Moonstruck again after 25 years - but what's the thrill in a few drops of water?

After Apollo 11, whatever happened to the Moonrush? Why don't we have a full-time base on our satellite? And why could the Lunar Prospector spacecraft make a crucial difference? Charles Arthur, Science Editor, explains why space buffs are getting excited about the prospect of finding a bit of water.

Lunar Prospector, which took off for the Moon early on Wednesday, may not be big: it's only 1.2 metres (4 feet) long and weighs just 295 kilograms (650 pounds). But it could really have a large impact. It could make the Moon the best thing since the Klondike Gold Rush.

Watching the spacecraft take off, Joseph Boyce, a scientist who worked on Nasa's Apollo moon missions, including the last one, in December 1972, was ecstatic. "After 25 years of having not been to the Moon by Nasa, it certainly feels good to be going back," Mr Boyce said. "I couldn't be more excited, more happy, more pleased."

Prospector is due to arrive at the Moon on Sunday, after a trip covering 240,000 miles (384,000 kilometres). Two days later, the probe will slip into orbit 60 miles (96 kilometres) above the surface and begin its year- long search for evidence of frozen water, as well as minerals and gases.

If you are over 35, then the excitement generated by the Moonshots 30 years ago meant you expected that by now we would be all over the Moon - taking holidays there, using it as a waystation to head for Mars or the stars, and digging it up (as in 2001: A Space Odyssey) to discover mysterious signs from extraterrestrial races.

Instead, it's a junkyard for equipment and items left behind by the 12 astronauts who walked on the Moon: landers, buggies, even golf balls. What happened to those dreams?

Although everyone was thrilled by the images of Neil Armstrong bouncing down the ladder from the Eagle lander, the news that the Apollo 11 mission brought back in July 1969 was of a place without water and without life. The lack of water makes living on the Moon impracticable, because would- be settlers would have to take all their oxygen and liquid requirements along in the launch vehicle. Water, in particular, is so heavy that getting it out of orbit is hugely expensive.

A Moon base would require about two metric tonnes of air and water per person per year.

Sending that to the Moon would cost roughly $2,500 per kilogram, according to a 1994 study; keeping each person alive would cost roughly $20 million per year. Though in 1989 President George Bush suggested setting up a full-time lunar station, the end of the Cold War three years later killed off the finances required for the idea.

But those equations all change if there is water already on the Moon: the cost of setting up the station and running it would be cut at least tenfold. And some scientists believe the moon's shadowy poles hold as much as 1 billion tons of water ice, a theory bolstered by the military's Clementine spacecraft, launched to the Moon in 1994.

With water, lunar settlers would be able to drink and breathe without having to bring their own supplies. The ice could be melted, and oxygen could extracted from it, by electrolysis, using electricity generated from solar panels.

But why should we want a Moon base? Because it is a terrific spot for launching missions to other planets, or even the stars. Launching something from the Moon takes comparatively little energy, since its gravity is just one-sixth that of Earth.

The raw materials could be found there - the rocks contain plentiful iron, magnesium and titanium, as well as uranium, calcium and aluminium - and the water, electrolysed into hydrogen and oxygen, could power a rocket. Neil Armstrong reckons that a lunar base would have a lot going for it.

"I'm certain that we'll have such bases in our lifetimes," he said on returning. "Somewhat like the Antarctic stations. There's certainly the problem of the environment and the vacuum, and the high temperature of day and night.

"Still, in some ways it's more hospitable than the Antarctic. There are no storms, no snow, no high winds, no unpredictable weather. As for the gravity - well, the Moon's a very pleasant kind of place to work in. Better than the Earth, I think."