PASADENA, California: Twenty-four hours after a NASA probe safely entered orbit around the moon, its twin was poised to do the same on Sunday. The back-to-back arrivals would cap a roundabout journey spanning 3 1/2 months and covering 4 million kilometers.

The Grail spacecraft - short for Gravity Recovery and Interior Laboratory - are on a hunt to learn what's inside the moon by meticulously mapping its lumpy gravity field from orbit.

On New Year's Eve, mission controllers at the NASA Jet Propulsion Laboratory in Pasadena, California, cheered after Grail-A flew over the south pole and braked into orbit. An engineer even blew a noisemaker to celebrate.

The space agency hoped for a similar flawless performance from Grail-B.

Chief scientist Maria Zuber of the Massachusetts Institute of Technology briefly relaxed after Saturday's success.

"Then I thought to myself, 'I have to go through this again. I can't believe it,'" said Zuber.

This time, Zuber will fly to Denver to watch the second half with engineers at Lockheed Martin Space Systems, which built the washing machine-size probes.

An expedition to the moon, Earth's closest neighbor at about 250,000 miles (400,000 kilometers) away, usually takes a few days. Grail took the road less traveled by launching on a small rocket that lengthened the trip, but was more economical.

Since the Space Age, the moon has been the focus of more than 100 missions. Despite all the exploration - including Apollo landings on the surface and hauling rock and soil back to Earth - aspects of the moon remain a mystery.

One of the enduring puzzles is its lopsided shape with the far side more hilly than the side that Earth sees. 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 graces 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.

Data collection won't begin until March after the spacecraft tweak their positions and are circling just 34 miles (55 kilometers) above the surface. While scientists focus on gravity, middle school students will get the chance to take their own moon pictures using cameras aboard the probes as part of a project spearheaded by Sally Ride, the first American woman in space.

There's already talk about trying to extend the $496 million mission, which was slated to end before the partial lunar eclipse next 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.

NASA's previous moonshot occurred in 2009 with the arrival of the Lunar Reconnaissance Orbiter and a companion spacecraft that deliberately crashed into the moon's south pole to measure the amount of water in a crater. Future lunar missions will be unmanned after the Obama administration last year scrapped a plan to send astronauts back in favor of going to an asteroid as a stepping stone to Mars.