The cheapest and simplest way to do anything interesting in space beyond low-Earth orbit is to visit a nearby asteroid. Oliver Morton suggested this idea in his contribution below, and mentioned our B612 Foundation's goal of changing the orbit of an asteroid. Indeed, it would be natural to combine asteroid deflection practice with actual human exploration of an asteroid. That way, the astronauts putting themselves at risk would know that at the same time they would have the potential of saving millions of lives on Earth.

Astronauts can play a useful role by setting up a mass driver that can throw rocks off the surface in order to move the asteroid in the opposite direction. In the long run, that may well be the most efficient way to ride asteroids. In the near future, plasma engines can be used to push an asteroid tugboat style in case of an unmanned mission. While fuel efficient and simple, plasma engines do consume lots of energy. In contrast, a mass driver presents a very low energy alternative, but the logistics for picking up the 'fuel' (rocks) are daunting. For a robotic mission to set up and anchor a mass driver, select and pick up the rocks and position them for launch presents challenges well beyond what has been demonstrated so far.

While we hear a lot about Mars as the next target beyond the Moon for human exploration, it is not often realized that it is energetically cheaper to visit an asteroid, even compared to visiting the Moon. Getting out of the gravitational grip of the Earth carries about the same cost as going to the Moon, while landing on an asteroid and taking off costs almost nothing: the escape speed of a 100 meter diameter asteroid is only a few cm/sec, and that of a 10 km diameter asteroid a few meters/sec. The challenge would be for an astronaut to stay put on the surface and not 'launch' herself inadvertently.

Getting to an asteroid will take only a few months, rather than years, if we pick one on an orbit close to that of the Earth. If we land on an asteroid just before it passes us, and then get off again soon afterwards in order to return, it would be as simple as switching between two moving walkways that have almost exactly the same speed.

Finally, missions to an asteroid would present far better prospects for the commercialization of space than missions to the Moon or Mars. Anything we find, from precious metals to possibly water (on inactive comets, say, that may pose as asteroids), can be easily brought back to Earth orbit, to provide raw material for the space station or for other space flights. It is energetically far more favorable to bring matter from an asteroid down to the space station than to send it up from the surface of the Earth. While it is too early to know what exactly we will find there, we may hope that an extraterrestrial 'gold rush' may help push the frontiers of exploration to include asteroids.

I recall attending a lecture given, back in the 1960s, by John Glenn, the first American to go into orbit. A questioner asked him what went through his mind while he was crouched in the rocket nose-cone, awaiting blastoff. He wryly replied "I was thinking that the rocket had twenty thousand components, and each was made by the lowest bidder".

Glenn was aware of the risk he was taking—so surely, would have been the astronauts who perished in Columbia. But their fate injects a dose of reality: space travel is not a routine exercise. We need to ask—as we do of any pioneering venture—whether the goals of manned spaceflight are inspiring or valuable enough to justify the hazards involved.

The Shuttle's 98 percent success record—two disasters in just over a hundred flights—is actually rather good by space standards. Must unmanned rockets have a worse record. (The French Ariane V rocket had two catastrophic failures in less than a dozen flights).

We don't yet know whether last week's accident could have been avoided by better maintenance. I suspect it could. But even with optimal precautions, the risks of going into space will remain high compared to those that most of us willingly and routinely accept.

Publicly-funded astronauts are, in a sense, acting on our behalf. We feel uneasy about civilians bearing such risks, when the issues aren't of life or death urgency, but primarily science or exploration. Nonetheless, some individuals—wealthy amateur mountaineers who join guided parties to climb Everest, or test pilots—willingly do things that are at least as dangerous as a Shuttle flight.

When I am asked about the case for sending people into space, my answer is that as a scientist I'm against it, but as a human being I'm in favour. Practical activities in space—for communications, science, weather forecasting and navigation)—are better (and far more cheaply) carried out by computers and robots. I am nonetheless an enthusiast for space exploration as a long-range adventure for (at least a few) humans .

The next humans to walk on the Moon may be Chinese—only China seems to have the resources, the dirigiste government, and the willingness to undertake a risky Apollo-style programme. I hope Americans or Europeans will sometime venture to the Moon and beyond, but this will be in a very different style, and with different motives..

The kind of vibrant manned programme that I'd one day like to see will require changes in techniques and style. First, costs must come down. Second, there must be an overt acceptance that the enterprise is dangerous.

A role model for the future astronaut is not a NASA employee, nor even a military test pilot, but someone more in the mould of Steve Fossett, the wealthy "serial adventurer" who, after several expensive failures, succeeded in his solo round-the-world balloon flight. He has a craving for arduous challenges, and is now trying to beat altitude and endurance records for gliders. In each venture, Fossett must knowingly accept accepts a risk of at least 1 percent. Were he to come to a sad end, we would mourn a brave and resourceful man, but there would not be a national trauma. We would know that he willingly too the risks, and it was perhaps the way he wanted to go. Future expeditions to the Moon and beyond will only, I think be politically and financially feasible if they are spearheaded by individuals prepared to accept high risks, and perhaps even privately funded.

