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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-stabilizing. 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 U.S. 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 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. Gregory Benford is a professor of physics and astronomy at the University of California, Irvine, a long-time advisor to NASA, and a novelist. Source |
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