther away. The Apollo missions took three days to get there, but flight times to Mars are measured in months. That would require an utterly reliable spacecraft. The vast distance imposes a communication delay, too. Whereas the Apollo astronauts could talk to their ground-based controllers more or less in real time, Martian astronauts would face delays of up to 40 minutes between asking a question and getting an answer.

And spending months in deep space would expose a crew to a chunky dose of radiation. Information from the Curiosity rover’s flight to Mars, just published in Science, suggests a crew could expect a radiation dose close to the maximum lifetime limit for NASA’s astronauts. If the sun were to have one of its regular temper tantrums, known as coronal mass ejections, which produce huge bursts of ionising radiation, that limit might be far exceeded.

When the stars are right

Both missions hope to use SpaceX’s Falcon Heavy, a planned upgrade of its existing Falcon rocket. But even with access to cheap rockets, flying to Mars without the financial firepower of a government requires clever, and drastic, cost-cutting. In Mr Tito’s case, this means that although his astronauts will fly within about 140km (90 miles) of the Martian surface, they will not actually land. His mission calls for a two-person craft—possibly a variant of SpaceX’s Dragon capsule, which is already used to ferry cargo to the ISS—to be sent on a “free return” trajectory to Mars. By clever design of its orbit, the craft can be sent to Mars with a single burn of its rocket engine. From thereon in, gravity takes over. The craft will swing around Mars and emerge on a trajectory precisely calculated to have it re-enter Earth’s atmosphere several months later, with only minimal course corrections needed along the way. Orbital mechanics is one of Mr Tito’s specialities. Before he left JPL to become an investment manager, he designed orbits for Mars probes.

Removing the requirement to land simplifies the task enormously. No complex manoeuvring into a Mars orbit is necessary. No landing craft is needed, nor any survival gear for use on the Martian surface. That saves fuel and, more importantly, launch mass (the overriding concern for any space mission). Mr Tito’s team hopes to wring further savings by launching in 2018, in order to take advantage of a rare set of celestial circumstances that allow a gravity-powered trip to Mars and back in 501 days, instead of the two years that would normally be required. That keeps food and water requirements low. “There is nothing technically infeasible about Tito’s plans,” says Robert Zubrin, who runs the Mars Society, which lobbies for human trips to Mars, and who has spent decades thinking about exactly how such missions might be executed. Mr Tito’s team has plenty of technical nous, as well as a deal with NASA to develop technology. It plans to recycle as much kit as possible from the ISS, whose life-support systems have proved themselves reliable for more than a decade. Of course, saying that something is technically feasible does not mean that it is easy. Mr Tito’s mission will push the bounds of space flight. A year and a half is long time to spend cooped up in a craft the size of a motor-home, but Dr Zubrin points out that similar feats have already been achieved. Valeri Polyakov, a Russian cosmonaut, spent 438 days in space in 1994 and 1995. Four other Russian and Soviet missions have been over 300 days long. Mr Tito hopes to keep personal conflict to a minimum by sending a married couple—ideally one past child-bearing age, to eliminate the risks of accidental pregnancy and radiation-induced infertility. The return to Earth could also be difficult. Inspiration Mars’s spacecraft will slam into Earth’s atmosphere at 14.2km a second, significantly above the 11km/s speed of Apollo. No existing heat shield could deal with such re-entry speed. But Taber MacCallum, the head of Paragon Space Development, an engineering firm, who is one of Mr Tito’s chief collaborators, says modern materials should be up to the task of making a new one.

Assuming the engineering questions can be solved, Mr Tito’s chief problems will then be time and money. His mission faces a hard deadline. If it is not ready by January 2018, the intricacies of orbital mechanics mean there will not be another chance of such a short trip until 2031. And even a stripped-down, Spartan mission that uses as much existing technology as possible and makes no attempt to land will be expensive. Inspiration Mars gives no official cost estimates, but Jeff Foust, the editor of the Space Review, an industry newsletter, thinks it could be done for “very roughly, around a billion dollars”, a sum that Mr Tito may try to raise through a personal donation, the sale of media rights, sponsorship deals and charitable appeals to his fellow tycoons.

It is a big task, and a dangerous one, but it is not beyond what technology allows. Dr Foust, for instance, gives Mr Tito about a one chance in three of succeeding. Even if everything does go according to plan, though, cynics might question the value of a billion-dollar, one-and-a-half year trip that comes within spitting distance of Mars but does not land. Dr MacCallum points out that even a fly-by would generate a great deal of publicity. “It would be a [Charles] Lindbergh” mission, says Dr Zubrin. “The point would be to prove it can be done.” (Though, since it would involve a pair of people making a hazardous journey for the first time, rather than a copy-cat, albeit solo, trip eight years later, a better comparison would be with Alcock and Brown, two British pilots who flew the Atlantic in 1919.)

Marooned on Mars

Mr Tito intends to simplify things by not landing. The Mars One project intends to simplify them by not taking off. Its four crew members, if they arrive intact, will live out the rest of their lives on Mars. They will build a settlement from their spacecraft and from inflatable living areas covered with regolith (the crushed rock that passes for soil on the planet). Nor will this be a one-off. Every two years the Martians will be joined by four more refugees from Earth, with the eventual aim of building up a full-fledged colony. As far as possible, they will produce food, water and materials in situ, though regular cargo launches from Earth will supplement these.