The space shuttle Discovery blasted off from Florida on Tuesday. This 13-day mission is in effect a second safety test for the entire US shuttle programme, after the loss of Columbia in February 2003. In July 2005, Discovery carried out a first post-disaster mission, amid lingering worries over possible damage at lift-off from fragments of foam insulation that hit the craft's heat resistant tiles. The launch appears to have been a complete success, with no sign of significant damage. It is thus more likely that the shuttle programme will continue until its scheduled retirement in 2010.
The fact remains, however, that man's presence in space, which started with Yuri Gagarin in 1961, reached its climax just eight years later when Apollo XI carried men to the moon, 250,000 miles from earth. The main point of the latest Discovery mission, at a modest altitude of 200 miles, is to test the ability to make in-flight repairs, and above all to ensure the seven-man crew returns safely.
Is there an alternative to manned space exploration?
Very much so. Each shuttle launch costs around $1.3bn (£720m), but the most important exploration today is carried out by unmanned craft, costing far less per individual mission. Nasa's most productive programme is the Hubble Space Telescope, which has provided invaluable insights into fundamental problems of astrophysics- Hubble's Ultra Deep Field is the most sensitive astronomical optical image ever taken. Hubble is approaching the end of its life, but a "Next Generation Space Telescope" is due to be launched in 2010. The Mars Pathfinder and Mars Exploration Rovers have also been huge successes, continuing to send back important data about the red planet to scientists.
What's the problem with sending people into space?
Quite simply, space is an extremely hostile environment for humans. Everything we need - food, water, the air we breathe - must be taken with us. These factors limit the length of any mission, while the sheer weight of such basic items significantly reduces the useful payload of shuttle missions. Discovery and its surviving sisters, Atlantis and Endeavour, are hopelessly limited vehicles, capable of reaching only low earth orbit. For that reason, the International Space Station (ISS) which they service must fly in the same low and relatively unproductive orbit. And as the fate of Challenger in 1986 and Columbia in 2003 constantly remind, manned space travel is dangerous. Fourteen astronauts died in the two missions.
Are there any advantages?
The case for manned exploration boils down to the eternal argument over human versus artificial intelligence. Computer-controlled robotic missions can gather vast quantities of data. But they are less good at evaluating it. They cannot make the on-the-spot, creative judgements on which avenues should be pursued and which abandoned. Just like chess-playing computers, robots waste time and energy evaluating possibilities, when a human on the spot would instantly know whether a line of exploration was worthwhile.
Furthermore, the publicity given to human space disasters masks the much higher failure rate of unmanned missions. Take unmanned probes to Mars; since 1960 roughly two out of three have failed. Challenger and Columbia notwithstanding, manned missions have had a 90 per cent success rate. And nothing gets people excited about space exploration - and makes them willing to pay for it - like dramatic human moments.
Do we need a permanent manned space station?
Increasingly, the view among scientists is No. The International Space Station (ISS) programme - a joint venture of Nasa, and the space agencies of Russia, Canada, the EU and Japan - may cost a final $110bn, even though the 18 shuttle missions needed to bring it to completion may never take place, leaving the station manned by a mere skeleton crew. Dependent on the shuttle (which itself has cost $145bn over 30 years), the ISS must operate in relatively low orbit, limiting its possibilities. Another shuttle disaster would probably spell the end for the station, even though the five partners have pledged to complete it by 2010.
None other than Michael Griffin, chief administrator of Nasa, has implied that the shuttle and the ISS were mistakes. "It is now commonly accepted that was not the right path," he has said. "We are now trying to change the path while doing as little damage as we can."
Some argue that little of major scientific value has been accomplished by the station. Its main value, critics say, is as a vehicle of international co-operation in troubled political times - or as the ultimate in exotic tourism. Already two individuals have paid $20m to be taken up to the ISS, although the grimiest flophouse on mother earth is Ritz-like by comparison.
So what is the future of space exploration?
Man may return to the moon and one day reach Mars. But machines will set the pace - if only for economic reasons. While Nasa looks for human exploits to rekindle enthusiasm for space, its European equivalent, the ESA, spends only one eighth of its budget on manned space projects. The spectacular successes of the two Mars Rovers cost less than $800m. In 2005, the US orbiter Cassini sent home sensational images of Saturn, while a European "suicide" probe has landed on Titan, Saturn's largest moon. A Nasa craft has been deliberately collided with a comet, while another has brought a cargo of stardust back to earth. And other spacecraft are heading for Mercury, Venus and Pluto, while Voyagers One and Two, launched in 1977, head for interstellar space.
Should manned space exploration continue?
* When it comes to exploration, people are better than machines at recognising what is of interest
* Men are needed to make repairs in space, such as those which extended the useful life of the Hubble telescope
* The drama of watching men and women explore new worlds appeals to something basic in human nature
* The costs of sending humans into space far exceeds any benefits gained by employing them rather than machines
* Humans need so much in the way of supplies - oxygen, warmth, protection, etc - that the extra weight forbids going to most places
* Space travel is intrinsically dangerous, and it better to lose machinery in the inevitable accidents than human lives