Dennis Tito and Mark Shuttleworth each spent 20 million dollars in return for a week in the International Space Station. A line-up of others was willing to follow them, even at that price. Such people won't, in the long run, restrict themselves to the role of passengers passively circling the Earth: they will yearn to go further.

Manned expeditions into deep space may one day be fundable by private consortia. Larry Ellison, who bankrolled a yachting challenge for the Americas Cup would already have the resources to initiate a cut-price project to take humans beyond Earth orbit.

The intrepid voyagers who set out from Europe in the 15th and 16th century to explore the then-open frontiers of our own world were mainly bankrolled by monarchs, in the hope of recouping exotic merchandise or colonising new territory. Some later expeditions, for instance Captain Cook's three 18th century voyages to the South Seas, were publicly funded, their mission being to survey new territory, discover new plants, and make astronomical measurements. For some early explorers—generally the most foolhardy of all—the enterprise was primarily a challenge and adventure: the motivation of present-day mountaineers, balloonists, round-the-world sailors and the like.

The first travellers to Mars (maybe thirty years from now), or the first long-term denizens of a lunar base, could be impelled by this same mix of motives. They would confront hostile environments: nowhere in our Solar system offers an environment even as clement as the Antarctic or the deep ocean. However, no space travellers would be venturing into the unknown to the extent that the great terrestrial navigators were: those early pioneers had far less foreknowledge of what they might encounter in the regions where ancient cartographers wrote "here be dragons". Nor would space travellers be denied contact with home, any more than explorers and lone sailors now are. There would admittedly be about a 30 minute turnaround for messages to and from Mars, because it takes that long for a radio signal to traverse the hundreds of millions of miles distance. But that is as nothing compared to the isolation of traditional explorers.

The stakes will be high for space explorers: they will be opening up entire new worlds, Maybe some would accept—as many Europeans willingly did when they set out for the New World—that there would be no return. A Martian base would develop more quickly if those constructing it were content with one way tickets.

The mountaineers George Mallory and Andrew Irvine both perished on Mount Everest in 1924, during a celebrated early attempt to reach its summit. A stone tablet in Irvine's memory bears a text from Pericles's famous funeral oration (in Hobbes's translation) .

"They are most rightly reputed valiant who perfectly understand what is dangerous and what is easy, but are not thereby diverted from adventuring".

This sentiment, I believe, would have resonated with the Columbia astronauts. In future decades, if humans venture to the Moon and beyond, they will have to go in this same spirit.

I've been dwelling on George's suggestion of a one way manned voyage to Mars. I think George is right that it has the potential to capture the world's attention as nothing ever has done—science, religion, sex, adventure, reality tv, all rolled into one. The child of Shakespeare, Newton, Darwin, Freud and Einstein. Why don't we make this an Edge project? John Brockman has the best of all address books. Edge could make it happen!

Beyond The Shuttle

A friend at NASA's Marshall Space flight Center in Huntsville, Alabama told me today that all his engineer friends were working on their resumes. After the Challenger disaster, NASA dithered for 2.5 years before using the shuttle again. How long this time?

Quite probably, a year—if ever. This second wreck calls into question the entire shuttle program. Voices already are calling for a wholly new approach. The shuttle has the worst safety record of any launch vehicle, and is the most expensive, costing half a billion dollars per mission.

And we now contemplate a war in Iraq that depends on our technical prowess. I doubt that Americans will be moved to doubt our military, just because an advanced spacecraft fell out of a clear blue sky. We are tougher than we may look—and more resolute. The country is reasonably united, and yet again the president has responded with the right sense of gravity. He does disaster well.

Still, it is a good time to reassess. Early results from the telemetry and the huge debris field suggest that the thermal tiles failed. One amateur observer saw something blowing off the shuttle as it passed over California, possibly red-hot tiles. We know that a piece of foam blew off the fuel tanks at launch, striking the shuttle's left wing, a location that seems implicated in the heating spike that the telemetry recorded just before the craft began to slew and tumble.

Reentry is a tricky negotiation between gravity and aerodynamics. Controlling descent angle is important to reduce mechanical and thermal stress on the spacecraft, and an error in the on-board computers can allow the angle to get so steep that the craft breaks apart. (Multiple computers should reduce the risk, but that has not saved computer-run aircraft like the SAAB 39 Gripen from the occasional crash.)

Whatever the fault, tiles or computers or human error, the crash occurred at what many engineers thought was the most dangerous portion of a shuttle's flight. This is not a fluke; the system was vulnerable, and it failed yet again.

Perhaps the only good thing about this disaster is that it will prompt NASA to rethink the design of manned spacecraft from first principles. Foremost is that the more complex a spacecraft is, the more things can go wrong.

The safest manned descent module was also the simplest: the Soviet "sharik" descent capsule, which was used by Vostok and Voskhod craft, and also in many unmanned missions since. It was just a sphere with the center of gravity on the side with the thickest ablative thermal shielding, so it was self-stabilising. Even if the retrorockets failed to separate, it could re-enter safely.

