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NASA probes to measure moon gravity

Associated Press / September 6, 2011

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CAPE CANAVERAL, Fla. - Four decades after landing men on the moon, NASA is returning to Earth’s orbiting companion, this time with a set of robotic twins that will measure lunar gravity while chasing one another in circles.

By creating the most precise lunar gravity map ever, scientists hope to figure out what is beneath the surface, all the way to the core. The orbiting probes also will help pinpoint the best landing sites for future explorers, whether human or mechanical.

Near-identical twins Grail-A and Grail-B are due to blast off Thursday aboard an unmanned rocket. Although launched together, the two washing machine-size spacecraft will separate and travel independently to the moon.

It will be a three- to four-month trip because of the small Delta II rocket used to boost the spacecraft. NASA’s Apollo astronauts used the mighty Saturn V rocket, which covered the roughly 240,000 miles to the moon in a mere three days. The mission costs $496 million.

“Nearly every human who’s every lived has looked up at the moon and admired it,’’ said Maria Zuber, a Massachusetts Institute of Technology planetary scientist and Grail’s principal investigator. “The moon has played a really central role in the human imagination and the human psyche.’’

For nearly three months, the probes will chase one another around the moon, meticulously flying in formation. Radio signals bouncing between the twins will provide their exact locations, even on the far side.