NASA kicked off the new year with a pair of probes circling the moon in the latest mission to understand how Earth's closest neighbour was 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; its twin Grail-B executed the same manoeuvres on New Year's Day. The arrivals capped a roundabout journey spanning 3½ months and covering 4.02 million kilometres.

Since the dawn of the Space Age, more than 100 missions launched by the United States, Soviet Union, Japan, China and India have targeted Earth's companion. NASA flew six Apollo missions that landed 12 men on the lunar surface and brought back more than 360 kilograms of rock and soil samples.

Despite all the attention, the moon remains mysterious. Mission chief scientist Maria Zuber of the Massachusetts Institute of Technology said researchers know more about Mars, which is farther away from the Earth, than the moon.

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 orbits the Earth 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.

Longer trip

Since the washing machine-size Grail probes — short for Gravity Recovery And Interior Laboratory —  were squeezed on a small rocket to save on costs, it lengthened the trip and took them 30 times longer to reach the moon than the Apollo astronauts, who took a direct three-day flight.

Previous spacecraft have attempted to study the moon's gravity —  about one-sixth Earth's pull — with mixed success. Grail 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 54.7 kilometres above the surface. While scientists focus on gravity, middle school students will get the chance to take their own pictures of the moon using cameras aboard the probes as part of a project headed by Sally Ride, the first American woman in space.

There's already chatter about trying to extend the $496-million US mission, which was slated to end before the partial lunar eclipse in June.