LOS ANGELES - The New Year's countdown to the moon has begun. NASA said Wednesday that its twin spacecraft were on course to arrive back-to-back at the moon after a 31/2-month journey.
"We're on our way there," said project manager David Lehman of the NASA Jet Propulsion Laboratory, which manages the $496 million mission.
The Grail probes - short for Gravity Recovery and Interior Laboratory - won't land on the lunar surface. Instead, they were poised to slip into orbit to study the uneven lunar gravity field.
Grail-A is due to arrive on New Year's Eve, followed by Grail-B on New Year's Day. Lehman said team members won't celebrate until both probes are safely in orbit.
It's been a long voyage for the near-identical Grail spacecraft, which traveled more than 2.5 million miles since launching in September. Though the moon is relatively close at about 250,000 miles away, Grail took a roundabout way to save on costs by launching on a small rocket.