Twin probes to circle moon to study gravity field
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Last Updated: Tuesday, December 27, 2011, 09:31
  
Los Angeles: The moon has come a long way since Galileo first peered at it through a telescope. Unmanned probes have circled around it and landed on its surface. Twelve American astronauts have walked on it. And lunar rocks and soil have been hauled back from it.

Despite being well studied, Earth's closest neighbor remains an enigma.

Over the New Year's weekend, a pair of spacecraft the size of washing machines are set to enter orbit around it in the latest lunar mission. Their job is to measure the uneven gravity field and determine what lies beneath straight down to the core.

Since rocketing from the Florida coast in September, the near-identical Grail spacecraft have been independently traveling to their destination and will arrive 24 hours apart.

Their paths are right on target that engineers recently decided not to tweak their positions.

"Both spacecraft have performed essentially flawlessly since launch, but one can never take anything for granted in this business," said mission chief scientist Maria Zuber of the Massachusetts Institute of Technology.

The nail-biting part is yet to come. On New Year's Eve, one of the Grail probes short for Gravity Recovery and Interior Laboratory will fire its engine to slow down so that it could be captured into orbit. This move will be repeated by the other the following day.

Engineers said the chances of the probes overshooting are slim since their trajectories have been precise. Getting struck by a cosmic ray may prevent the completion of the engine burn and they won't get boosted into the right orbit.

"I know I'm going to be nervous. I'm definitely a worrywart," said project manager David Lehman of NASA's Jet Propulsion Laboratory, which manages the USD 496 million, three-month mission.

Once in orbit, the spacecraft will spend the next two months flying in formation and chasing one another around the moon until they are about 35 miles (56 kilometers) above the surface with an average separation of 124 miles (200 kilometers). Data collection won't begin until March.

Previous missions have attempted to measure lunar gravity with mixed success. Grail is the first mission dedicated to this goal.

Bureau Report


First Published: Tuesday, December 27, 2011, 09:31


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