Associated Press
In this undated image provided by NASA on Wednesday, Dec. 28, 2011, shows two sides of the moon. Twin NASA probes traveling for the past 3 1/2 months are scheduled to arrive at the moon during the New Year's weekend to study lunar gravity.

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 3½-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 was scheduled 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.

Once at the moon, the probes will spend the next two months tweaking their positions before they start collecting data in March. The pair will fly in formation at an altitude of 34 miles above the surface, with an average separation of 124 miles.