Smart-1 set for death plunge

Artist's impression image shows Smart-1 travelling to the Moon using a new solar-electric propulsion system. PHOTO: AFP

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 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.