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.