EVERYTHING about space flight is superlative. Even relatively modest rockets are hundreds of feet high. The biggest (the Saturn V, which launched astronauts to the Moon) remains the most powerful vehicle ever built. But space flight is superlatively expensive, too. One reason is that, for all their technological sophistication, rockets are one-shot wonders. After they have fired their engines for a few minutes they are left to fall back to Earth, usually splashing ignominiously into the ocean. Rocket scientists have therefore long dreamed of making something able to fly more than once. Such a reusable machine, they hope, would slash the cost of getting into space. The only one built so far, America’s space shuttle, proved a dangerous and costly disappointment, killing two of its crews and never coming close to the cost savings its designers had intended. But hope springs eternal, and several of America’s privately run “New Space” firms are planning to try again.

The furthest advanced is SpaceX, founded by Elon Musk, an internet mogul. On April 18th it is due to launch one of its Falcon 9 rockets on a cargo-carrying trip to the International Space Station (ISS), something it has done twice before. This time, though, the main story is not the ISS mission, but the modifications the firm has made to the rocket itself.

The most notable are the four landing legs folded up along the side of its first stage. If everything goes to plan, once that stage has finished its job and detached itself from the rest of the rocket, it will fire its engines again. Instead of crashing into the sea, it will make a controlled descent, deploy its legs, slow almost to a stop off the coast of Cape Canaveral, and then drop itself delicately into the drink. Mr Musk gives himself a slightly-less-than-even chance of pulling this off.

Will you walk with me, Grasshopper?

If it does work, though, it will be the most dramatic demonstration yet of technology that the firm has been working on for several years. In 2012 SpaceX began flying an unwieldy-looking legged test rocket called Grasshopper. This was able to hover, manoeuvre around in mid-air, and land itself back on the pad that launched it.

Then, last September, it attempted to organise the controlled descent of a legless first stage. In what the firm’s engineers call a useful failure, the rocket’s engines restarted as planned, but as the stage descended it began spinning, flinging its remaining fuel against the walls of its tanks and starving its motors, causing it to crash.

This week’s test is intended to end up with the rocket in the ocean, chiefly for safety reasons in case something does go wrong. But SpaceX’s ultimate goal is to have the first stage fly all the way back to the pad it was launched from, and to land itself facing upwards. It will then be taken away, serviced, refilled with rocket fuel and readied to fly again. The firm wants, one day, to recover the Falcon’s second stage, too—though the greater altitude and speed the second stage reaches makes this a far tougher proposition.

Still, being the biggest, the first stage is the most expensive part, so retrieving it should make a huge difference to launch costs. SpaceX already offers some of the lowest prices in the business. Its launch costs of $56m are around half those of its competitors. Mr Musk has said in the past that a reusable rocket could cut those costs by at least half again.

If SpaceX can make its technology work, that will be the biggest advance in rocketry for decades. Whether it will translate into higher demand for space flight is less clear. Jeff Foust, who edits the Space Review, an industry newsletter, argues that even dramatically lower launch costs will do little to change the economics of the industry, at least for the governments and firms that make up almost all of its current customers. Launch costs, as Mr Foust points out, are but a small part of the total cost of developing, building and running a satellite network.

Mike Gold, an executive at Bigelow Aerospace, a firm that makes inflatable space stations—and which has an agreement with SpaceX to launch its products—thinks that most of the interest will come from people and organisations so far denied access to space. “Putting a big rocket like the Falcon in range of mid-size companies, research institutions and even wealthy private individuals, that’s a game-changer,” he says. “When the laser was first invented, no one had any idea what it might be used for. Today they’re everywhere. We’re still at that early stage with cheap rockets.”

Perhaps. But although SpaceX is a commercial firm, simple profitability is not its only goal. Mr Musk has been perfectly frank about his long-term aim: “to die on Mars, preferably not on impact.” After the Falcon 9, the firm plans a beefier version called the Falcon Heavy. That, in turn, would be a dress rehearsal for something called the Mars Colonial Transporter.

Mr Musk wants to build a machine that would let him offer prospective colonists a (one-way) trip to the Martian surface for about $500,000—or, as he puts it, roughly the cost of a nice house in California. Perfecting reusability is essential for achieving that dream.

If you build it, will they come?

Hard-headed commentators may roll their eyes at such ambition. And history suggests reusability is difficult to do properly. The shuttle itself, for instance, was intended to fly every week. In the end, it made only 135 trips over the course of 30 years. There is a credible case that it proved more expensive, in the long run, than old-fashioned throwaway rockets would have done. Yet SpaceX has already shaken up an industry once mired in stifling conservatism. A successful fully reusable rocket would just be the latest example in a long tradition of it confounding its critics.