“I’D LIKE to die on Mars. Just not on impact.” Elon Musk has never been shy about his reasons for founding SpaceX, a rocketry firm that has become the flag-carrier for a buccaneering “New Space” industry. Although two recent rocket explosions have dented its halo, its launch prices are among the lowest in the world. It has pioneered the technology of returning expended rocket stages to Earth for later reuse, landing them back on special pads or on ocean-going barges, which should cut costs still further. As a result, it has a thick book of orders from private firms and the American government to fly satellites into orbit and cargo—and, eventually, astronauts—to the International Space Station.

But building better rockets has never been the real point. Mr Musk, who grew up on a diet of science fiction and video games, sees the various companies he has founded as ways to help solve some of the world’s biggest problems. Tesla, an electric-car maker, and Solar City, a solar-power firm, were set up to encourage a switch to cleaner forms of energy. SpaceX’s goal is loftier. Mr Musk has repeatedly said that he believes that human beings must learn how to live on places other than Earth, as an insurance policy against a planet-wrecking disaster. A series of trips to Mars, paving the way for a permanent colony, has been the firm’s long-term goal ever since its beginning in 2002.

On September 27th, in a speech to the International Astronautical Congress in Mexico that veered between hard technical specifics and wild speculation, Mr Musk outlined details of his grand vision. The idea was “to make Mars seem possible—like something we can do in our lifetimes. Is there a way that anyone could go, if they wanted to?” His goal, he said, was, in the coming decades, to allow people to buy tickets to Mars for something in the region of $200,000—about the median cost of an American house today.

It sounds fantastical. The Apollo flights that took 12 astronauts to the surface of the Moon—around a six-hundredth of the distance to Mars—consumed at their peak around 4.4% of America’s federal budget. Since then human space flight has been stuck in Earth orbit. America’s Space Shuttle, which made its last flight in 2011, was an expensive fiasco; the International Space Station, which cost roughly $150 billion to construct, is one of the most expensive human artefacts ever built and hosts a crew of at most six people. Attempts by private firms such as Virgin Galactic to fly people into the lowest reaches of space have, so far, come to nothing.

Yet the mood among the space fraternity is that Mr Musk has earned the right to a hearing. When SpaceX was founded the idea of a startup successfully launching a rocket—let alone making big inroads into the business—seemed ludicrous to many. Mr Musk has proved his doubters wrong. A permanent colony on Mars is undeniably a grand ambition, but his ideas about how to begin, and how to build the necessary infrastructure to get people there, are worth pondering.

Mars is a harsh mistress

Mr Musk is far from the first person to advocate going to Mars. Wernher von Braun, who built Germany’s V2 missiles and America’s Saturn V moon rockets, published plans for such a trip in 1953. After Apollo’s success he lobbied for America to execute it as a follow-up. That did not happen, but the idea never went away. A mission to Mars, sometime in the 2030s, is the notional end point of Barack Obama’s space strategy. China has talked, in vaguer terms, of doing something similar by mid-century. Science-fiction authors and rocket scientists have made detailed technical studies, and the topic is a perennial favourite at space-flight conferences.

Besides the sense of prestige and derring-do, the reason Mars makes an attractive target is that it is, Earth aside, the friendliest world in the neighbourhood. Not that friendly, admittedly. Its average surface temperature is around -60°C, its atmosphere is unbreathably thin and made mostly of carbon dioxide, and its soil (technically, “regolith”) is rich in perchlorates, an unpleasant family of chemicals that terrestrial life does not like. Nevertheless, compared with the other six planets in the solar system it is a paradise. The outer four—Jupiter, Saturn, Uranus and Neptune—are gas giants, lacking a solid surface on which to land. Mercury is an airless world similar to the Moon and Venus suffers from the opposite problem, a crushingly thick (and oxygen-free) atmosphere.

