SPACE TRAVEL is an exceedingly dangerous business. The Earth’s gravity is so strong that the only way to escape it is to strap men and machines into a huge firework filled with high explosive and point it at the sky. Flying into orbit and beyond is the engineering equivalent of Samuel Johnson’s remark about dogs walking on their hind legs. It is not done well, but it is surprising to find it done at all.

Two decades ago this had all been forgotten. The near-disaster of Apollo 13 was a distant memory, yet to be revived by Tom Hanks, and a fleet of four shuttles had flown two dozen missions with barely an incident. Spaceflight seemed different only in degree from hopping on a 747. So well had Nasa’s PR machine sold this new chapter of the space age that the agency feared becoming a victim of its own hype. It had done such a good job of persuading us that space travel was routine that we were losing interest.

Nasa’s answer was to put a teacher in space. When the 25th shuttle mission launched 20 years ago last Saturday, it was the agency’s biggest media event in years. Millions tuned in to follow Christa McAuliffe’s progress on TV. They saw one of the most shocking pieces of news coverage in television history. Only the destruction of the twin towers can match the chilling impact of the explosion that tore Challenger apart 73 seconds into its flight.

The disaster should have been a turning point for humanity’s exploration of the heavens. Nasa, however, was not for turning. The culture of corner-cutting and complacency that doomed Challenger lived on, to be blamed again for the loss of Columbia three years ago. A still more important lesson was also ignored. The tragedy should have been taken as a warning that technology has yet to provide a safe way of reaching for the sky, and prompted a searching re-evaluation of the rationale for human spaceflight. But encouraged perhaps by Ronald Reagan’s inimitably moving eulogy to the lost crew, Nasa never countenanced this debate. It took it as a given that the shuttle programme must go on, so as not to betray the memory of the brave seven who “slipped the surly bonds of Earth to touch the face of God”.

This was a terrible mistake. Even before the disaster it was plain that the shuttle was worth neither its financial or human cost. While the Apollo programme explored a new frontier, the shuttle went boldly into low earth orbit where plenty of men had gone before, at a cost of $150 billion and counting. Its clunky design and unreliable technology were out of date before its maiden flight. It was the Trabant of spaceflight — a cultural icon for sure, but a badly made deathtrap.

High costs and high risks can be worthwhile in the pursuit of great endeavours. The shuttle’s achievements, however, have been trivial. As a platform for scientific experiments, it has accomplished little of value. Its cost meant it could never compete with rockets for launching commercial satellites, forcing Nasa to cook up tasks to justify its existence. The Hubble Space Telescope was designed with the shuttle in mind, when a cheaper and better version could have been launched by rocket. Nasa then spent $100 billion on the International Space Station just to give the shuttle something to do.

The waste is all the more criminal when set against what might have been. By refusing to cut its losses on the shuttle when Challenger blew up, Nasa diverted vast resources away from a field in which it truly excels. Since going to the Moon, human beings have done nothing of scientific consequence in space: robots are the great space explorers now. Unmanned probes can be dispatched farther and faster than astronauts, and no one dies when they fail. From Voyager and Viking to the Mars rovers and Stardust, which returned triumphantly a fortnight ago after catching pieces of a comet’s tail, Nasa’s robots have transformed our knowledge of the solar system. The Hubble, Chandra and Spitzer space telescopes have looked out further still, revealing secrets of the wider cosmos. Robots have made almost every significant discovery of the space age. It is tantalising to consider how much more they could have learnt with even half the funds squandered on the shuttle.

The shuttle, at least, is now on its last legs. Last year’s Discovery mission nearly ended in a third tragedy, and the last of 18 remaining flights is scheduled for 2010. The folly of manned spaceflight, however, is about to get worse. Egged on by President Bush, Nasa has committed more than $100 billion to putting men back on the Moon by 2018. The next step is supposed to be men on Mars. Robotic missions, meanwhile, are being squeezed. The New Horizons probe that launched last week for Pluto, the only planet robots have yet to visit, was very nearly cancelled, and the Dawn mission to study asteroids has been postponed indefinitely. A trip to Europa, a moon of Jupiter that might harbour life, is also under threat.

As Nasa remembers the Challenger astronauts, it should pay them a tribute that is 20 years overdue. The shuttle’s next flight should be to the Smithsonian’s Air and Space Museum, and the grandiose Moon and Mars plans belong in the bin. The bad-taste joke of 1986 was that Nasa stood for Need Another Seven Astronauts. In truth, it doesn’t need any at all.

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