Human colonies on the moon, space observatories that can peer into the depths of the universe, a staging post for the planets and beyond. There is nothing like the discovery of a drop of water to set the imagination racing.

Lunar Prospector, a satellite belonging to the US National Aeronautics and Space Administration, has been analysing the moon's surface for only a few weeks but has already confirmed what many scientists long believed, that water ice would be found on the moon.

Early estimates suggest that there could be 100 million metric tonnes up there, enough to keep a moon colony 2,000-strong going for a century without having to recycle.

It does not really matter that the water is spread out paper thin across the north and south lunar poles, and that special "mining" equipment would have to be developed to recover it. What matters is that it is there and in reasonable quantities.

Its existence opens up all sorts of opportunities for those willing to exploit it, whether commercial, government or military. Water can be drunk or can be breathed after being split into hydrogen and oxygen. Space-launch vehicles burn these two gases to yield the tremendous energy needed to get into orbit, and the gases also power fuel cells which produce electricity.

The great advantage is that now explorers to the moon will not have to bring this vital commodity with them: they can recover it from the lunar soil.

Its presence means that much longer missions to the surface will be possible, allowing more time to prospect for other materials such as metal ores that could be used to build new structures, to recover more water, to create bigger colonies, to allow more explorers to open up more opportunities . . . and so the system grows.

But who will get there first? The US military has always been interested, and much of the early American space programme was driven by military considerations. And it was a 1994 US military radar satellite, Clementine, which provided the first indications that there was water at the lunar poles.

However, the world's space programmes have a much more commercial edge than when man first walked on the moon almost 30 years ago. Companies which want to get into space just buy a "seat" on a launch vehicle.

The Chinese, the Russians, the Americans and the Europeans all sell launch capacity to commercial interests for satellites. Why not for the moon?

The Russians and the Americans are the only nations with launchers powerful enough to carry people as far as the moon, but the European Space Agency is developing its own heavy-lift Ariane 5. Large European companies such as AEA Technology and Matra Marconi Space have either expressed interest, or are already involved, in joint ESA activities to get them to the moon.

There are no protocols yet that would restrict free enterprise from grabbing a piece of the action if the investment funding was there. First on the list of gadgets to be developed would be a fully automatic, fuel-cell-powered, self-propelled water digger to keep astronaut/explorers in all the water they might need.

Next on the list would be semi-permanent settlements from which intensive prospecting could be carried out. And if the California or Yukon gold rushes were anything to go by, important discoveries on the moon would quickly attract more prospectors of all sorts.

Scientists, of course, would also be clamouring for access to the moon to tap its potential as a prime space observatory. Darkside-of-the-moon observations of distant galaxies with no atmosphere to obscure vision would make even ordinary telescopes tremendously powerful.

Then there is the whole question of how water got to the moon in the first place.

It could not have been when the moon formed as a superheated blob of molten rock. Most scientists believe water was delivered by incoming comets, the "dirty snowballs" of the cosmos.

Studying the lunar water would provide a rich source of information for specialists trying to understand how life might have begun on Earth.