A small army of terrestrial astronomers will be training their telescopes on the southwestern side of the Moon's face, hoping to glimpse the cloud of dust that will signal SMART-1's mortal impact in a plain called the Lake of Excellence.

"It will be a very shallow trajectory," says mission scientist Bernard Foing.

"It's possible that much of the probe's structure will be preserved from the impact, accidentally creating a sculpture or a monument for future generations which says 'there you are, that was Europe's first attempt to explore the Moon.'"

The spectacular end, scheduled for 0542 GMT on Sunday, is intended to be a more useful alternative to letting the European Space Agency (ESA) craft crash anywhere or at any time through orbital decay and lack of fuel.

Over the past three years, operating with a full-time staff of just seven and a total budget of just 120 million euros (151 million dollars), the little probe has been patiently testing new technology that one day could help put Man on Mars.

Scientists also believe it will yield a fresh look at the Moon, revealing Earth's satellite as a place of surprising complexity and promise rather than a lifeless rock with little to offer except grey dust.

"SMART-1 is the vanguard" of future space missions, said the craft's operations manager, Octavio Camino-Ramos.

Driving the probe since it was placed into orbit in September 2003 is an ion thruster, an engine type that has only been used once before -- with the US craft Deep Space 1, launched in 1998 to rendezvous with an asteroid and then a comet.

Ion engines are fuelled by xenon gas. The gas atoms are charged by electric guns powered by solar panels and are then expelled from the rear of the spacecraft, delivering a tiny thrust, visible as a ghostly blue glow.

Compared with the blast, roar and smoke of chemical rockets, ion engines seem almost laughably puny.

But chemical engines burn out after a couple of minutes, whereas an ion engine can push on gently for months or even years, for so long as the Sun shines and the small supply of propellant lasts.

In the frictionless vacuum of space, ion engines, by sipping patiently at their fuel, can slowly but relentlessly build up speed.

That makes them ideal for long-range missions where time is not a big factor but payload space is precious.

Unlike the Deep Space mission's essentially straight-line trajectories, SMART-1 had to carry out a complex series of manoeuvres. It had to loop again and again around the Earth to gain extra speed yet also juggle with the Moon's gravitational tug, all the time using a tiny engine that delivered the same power as someone picking up an A4-sized sheet of paper.

Camino says the experience was "an adventure," and compared it to navigating a small sailboat buffeted by swirling winds and currents. But the tiny thruster performed splendidly, even though it needed 14 months for a trip that took Apollo only three days.

"We have shown that ion propulsion works," Camino said in an interview, predicting that this will be the thruster of choice for two ESA missions in the next decade -- BepiColombo, to explore Mercury, and Solar Orbiter, which will swoop close to the Sun.

And it is also likely to be the propulsion for hauling big cargo containers to the Moon to help build the first lunar settlement, or to Mars, to support the first manned mission there, he said.

Other SMART-1 innovations are a new communications system, new-generation solar panels and a package of sensors and scanners that Foing hailed as "a miracle of miniaturisation -- seven instruments weighing just 19 kilos" (41.8 pounds).

Weighing as little as a tenth of conventional instruments being carried aboard space probes, SMART's X-ray telescope and infrared spectrometer have been carrying out the most detailed map of the Moon's elements and minerals, said Foing.

SMART-1 has also sought out locations at the lunar poles that are in permanent shade and could be worth exploring if, as some scientists hope, water exists on the Moon or just below its surface.

Finding water would be a key to a future lunar colony, providing it with the means of physical survival as well as fuel, in the form of hydrogen.

Foing said the Moon had been given a bad rap.

"We do have a detailed knowledge of the Moon, but only of nine locations exactly, and all of them are on the side of the Moon that is visible from Earth," he said, referring to the Apollo missions and the scouting trips that preceded them.

"In fact, the Moon turns out to be far more diverse than that. An example: on the dark side of the Moon is a crater that is 2,500 kilometers (1,500 miles) wide. It's the biggest known impact crater in the Solar System -- and it was only discovered in 1994."

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