In this follow up article to last weeks Danger in Space, Sir Martin Rees reflects on the recent space tragedy, and ponders the question of the dangers of men travelling into space.

by Sir Martin Rees, Astronomer Royal

Space is an unexplored frontier. The fate of the Space Shuttle Columbia reminds us that those who venture beyond the Earth confront real danger. The astronauts themselves have always been mindful of the hazards. 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 survived to become a US senator, as well as an inspiration to elderly Americans when he ventured into space again, at age 77.

Other astronauts have been less lucky. There were probably fatalities in the early Soviet space ventures, though these were never publicised at the time; three of Glenn's fellow-astronauts perished while testing NASA's Apollo capsule; the near disaster that befell James Lovell and his crew in Apollo 13 is familiar to a younger generation through the excellent film starring Tom Hanks. An earlier shuttle, the Challenger, exploded, killing its crew, in 1986.

John Glenn was aware of the risk he was taking - so surely, would have been the astronauts who perished last Saturday. 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 catastrophic failures 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.

When the Challenger exploded in 1986, the trauma was deepened because one of those killed was a schoolteacher, Christa McCauliffe. The American public had been falsely lulled into a view that spaceflight was 'routine' and that astronauts faced little more risk than passengers in a commercial jet. But the memory of that accident had faded and Americans had come to believe that space was a place for tourists. 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.

NASA Hurtling out of the conflagration at 78 seconds are the Challenger's left wing, main engines (still burning residual propellant) and the forward fuselage (crew cabin).

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 (including the marvellous GPS system) - 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. Present procedures are as extravagant as air travel would be if the plane had to be rebuilt after every flight; the Space Station is cumbersome and inflexible. 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 (like our own Richard Branson) a craving for arduous challenges , and is now trying to beat altitude and endurance records for gliders.

NASA Touring the Johnson Space Center on a familiarization tour, from left to right, are backup Soyuz 4 Commander Cosmonaut Gennady Padalka, a Russian cosmonaut; Soyuz 4 Space Flight Participant Mark Shuttleworth, a South African businessman who flew to the International Space Station.

In each venture, Fossett must knowingly accept 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. American financier Dennis Tito and the South African software magnate 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, the multi-billionaire CEO of Oracle who bankrolled a yachting challenge for the Americas Cup would already have the resources to initiate a project to explore the Moon and beyond. The maverick engineer Robert Zubrin has proposed a cut-price Mars mission for ten billion dollars. In earlier centuries, adventurers explored the then-open frontiers of our own world. Columbus, Magellan, and the other intrepid voyagers who set out from Europe in the 15th and 16th century 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. Many of these explorers never returned. Even when they did, their crews had suffered severe attrition through disease and accident. 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 range of motives. The risks would be high. 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".

Artist Pat Rawlings' concept of the type of work and hazards future astronauts will encounter on outer worlds: Mars Rover Repair

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. It generally took months for them to send messages home; and the heroism of some - Captain Scott and other polar pioneers among them - is known to us only because their diaries survived.

The exploration will never involve more than a tiny number - it is absurd to regard emigration into space as a solution to population pressure on Earth. Neither Mars nor anyone else in our Solar system offers an environment even as clement as the Antarctic or the deep ocean. But, as in pristine parts of our own Earth, any human presence leads to issues of environmental ethics. Would it be acceptable to exploit Mars, as happened when (with tragic consequences for the Native Americans) the pioneers advanced westward across the United States? Or should it be preserved as a natural wilderness, like the Antarctic? The answer would I think depend on what the pristine state of Mars actually is. If there were any life there already then there would be widely voiced views that it should be preserved as unpolluted as possible. What might actually happen would depend on the character of the first expeditions. If they were governmental (or international), Antarctic-style restraint might be feasible. On the other hand, if the explorers were privately funded adventurers of a free-enterprise (even anarchic) disposition, the Wild West model would be more likely to prevail.

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. Many could be found who would sacrifice themselves in a glorious and historic cause; by foregoing the option of ever coming back home. 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 the famous funeral oration by the Athenian general Pericles in the 5th century BC: "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 astronauts who died on February 1st 2003. In future decades, if humans venture to the Moon and beyond, they will have to go in this same spirit.