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Monday, August 2, 1999 Published at 15:04 GMT 16:04 UK


Lunar crash proves nothing

Impression of the probes last moments

By BBC News Online Science Editor Dr David Whitehouse

It seems that Lunar Prospector's violent collision with the Moon left no visible cloud of dust. Researchers will still analyse their images to see if they can find traces of water in a vapour plume possibly created by the impact.

[ image: Impact point]
Impact point
But even if they do not find anything it does not mean that there is no water at the Moon's poles. The Lunar Prospector impact was no "make-or break" mission for lunar water and many have not realised just how low the chances of success were.

The spacecraft struck the Moon with some force, equivalent to a two tonne car travelling at 1,600 km/h (1,000 mph). The impact point was deep in a shadow in the eternally-dark depths of a crater.

But we know from the Apollo landings that the lunar surface can be variable. It is highly likely that the spacecraft struck a big rock and shattered without disturbing much dust.

Also, if the lunar ice is patchy it may simply have missed the icefield or struck the Moon at such a shallow angle that it just rolled over and over until it stopped.

No news: good news?

It is going too far to say that the non-detection of the impact makes the case for lunar ice less likely.

Even if it had kicked up a dust and water plume it may have been undetectable.

Given the most favourable circumstances, the dust plume would have stretched only 22 km (14 miles) above the Moon's surface and would have been 100,000 times fainter than the brightness of the lunar limb.

What is more, the water plume would never have been detectable from Earth. Only one satellite in Earth orbit had any chance of seeing it.

Water of life

The search for water is more than a scientific curiosity. Large stores of ice on the moon would make it much easier and cheaper to establish a base there, or to use the moon as a staging post to more distant bodies.

The observations made by Lunar Prospector from lunar orbit provide extremely strong, almost compelling, evidence that there is water at the Moon's poles.

To prove it will require a probe to land on the surface and do some digging.

At present no one plans to do this. The European Space Agency was considering such a proposal - but then cancelled it.

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