"With chemical propellant, you take a big rocket to give a tiny spacecraft a big push to start, and then it coasts," Rayman said in a telephone interview from JPL, in Pasadena, Calif. "With ion propulsion, you use a smaller rocket, and the spacecraft does its own accelerating. It's not a hot rod, but over time, it can make faster flights."
Also, instead of relying on slingshot planetary "flybys" and brief "burns" of limited onboard fuel to alter course and speed, an ion engine-powered spacecraft is master of its own destination, a true spaceship able to add speed and make course changes almost at will.
As long as it has plenty of time.
Instead of flying directly to the moon, SMART-1 made 331 ever-expanding orbits of Earth, ultimately traveling more than 52 million miles to lunar rendezvous. During the voyage the spacecraft used the ion engine 289 times in burns lasting from "a few hours to about 10 days," Racca said. The spacecraft carried nearly 52 quarts of xenon at launch, designed to provide 7,000 hours of thrust at full power. At the time of its last burn Oct. 27, it had used 3,648 hours of fuel.
SMART-1 goes into lunar orbit today, 25,000 miles above the lunar surface, and between now and the middle of January, it will use the ion engine to slow down and make adjustments, eventually settling into an elliptical polar orbit at an altitude of 250 miles to 2,500 miles. That's when the six-month science mission will begin.
Although researchers have advanced many theories for the moon's origins, the current favorite holds that a Mars-size body sideswiped the young Earth shortly after its formation, flinging vaporized rock, dust and debris into space, where some of it clumped together by the force of gravity to create the moon.
Earlier moon explorations have established that moon rocks are very similar to Earth rocks, but the moon, unlike Earth, shows no evidence of a significant iron core. SMART-1 hopes to make an "inventory" of the elements on the moon, Foing said in a telephone interview, building on research begun during the Apollo era and continued more recently by the U.S. Clementine and Lunar Prospector missions of the 1990s.
The spacecraft will also try to use its infrared spectrometer to peek into the moon's polar craters and analyze the reflected light to see if it can find ice. This is a difficult task, because the spectrometer needs light, but ice, by definition, can survive on the moon only in sunless environments.
Mineral mapping and the search for ice in the moon's shaded recesses could be crucial for future lunar exploration.
"We're trying to understand the moon's resources and get them to the service of humanity," said Lunar Prospector lead scientist Alan Binder, speaking by telephone from Tucson, where he directs the Lunar Research Institute. "Also, the moon is clearly the jumping-off point for the expansion of humanity into the solar system."