The cruise to the moon took 3 1/2 months and covered 2 1/2 million miles _ far longer than the direct three-day flight by Apollo astronauts.
Over the New Year's weekend, a pair of NASA spacecraft arrived back-to-back at their destination in the first mission devoted to studying lunar gravity.
Mission controllers at the NASA Jet Propulsion Laboratory did not toast champagne _ there's a no alcohol policy on campus _ but several belatedly heralded the new year by with noisemakers.
"We can start celebrating the new year now," project manager David Lehman said Sunday after attending a post-mission fete where cake and sparkling cider were served.
The tricky arrivals occurred 24 hours apart. The drama unfolded on New Year's Eve when Grail-A flew over the south pole, fired its engine and dropped into lunar orbit. Its twin Grail-B repeated the maneuvers on New Year's Day.
Cheers and applause filled mission control when each probe signaled it was healthy and circling the moon.
"Everything worked much better than I hoped," Lehman said.
The moon has long been an object of fascination. Galileo spotted mountains and craters when he peered at it through a telescope. Poets and songwriters looked to the moon as a muse.
Since the late 1950s, more than 100 missions launched by the United States, Soviet Union, Japan, China and India have targeted Earth's companion. NASA flew six Apollo missions that landed twelve men on the lunar surface and brought back more than 800 pounds of rock and soil samples.
Despite all the visits, the moon remains mysterious. Mission chief scientist Maria Zuber of the Massachusetts Institute of Technology said researchers know more about Mars, which is farther away from the Earth, than the moon.
One of the enduring puzzles is its lopsided shape with the far side more hilly than the side that always faces Earth. Research published earlier this year suggested that our planet once had two moons that crashed early in the solar system's history and created the moon that hangs in the sky today.
Scientists expect to learn more about how the celestial body formed using Grail's gravity measurements that will indicate what's below the surface.
Since the washing machine-size Grail probes _ short for Gravity Recovery And Interior Laboratory _ were squeezed on a small rocket to save on costs, it lengthened the trip and took them 30 times longer to reach the moon than the Apollo astronauts.
Previous spacecraft have attempted to study the moon's gravity _ about one-sixth Earth's pull _ with mixed success. Grail was expected to give scientists the most detailed maps of the moon's uneven gravitational field and insight into its interior down to the core.
Data collection won't begin until March after the near-identical spacecraft refine their positions and are circling just 34 miles above the surface. While scientists focus on gravity, middle school students will get the chance to take their own pictures of the moon using cameras aboard the probes as part of a project headed by Sally Ride, the first American woman in space.
There's already chatter about trying to extend the $496 million mission, which was slated to end before the partial lunar eclipse in June. Scientists initially did not think the solar-powered probes would survive that long, but changed their minds during the long cruise to the moon after getting new data.
Researchers expect Grail to return a plethora of data, but that information won't be a guide to manned lunar trips anytime soon. The Obama administration last year scrapped a plan to return astronauts to the lunar surface in favor of landing on an asteroid as a stepping stone to Mars.
Follow Alicia Chang's coverage at http://www.twitter.com/SciWriAlicia