Time was you could get to the moon in a few days, and you can still do it if you have a really big rocket. But SMART-1, a washing machine-size spacecraft carrying an experimental engine, has taken the long way around.
And around, around, and around, flying more than 13 months to make 331 loop-de-loops around Earth in ever-expanding spirals until today, when it is scheduled to enter lunar orbit. Give it another couple of months to get comfortable, and it should be ready to collect data on the moon's composition and begin searching for ice at the lunar poles.
Nothing happens fast on this European Space Agency spacecraft, which left Kourou, French Guiana, aboard an Ariane 5 rocket on Sept. 27, 2003, climbed into Earth's orbit, then used the gentle yet insistent thrust of its ion propulsion engine to gradually scale the heavens to the moon's embrace.
"It's like the turtle and the hare, and ion propulsion is the turtle," said engineer Giuseppi Racca, the mission's project manager, comparing it to the explosive chemical reactions that power conventional rockets. "It's faster than chemical propulsion, but only if you travel a long way."
But that's all right. SMART-1 was designed to test ion propulsion, potentially a workhorse technology that could power spaceships on prolonged tours of the heavens, or pre-position supplies for astronauts to pick up on a later flyby or after they land on a distant body.
NASA flew the first mission to rely on ion propulsion, Deep Space 1, in 1998. A mission set to begin in 2006, called Dawn, will use ion propulsion to explore the asteroids Ceres and Vesta in a multi-year journey that would be prohibitively difficult and expensive using chemical fuels.
"SMART-1 is a good way to use the moon as a test bed because it's close by," planetary geologist G. Jeffrey Taylor, a lunar specialist from the University of Hawaii at Manoa, said in a telephone interview. "The mere fact that they could get there makes the mission a success."
As "the cherry on the cake," said physicist Bernard Foing, the project's chief scientist, SMART-1 also hopes to use its spectrometers to gather information about how the moon was formed around 4.5 billion years ago.
SMART-1, short for Small Missions for Advanced Research in Technology, weighs about 800 pounds. It was built for ESA by the Swedish Space Corp., an aerospace firm. So far it has cost about $110 million, Racca said.
Cheap, however, does not mean unsophisticated. A standard moonshot involves putting a rocket on a launchpad and lighting the fuse. The target is about 240,000 miles away, and it takes about three days to get there. Apollo 11, the first mission to put two men on the moon, reached lunar orbit in just under 76 hours.
SMART-1 has a different idea. Instead of using chemical propellant, its ion propulsion expels the positively charged atoms, or ions, of the gas xenon, accelerated by an electric field inside the spacecraft's engine.
"There's no combustion," Racca said in a telephone interview from his ESA office in Noordwijk, the Netherlands. "We split the atoms with electricity to get ions, accelerate them at high speed and eject them." Ejection -- firing the ions out the back of the spacecraft -- is what drives the spacecraft forward. SMART-1 generates the electricity by converting sunlight with outsize solar arrays that give the spacecraft a 45-foot wingspan.
The bad news about ion propulsion is that it does not produce a lot of thrust. "The ion engine varies from gentle to exceedingly gentle," said physicist Marc Rayman, of NASA's Jet Propulsion Laboratory. "It pushes like a piece of paper pushes on your hand."
But there are plenty of advantages. Rayman, chief engineer for Dawn and former project manager of Deep Space 1, noted that objects don't slow down in space, so acceleration is cumulative.
"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."