|Grail is expected to help researchers better understand why the moon is asymmetrical and how it formed [AFP]
A NASA spacecraft has flown over the moon's south pole, fired its engine and slipped into orbit in the first of two back-to-back mission to unveil the moon's inner secrets.
Grail-A began its first orbit shortly after 2141 GMT and its sister craft Grail B is to do the same at 2205 on Sunday.
"This is great, a big relief," deputy project scientist Sami Asmar told a roomful of family and friends who gathered at the NASA center to watch the drama unfold.
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.
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.
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.
Understanding the moon
"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 Grail-A and Grail-B spacecraft will spend the next two months refining their positions until they are just 55 kilometres above the surface and flying in formation.
Data collection will begin in March.
Despite the latest focus on the moon, NASA will not be sending astronauts back anytime soon.
The Obama administration last year nixed a lunar return in favour of landing humans on an asteroid and eventually Mars.
A jaunt to the moon is usually speedy. It took the Apollo astronauts three days to zip there aboard the powerful Saturn V rocket.
Since NASA wanted to economise by launching on a small rocket, it took Grail a leisurely 3 1/2 months to make a roundabout trip.
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.