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Europe's First Moon Probe Arrives at Destination with Revolutionary Engine

Europe's first mission to the Moon has arrived and begun orbiting, 14 months after its launch from French Guiana. European Space Agency officials confirmed Tuesday the SMART-1 spacecraft reached its destination on schedule, Monday, and will take several weeks to settle into a lower orbit from which it can gather geological data. But the orbiter's main goal has been to test a revolutionary type of engine.

SMART-1 is finished with the first phase of its mission and has proven its new engine over the indirect 84 million kilometer journey to the Moon. The engine was designed to save enormous amounts of fuel at the expense of speed and time, a technology believed useful for unmanned deep space missions when rapid results are not essential.

The spacecraft is a one-meter-square box propelled by a so-called ion engine envisioned on the science fiction television program Star Trek, almost 40 years ago. Instead of carrying tons of heavy solid hydrogen and oxygen as fuel, SMART-1 was loaded with only about 82 liters of xenon gas, commonly found in camera flash bulbs.

The head of the European Space Agency's Propulsion Division, Giorgio Soccaccia, says SMART-1 has used only 50 liters of this supply so far, showing itself to be extremely efficient in its fuel use. "So if you make a rough calculation, considering the density of xenon, it is something on the order of two million kilometers per liter of propellant," he said. "I'd like to have that for my car, to be honest. So the end result is that we are now in Moon orbit."

Powered by solar energy panels, an electron gun bombards the gas, dislodging one electron from each of the xenon atoms. Without its negatively charged electrons, the gas becomes positively charged and attracted to negatively charged plates at the rear of the fuel chamber. The gas rushes toward the plates and spits out the back of the chamber with a ghostly blue glow instead of flames. The rearward gas movement thrusts the spacecraft in the opposite direction without combustion. The pressure of the thrust is minuscule, about 10,000 times less than a conventional rocket, like the weight of a piece of paper resting on your hand.

Mr. Soccaccia says the lack of heavy fuel mass allows more cargo weight. "To give you a rough idea, in terms of mass efficiency, an ion engine can be five to 10 times more economical in fuel consumption than a classical engine," explained Mr. Soccaccia. "This means basically you would be in a position to have a much larger scientific payload on a spacecraft in the future, which is so very welcome, of course."

In contrast to the three-day American missions to the moon, the 14-month SMART-1 flight circled Earth 331 times, each time expanding its orbit until the moon's gravity could capture it. It picked up speed along the way, based on the principle that objects in space do not slow, but accumulate speed gradually.

An ion engine was first used on the U.S. Deep Space-1 mission, launched in 1998. The deputy manager of that mission, Marc Rayman, says the efficiencies of an ion engine promise to make deep space ventures more affordable.

"Ion propulsion is applicable to any place we want to get to in the solar system that previously has been unaffordable or physically impossible to get to with what we had," said Marc Rayman. "The set of technologies we have, by allowing us to make the spacecraft less expensive, launch on more affordable launch vehicles, means that now we can be more rapid in responding to scientific questions that we devise and sending spacecraft to more places in the solar system and conducting more detailed studies."

The European SMART-1 spacecraft now enters the second phase of its mission. It is beginning to lower its initial 40,000 kilometer orbit around the Moon's poles to an oval shaped one ranging in altitude from 300 to 10,000 kilometers. Starting in January, it will gather data on the Moon's origin and geological activity, map its composition, search for possible water ice and seek potential locations for future robotic and human bases.