A NASA spacecraft fired its engine and slipped into orbit around the moon on Saturday in the first of two back-to-back arrivals over the New Year's weekend.
Ground controllers at the NASA Jet Propulsion Laboratory erupted in cheers and applause after receiving a signal that the Grail-A probe was healthy and circling the moon. An engineer was seen on closed-circuit television blowing a noisemaker to herald the New Year's Eve arrival.
"This is great, a big relief," deputy project scientist Sami Asmar told a roomful of family and friends who had gathered at the NASA center to watch the drama unfold.
The celebration was brief. Despite the successful manoeuvre, the work was not over. Its twin Grail-B still had to enter lunar orbit on New Year's Day.
The Grail probes - short for Gravity Recovery and Interior Laboratory - have been cruising independently toward their destination since launching in September aboard the same rocket on a mission to measure lunar gravity.
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 forward in 2011 suggests that Earth once had two moons that collided early in the solar system's history, producing the hummocky region.
By mapping the uneven lunar gravity field that will indicate what's below the surface, Grail is expected to help researchers better understand why the moon is asymmetrical and how it formed.
"It seems that the answer is not on the surface," says 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 of Earth's pull - with mixed results. Grail is the first mission devoted to this goal.
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