Simple ballistic craft that do not fly are also (relatively) simple. With a spaceplane like the shuttle, however, you are not only committed to a complex shape, you are also committed to using brittle ceramic materials for thermal shielding. The first item on NASA's agenda will be to revisit the tiles issue.

The ceramic tiles not only make overhaul very time-consuming and expensive—specialists affix each tile by hand, managing to do a few per day, and there are thousands— they are also literally impossible to check for inner defects. Unlike metal components, you cannot test them for small cracks that may cause failure.

One way around this is to use many small ceramic tiles, so the spacecraft can survive losing individual tiles. But if several adjacent tiles are lost, it will cause catastrophic failure during reentry. Maybe that happened; it is consistent with what we know now (or are likely to know for several months).

A second line of defense is to have the crew in a detachable unit that can land safely. This would be straightforward in a ballistic craft, but with an aerodynamic spaceplane it is difficult to squeeze such a unit into the nose. On the B-58 bomber the crew had small individual pods that enabled them to eject safely at supersonic speeds, but the weight penalty ruled out this option for the shuttle.

Ideally, you need a descent module that can take a lot of punishment. But a big spaceplane would get impossibly heavy if it was stressed for this. This is another argument for small sixties-era crew capsules.

Ironically, the Soviet "Buran" shuttle could lift loads to orbit without any crew at all, and might make a viable alternative to the US shuttle. But the only remaining craft got badly damaged when a corroded hangar roof fell down on it last year—a symbol of the Russian program's decay.

The safest manned spacecraft built was also among the cheapest and simplest. The lunar lander used pressurized tanks, eliminating the need for turbo pumps, and the fuel and oxidizer self-ignited when mixed, making the engine very reliable.

NASA considered mass-producing similar, simple rockets in the sixties as an option to make space flight cheaper. Political considerations favored the more spectacular spaceplane solution. To date, this decision has killed two shuttle crews and cost billions.

In the end, the next months will try NASA as never before. It has tried to convince its public that going into space is safe, when it is not. Once is an accident, twice is a defect.

The shuttle's justification these days has been its role in supporting a space station that now does little science. The station runs with the minimum crew of three, to save money while forgetting science. The Russian Soyez vehicle could cycle crews and probably will be used to bring down the three up there now.

The station program can limp along for a few years with two flights a year, to cycle crews every half year and not abandon the station entirely. A Russian Proton rocket can continue to boost the station up as its orbit decays from atmospheric friction, as we now do routinely.

This can go on until NASA can decide what to do. Its habit is not to be truly decisive, but now its back is to the wall. It must confront the big question:

What is the American destiny in space? The station is not a destination; it is a tool. But for what?

NASA has played up the station as "a stepping-stone to the planets" — but it cannot perform the two experiments we know must be done before any manned ventures beyond Low Earth Orbit begin.

These are, first, development of a true closed biosphere in low or zero gravity. The station recycles only urine; otherwise, it is camping in space, not truly living there.

Second, we must develop centrifugal gravity. Decades of trials show clearly that zero-g is very bad for us. The Russians who set the endurance records in space have never fully recovered. Going to Mars demands that crews arrive after the half year journey able to walk, at least. No crew returning from space after half a year ever have, even for weeks afterward. So we must get more data, between one gravity and none. Mars has 0.38 g; how will we perform there? Nobody knows.

Spinning a habitat at the other end of a cable, counter-balanced by a dead mass like a missile upper stage, is the obvious first way to try intermediate gravities. The International Space Station has tried very few innovations, and certainly nothing as fruitful as a centrifugal experiment. Until a livelier spirit animates the official space program, the tough jobs of getting into orbit cheaply, and living there self sufficiently, will probably have to be done by private interests who can angle a profit from it. But not right away.

This is an historic moment, one of great opportunity. NASA can either rise to the challenge and scrap the shuttle, or just muddle along. An intermediate path would use the shuttles on a reduced schedule, while developing a big booster capable of hauling up the big loads needed to build more onto the station. This would be cost-effective and smart.

The past Director of NASA said to me a few years ago that he thought the agency had about a decade to prove itself. Around 2010 the Baby Boomers will start to retire and the Federal budget will come under greater pressure. Space could go into a slow, agonizing withering. He thought this was a distinct possibility if NASA did no more than fly around in cycles over our heads. "It has to go somewhere else," he said.

The obvious target that has huge scientific possibility is Mars. Did life arise there, and does it persist beneath the bleak surface? No robot remotely within our capability can descend down a thermal vent or drill and find an answer. Only humans are qualified to do the science necessary, on the spot.

A Mars expedition would be the grandest exploit open to the the 21st Century. It would take about 2.5 years, every day closely monitored by a huge Earthside audience and fraught with peril.

This is what we should be doing. Such an adventure would resonate with a world beset by wars and woes. It has a grandeur appropriate to the advanced nations, who should do it together.

The first step will be getting away from the poor, clunky shuttle, a beast designed 30 years ago and visibly failing now. How we respond to the challenge of this failure will tell the tale for decades to come, and may become a marking metaphor for the entire century.

As well, the engineers at NASA would be overjoyed to have a larger prospect before them, something better than patching up an aging shuttle that, in the end, was going nowhere.