PASADENA, CALIF. (AP) - One of two NASA spacecraft en route to the moon fired its engine Saturday, slowing its speed to slip into lunar orbit on New Year's Eve.
Family and friends of the mission team gathered at the NASA Jet Propulsion Laboratory to watch the drama unfold. Deputy project scientist Sami Asmar told the audience the engine burn occurred on time.
The maneuver was expected to last about 40 minutes. Deep space antennas in the California desert and Madrid were keeping tabs on the spacecraft and feeding real-time information to ground controllers. Its twin Grail-B will execute the same move on New Year's Day.
The duo will measure the moon's lumpy gravity field to determine what's inside straight down to the core.
Grail _short for Gravity Recovery and Interior Laboratory _ 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. Despite the attention the moon has received, scientists don't know everything about Earth's nearest neighbor.
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 forth earlier this year suggested that Earth once had two moons that collided early in the solar system's history, producing the hummocky region.
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.
"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 spacecraft will spend the next two months refining their positions until they are just 34 miles above the surface and flying in formation. Data collection will begin in March.
The $496 million mission will be closely watched by schoolchildren. An effort by Sally Ride, the first American woman in space, will allow middle school students to use cameras aboard the probes to zoom in and pick out their favorite lunar spots to photograph.
Despite the latest focus on the moon, NASA won't be sending astronauts back anytime soon. The Obama administration last year nixed a lunar return in favor of landing humans on an asteroid and eventually Mars.
A jaunt to the moon _ about 250,000 miles away from Earth _ is usually speedy. It took the Apollo astronauts three days to zip there aboard the powerful Saturn V rocket. Since NASA wanted to economize by launching on a small rocket, it took Grail a leisurely 3 1/2 months to make the trip covering 2 1/2 million miles.
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.
Follow Alicia Chang's coverage at http://www.twitter.com/SciWriAlicia