Los Angeles --

NASA celebrated the new year with a pair of probes circling the moon in the latest mission to understand how Earth's closest neighbor formed.

There was no Champagne popping in the mission control room at the NASA Jet Propulsion Laboratory when the Grail spacecraft arrived back to back over the New Year's weekend, but several scientists and engineers celebrated by blowing noisemakers.

"It's a really good feeling to have not one but two of our twins in orbit," project manager David Lehman said Sunday after the mission was deemed successful.

The action began on New Year's Eve when Grail-A swung over the south pole, fired its engine and braked into orbit around the moon. Not to be outdone, its twin, Grail-B, executed the same maneuvers on New Year's Day.

Scientists expect to learn more about how the celestial body formed using Grail's gravity measurements, which will indicate what's below the surface.

Previous spacecraft have attempted to study the moon's gravity - about one-sixth Earth's pull - with mixed success. Grail - short for Gravity Recovery and Interior Laboratory - 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.

Discussions are already taking place about trying to extend the $496 million mission, which was slated to end before a partial lunar eclipse in June. Scientists initially did not think the solar-powered probes would survive that long, but they changed their minds during the long cruise to the moon after receiving new data.

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