Mars, though, sits on the edge of the sun’s “habitable zone”, the band of space at the right distance away from a star to let liquid water exist on a planet’s surface—and despite Mars’s subfreezing average temperature, parts of its surface are occasionally warm enough to permit just that. An optimist might reckon it only a bit less friendly than Antarctica, a place that does play host to a permanent human presence. Antarctica, though, is within a few hours’ flight of civilisation and more hospitable climes. Depending on their relative positions in their orbits, Mars can be anything from 75m km to 375m km from Earth—so far away that even light (and therefore radio messages) take many minutes to cross the distance. To get there will mean building a spaceship that can keep its occupants alive for a journey that will last for months, and a rocket that can send it on its way. It was on this subject that Mr Musk spent most of his talk.

The “Interplanetary Transport System” (ITS) comes in two parts. The spaceship itself would be 49.5 metres long, with room for around 100 passengers. It would be lofted into orbit by a gigantic, reusable carbon-fibre launch vehicle (referred to internally at SpaceX as the “BFR”, for “Big Fucking Rocket”) that would be, by some distance, the most powerful ever built (see chart). Once the spaceship had been placed into orbit, the rocket would fly itself back to its landing pad, using the technology that allows SpaceX’s present series of Falcon rockets to pull off the same trick. Further flights of the BFR would carry fuel, passengers and cargo to the orbiting spaceship. Once fully provisioned, that ship would set off for Mars.

The journey would take around six months. This is a long time, but as Mr Musk pointed out, not unprecedented: passengers on sailing vessels once endured similarly long journeys. Upon arrival the ship would enter the Martian atmosphere in the same sort of way that the Apollo command module did when returning to Earth, using the planet’s atmosphere to shed speed. Unlike Apollo, though, Mr Musk’s vehicle would make a rocket-powered landing on the surface.

One of these ships, landed permanently on Mars, could conceivably serve as the first Martian habitat. But the long-term goal is to send the spaceship home. Solar-powered machinery placed on the surface by an earlier mission would combine carbon dioxide in the planet’s atmosphere with water (which, as ice, appears to be fairly common beneath the regolith) to form methane and oxygen—an idea advanced by Robert Zubrin, an American engineer, in the 1990s. This fuel would be loaded into the spaceship’s tanks for the return journey to Earth. (The low gravity of Mars, which is around a third as strong as Earth’s, means the spaceship could take off without the need for a booster.)

Every part of the ITS has been designed to be cheap. Compared with the Apollo missions, said Mr Musk, he would need to cut costs per person 50,000-fold to hit his $200,000 ticket price. Part of that would be done, as described, by making fuel from Martian resources, rather than lugging it all the way from Earth. But the bulk of the savings come from making every part of the system as reusable as possible, an approach that has been emphasised by SpaceX and its New Space rivals, particularly Blue Origin, a rocket firm founded by Jeff Bezos, the owner of Amazon.

One reason rockets are so expensive is that they are, conventionally, single-use machines. Once they have done their job, they are either abandoned in space or left to crash back into the ocean. Rocket fuel is cheap. It is the rockets themselves that are costly. A reusable rocket would permit those construction costs to be spread across several flights. A BFR, reckoned Mr Musk, might be good for 1,000 flights before it needed replacing. The spaceships themselves might make a dozen trips to Mars each, limited by the travel time and the finite number of viable “launch windows” during which they could be sent.

Theorising is the easy part. For now SpaceX is spending only “a few tens of millions” of dollars on designing the BFR and the spacecraft. But there has been progress: the snazzy videos that punctuated Mr Musk’s speech were generated, he said, from real design blueprints. He showed a video of a test-firing of SpaceX’s new methane-burning “Raptor” engine, 42 of which would power the BFR. The firm has a lot on its plate at the moment, not least tracking down the cause of an explosion that destroyed a Falcon 9 (and the satellite it was carrying) on September 1st and preparing for its first manned flight. Once those are done, and the firm’s next commercial rocket—an upgraded version of the Falcon 9 called the Falcon Heavy—is ready, he plans to devote more time and money to the ITS.

Timescales could be only rough guesses, he said. But the first flight of the BFR should take place around 2020, with the first spaceship sent for orbital testing a year or two later. In the meantime SpaceX is planning to launch a smaller, uncrewed spacecraft—one of its existing Dragons, as depicted above—to Mars in 2018, and at the congress Mr Musk promised that he would follow up with a steady stream of further missions every two years, when the positions of Earth and Mars make the latter most reachable.

