Space travel is a risky business. Martin Rees considers who might be prepared to leave Earth to explore the heavens.

The disintegration of the space shuttle Columbia in February this year reminds us that those who venture beyond the Earth confront real danger. The astronauts themselves have always been mindful of the hazards.

I recall a lecture by John Glenn, the first American to go into orbit. When asked what went through his mind while he was crouched in the rocket nose-cone, awaiting blast-off, he replied: "I was thinking that the rocket had 20,000 components, and each was made by the lowest bidder."

Glenn survived but other astronauts have not been so lucky. There were probably fatalities in the early Soviet space ventures; three Americans perished on the ground; near disaster befell James Lovell and his Apollo 13 crew; and shuttle Challenger exploded in 1986, killing its crew.

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We need to ask - as we do of any pioneering venture - whether the goals of manned space flight are inspiring or valuable enough to justify the hazards involved. The shuttle's 98 per cent success record - two catastrophic failures in just over 100 flights - is actually rather good by space standards. Most unmanned rockets have a worse record. But nonetheless the risks will remain high compared with those that most of us willingly and routinely accept.

When Challenger exploded in 1986, the trauma was deeper because one of those killed was a schoolteacher, Christa McCauliffe. Millions of American schoolchildren watched on TV; the public had been falsely lulled into a view that astronauts faced little more risk than passengers in a commercial jet. Publicly funded astronauts are, in a sense, acting on our behalf, and we feel uneasy about exposing civilians to substantial risks, when the issues are not of life or death urgency, but primarily tied to science or exploration.

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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. Moreover, the unmanned programme excites more public interest than the "routine" manned programme we now have.

Only disasters now attract more media coverage than data from the Hubble Space Telescope or the robotic Pathfinder on Mars, a success that will surely be repeated by Beagle 2. The International Space Station is neither practical nor inspiring: the US scientific community campaigned insistently against it until opposition seemed hopeless.

Thirty years after men walked on the Moon, a new generation of astronauts is going round and round the Earth at great expense. The three-strong crew are primarily engaged in "housekeeping" tasks, with little for serious or interesting projects.

I am nonetheless an enthusiast for space exploration as a long-range adventure for (at least a few) humans. Indeed, the only case for continuing with the space station is that it ensures that the 40 years of experience built up by the US and USSR is not dissipated.

It is unlikely that the West will ever be motivated to revive an Apollo -scale programme. 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 such a risky mission. I hope Americans or Europeans will venture to the Moon and beyond at some time, but this may be in a very different style and with different motives from those of today.

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 space travel is a dangerous pursuit.

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A role model for the future astronaut would be someone in the mould of Steve Fossett, the wealthy "serial adventurer" who, after several expensive failures, succeeded in a solo round-the-world balloon flight and is now seeking to break gliding records. Were Fossett to come to a sad end, we would mourn a brave and resourceful man, but there would not be a national trauma.

Future expeditions to the Moon and beyond will be politically and financially feasible only if they are spearheaded by individuals prepared to accept high risks. US financier Dennis Tito and South African software magnate Mark Shuttleworth each spent $20 million (?12 million) for a week in the International Space Station. Such people will not, 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 multibillionaire chief executive officer of Oracle who bankrolled an Americas Cup challenge, already has the resources to initiate a project to explore the Moon. The maverick engineer Robert Zubrin has proposed a cut-price Mars mission for $10 billion.

The first travellers to Mars (maybe 30 years from now) could be impelled by a 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".

Before humans venture into deep space, the entire solar system will have been explored and mapped by flotillas of tiny robotic craft, controlled by the ever-more powerful and miniaturised "processors" that nanotechnology will make possible. There would be about a 30-minute turnaround for messages to and from Mars, because it takes that long for a radio signal to traverse the distance of hundreds of millions of miles. But that is as nothing compared with 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 stakes will be high for space explorers: they will be opening up entire new worlds. Some might accept that there would be no return.

The exploration will never involve more than a tiny number - it is absurd to regard emigration into space, even in the very long term, as a solution to population pressure on Earth. Neither Mars nor anywhere else in our solar system offers an environment even as clement as the Antarctic.

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 North American continent? Or should it be preserved as a natural wilderness? The answer would depend on what the pristine state of Mars actually is.

If Beagle 2 finds life already there, then there would be widely voiced views that it should be preserved as unpolluted as possible. What might happen would depend on the character of the first expeditions. If they were governmental (or international), restraint might be feasible. On the other hand, if the explorers were privately funded adventurers of a free-enterprise disposition, the Wild West model would be more likely to prevail.

The populations in space may eventually expand exponentially, but this would be because of their internal growth. Space pioneers will be impelled by an exploratory urge. But their initiative will have epochal consequences - the first stages towards post-human evolution beyond the Earth.

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Sir Martin Rees is astronomer royal, professor of astronomy at the University of Cambridge and author of Our Final Century, published by William Heinemann (?17.99). He will be speaking at the Cheltenham Festival of Science on June 4.