When I am asked about the case for sending people into space, my answer is that, as a scientist, I'm against it.
Most of what astronauts do in space can be done better and more cheaply now by computers and robots. Each advance in robotics and miniaturization only widens the efficiency gap between man and machine in space.
Circling the Earth for months on end, the International Space Station is nothing more than a huge turkey in the sky. Now that only two astronauts are aboard the craft, the pursuit of any serious projects is even less likely; most of the work will involve routine maintenance and other housekeeping tasks. And, of course, the recent space shuttle tragedy has put even this program in jeopardy.
But as a human being, I hope manned space exploration will continue. It's trite but true: It is in our nature to want to know what lies beyond.
I recall the excitement that people of my vintage felt at seeing those murky images of Neil Armstrong's "one small step." To young people today, the Apollo program is ancient history; just as they know the Egyptians built the pyramids, so they know that Americans once walked on the moon. Indeed, the national imperatives might seem to them as bizarre in the one case as the other.
Preferring to err on the side of hope, I believe we will continue sending people into space, which raises important questions: Who will go? And where might they travel?
Manned space flight began as a competition between the United States and the Soviet Union but lost its Cold War associations by the time the Iron Curtain fell in 1989. At least in the West, the motive to pursue space travel as a means of projecting national pride and power has now eroded.
Yet I suspect the next major project may be an attempt by China to put people on the moon, a mission meant to herald the country's emergence as a global force. China certainly has the means, and with its dirigiste government, it probably also has the will to undertake an Apollo-like effort.
Should China succeed at a lunar landing, it would be a leap forward for the Chinese people, but nothing more than a throwback as manned space flight goes. Multilateral efforts, symbolized by the International Space Station, have perhaps also had their day (in no small part because of bureaucratic inertia; exploration by committee can be a drag on progress).
The kind of vibrant program I wish to see, taking us to the moon and beyond - with Mars the ultimate destination - will require two big changes: The cost of space travel must decrease, and travelers should venture into space on behalf of themselves or private consortia, rather than nations.
My prototype for a future astronaut is neither a civilian NASA employee nor a military test pilot, but someone in the mold of Steve Fossett, the wealthy serial adventurer who, after several expensive failures, finally managed a solo, round-the-world balloon flight in 2002.
Fossett obviously craves dangerous challenges - he is now attempting to beat the altitude record for gliders - and is willing to risk his life in pursuit of adventure. As temperament goes, he has the right stuff. And as affiliation goes, he also has the right stuff. Paying his own way, he doesn't represent a nation; he represents humanity.
Space travel carries substantial risks, and people will lose their lives once they venture into deep space. True, the level of risk will be no higher than it was for the classic explorers.
In fact, space adventurers will not leap into the unknown to the extent the great terrestrial navigators did. By the time Mars is within reach, the solar system will have been explored and mapped by flotillas of tiny robotic craft, controlled by the ever more powerful and miniaturized processors that nanotechnology will make possible.
When nations send people to space, disasters become national traumas - and nations lose some will to explore.
By contrast, were a private adventurer like Fossett to come to a sad end in space, we would mourn a brave and resourceful man, but his death would not be considered a catastrophe on the scale of the Columbia or Challenger accidents. Nor would it provoke nearly as much hand-wringing. It would be seen for what it was: a personal tragedy.
For the privatization of space travel to proceed, however, the costs must come down. The current high price tag deters nations from committing more resources to their space programs. California financier Dennis Tito's pioneering $20 million voyage two years ago aboard a Russian craft, the Soyuz TM-31, clearly opened the door to space tourism, but even low-orbital flight is still open only to the seriously rich.
But just as technology has lowered the cost of travel on Earth, it will also lower the cost of travel in space.
Right now, propelling one ton of payload into orbit requires several tons of chemical fuel. Using nuclear or solar energy, we may be able to create more efficient propulsion systems. One futuristic possibility is the so-called space elevator - a rope made of carbon nanotubes extending 35,000 kilometers into space and held vertical by a geostationary satellite. This system would allow payloads and passengers to be hoisted to geostationary orbit by power supplied from the ground. The rest of the voyage could be made using a low-thrust rocket.
Still, while I am optimistic about the ability of private enterprise to colonize the moon and lead us to Mars, I am less sanguine about what space pioneers will do once they establish a presence there. Will they be as scrupulous in preserving the natural environment as, say, the governments involved in the Antarctic project have been? Or will they simply exploit the planets they conquer, much as was done to the American West?
Ultimately, how we get there is less important than what we do when we arrive.
Martin Rees is Britain's astronomer royal and a Royal Society research professor at Cambridge University's King's College. He is the author of the new book "Our Final Hour: The Threat to Humanity's Survival," an apocalyptic guide to emerging threats to humanity. This story originally appeared in the July/August issue of Foreign Policy magazine.