LOS ANGELES (AP) - A NASA spacecraft fired its engine Sunday to enter lunar orbit, hoping to join its twin already circling the moon.
If all goes as planned, the back-to-back arrivals over the New Year's weekend would cap a roundabout journey spanning 3 1/2 months and covering 2 1/2 million miles.
The Grail spacecraft _ short for Gravity Recovery and Interior Laboratory _ are on a hunt to learn what's inside the moon by meticulously mapping its lumpy gravity field from orbit.
On New Year's Day, the Grail-B spacecraft swooped over the moon's south pole and executed a 40-minute maneuver to brake into orbit as deep space antennas tracked its progress.
The move came a day after mission control at the Jet Propulsion Laboratory cheered at the success of its twin Grail-A. An engineer even blew a noisemaker to ring in the New Year's Eve arrival.
An expedition to the moon, Earth's closest neighbor at about 250,000 miles away, usually takes a few days. Grail took the road less traveled by launching on a small rocket that lengthened the trip, but was more economical.
Since the Space Age, the moon has been the focus of more than 100 missions. Despite all the exploration _ including Apollo landings on the surface and hauling rock and soil back to Earth _ aspects of the moon remain a mystery.
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 collided early in the solar system's history and created the moon that graces the sky 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.
Data collection won't begin until March after the spacecraft tweak their positions and are circling just 34 miles above the surface. While scientists focus on gravity, middle school students will get the chance to take their own moon pictures using cameras aboard the probes as part of a project spearheaded by Sally Ride, the first American woman in space.
There's already talk about trying to extend the $496 million mission, which was slated to end before the partial lunar eclipse next June. Scientists initially did not think the solar-powered probes would survive that long, but changed their minds during the long cruise to the moon after getting new data.
NASA's previous moonshot occurred in 2009 with the arrival of the Lunar Reconnaissance Orbiter and a companion spacecraft that deliberately crashed into the south pole to measure the amount of water in a crater. Future lunar missions will be unmanned after the Obama administration last year scrapped a plan to send astronauts back in favor of going to an asteroid as a stepping stone to Mars.
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