Ticket to ride

If you wanted to come up with a way of transporting large numbers of people to Mars, then the ITS—or something like it—seems a good place to start. But the bigger question is why you might want to do that in the first place.

For decades human space flight has been an activity in search of a justification. In the 1960s the justification was that it was war by other means. The Apollo missions to the Moon, and the failed Soviet attempt to do likewise, were exercises in cold war propaganda, designed to prove that capitalism was better than communism, or vice versa. After America won the space race interest and funding dwindled, and advocates of human space flight were reduced to homilies about inspiring people to take up careers as scientists. Mr Musk instead makes an existential argument—spaceflight, he says, should be seen as an insurance policy. Only by becoming a multi-planet species, he argued, can humanity make itself safe from the sorts of disasters that might wreck Earth: asteroid strikes, say; or malevolent robots; or the gamma radiation from a nearby supernova. That means planting colonies on other planets, and nurturing them until they can survive without resupply from Earth. Various other luminaries, such as Stephen Hawking, a revered British physicist, have expressed similar worries.

This argument sometimes turns quasi-mystical. Life seems to have begun on Earth shortly after the planet formed, which suggests that its emergence is easy and it should therefore be common elsewhere. But that does not seem to apply to intelligent life. Even at relatively slow speeds, an intelligent spacefaring species ought to be able to colonise a galaxy in a few hundred million years. The fact that there is no sign of this having happened in mankind’s own galaxy, the Milky Way, is sometimes taken as evidence that intelligence is either vanishingly rare, or else tends to blow itself up before it can leave its home planet. Either way, the mystics suggest, mankind has a duty as an intelligent species to look after itself.

In the very long run, the doom-mongers are right: human beings may indeed have to migrate, assuming any are still around. Around a billion years from now the sun, which has been brightening slowly ever since its formation, will be shining fiercely enough to make Earth uninhabitable. If human beings want to survive, they will have to leave. (At the same time, the brighter sun would make Mars much more salubrious.)

Critics argue that the shorter-run threats are either unlikely (malevolent robots); or would destroy a Martian colony too (gamma-ray bursts); or could be avoided by any species capable of planning to colonise other planets (killer asteroids can be deflected). But even if an interplanetary insurance policy were a prudent idea, could it be taken out?

It was here in Mr Musk’s presentation that the technical details ran out and the speculation began in earnest. Living on Mars would be difficult: harder, probably, than getting there in the first place. Colonists would have to spend all their time in pressurised buildings. Communication with Earth would be doable but tedious. It might be possible to modify terrestrial plants to grow in a high-pressure version of the Martian atmosphere, but no one has tried. A closed, miniature ecosystem would have to be devised to recycle nutrients and waste products. A series of Earth-bound experiments, called Biosphere 2, which tried that out in the 1990s were abandoned after plants died, food became scarce and oxygen levels started dropping.

When asked what such a habitat might look like, Mr Musk said that was not for him to say. “We see ourselves as being like the Union Pacific railroad,” he said, referring to the railway that opened up the American West in the 1860s. His focus was on building an affordable transport system. What the settlers did when they arrived would be up to them. He had given it some thought—he opined that you might need a million people to maintain the sort of industrial base necessary for true independence from Earth. But specifics were in short supply.

One such specific was how much, in the end, such a scheme might cost. Working with Mr Musk’s own numbers, sending 1m people to Mars would cost in the order of $200 billion. That is too much for even his deep pockets. There are other rich men also keen on space flight, who might be persuaded to cough up. But he was candid that, for the scheme to work, governments would also have to open their chequebooks. That seems unlikely at the moment.

Home away from home?

One final question is why anyone would want to go to Mars, even if a ticket could be had by selling your house. Appeals to the future of the species are unlikely to motivate many individual members of that species to give up everything and move. Space advocates point to humans’ history of migration, saying that colonising Mars would be like the Polynesian conquest of the Pacific, say, or the European migrations to the New World. But historical migrations have happened either because those involved had no choice, or because there were big rewards from doing so. The founders of Virginia were seeking profit. The Pilgrim Fathers were fleeing religious persecution. And the lands they arrived at were not intrinsically hostile to human survival.