LOS ANGELES: NASA is counting down the seconds until its twin spacecraft bound for the moon make back-to-back arrivals over the New Year's weekend.

The washing machine-sized probes have been cruising independently toward their destination since launching in September aboard the same rocket on a mission to measure lunar gravity.

Approaching the moon from the south pole, the Grail spacecraft - short for Gravity Recovery and Interior Laboratory - won't land on the surface, but will survey from orbit.

On New Year's Eve, Grail-A was poised to fire its engine for more than a half hour to slow itself and get captured into orbit. Grail-B will follow suit on New Year's Day.

Deep space antennas in the California desert and Madrid will track the tricky maneuvers and feed real-time updates to ground controllers.

"The anxiety level is heightened," project manager David Lehman of the NASA Jet Propulsion Laboratory said earlier this week.

Grail is the 110th mission to target the moon since the dawn of the Space Age including the six Apollo moon landings that put 12 astronauts on the surface. Despite the attention the moon has received, scientists don't know everything about Earth's nearest neighbor.

Why the moon is ever so slightly lopsided with the far side more mountainous than the side that always faces Earth remains a mystery. A theory put forth earlier this year suggested that Earth once had two moons that collided early in the solar system's history, producing the hummocky region.

Grail is expected to help researchers better understand why the moon is asymmetrical and how it formed by mapping the uneven lunar gravity field that will indicate what's below the surface.

"It seems that the answer is not on the surface," said chief scientist Maria Zuber of the Massachusetts Institute of Technology. "We think that the answer is locked in the interior."

Previous lunar missions have attempted to study the moon's gravity - which is about one-sixth Earth's pull - with mixed results. Grail is the first mission devoted to this goal.

Once in orbit, the near-identical spacecraft will spend the next two months refining their positions until they are just 34 miles above the surface and flying in formation. Data collection will begin in March.

The $496 million mission will be closely watched by schoolchildren. An effort by Sally Ride, the first American woman in space, will allow middle school students to use cameras aboard the probes to zoom in and pick out their favorite lunar spots to photograph.

Despite the latest focus on the moon, NASA won't be sending astronauts back anytime soon. The Obama administration last year nixed a lunar return in favor of landing humans on an asteroid and eventually Mars.

A jaunt to the moon - about 250,000 miles (400,000 kilometers) away from Earth - is usually speedy. It took the Apollo astronauts three days to zip there aboard the powerful Saturn V rocket. Since NASA wanted to economize by launching on a small rocket, it took Grail a leisurely 3 1/2 months to make the trip covering 2.5 million miles (4 million kilometers).

NASA's last moonshot occurred in 2009 with the launch of a pair of spacecraft - one that circled the moon and another that deliberately crashed into the surface and uncovered frozen water in one of the permanently shadowed lunar craters.