The twin Gravity Recovery And Interior Laboratory probes will spend 84 days creating a high-resolution map of the lunar gravitational field to gather data about about the moon's internal structure and composition, the agency said in a release Wednesday.
"The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do," Maria Zuber, GRAIL investigator at the Massachusetts Institute of Technology said, "but it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."
The GRAIL probes, named Ebb and Flow, entered lunar orbit on New Year's Eve and New Year's Day respectively.
"We are in a near-polar, near-circular orbit with an average altitude of about 34 miles right now," David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said.
"During the science phase, our spacecraft will orbit the moon as high as 31 miles and as low as 10 miles.
The twin spacecraft "will get as close to each other as 40 miles and as far apart as 140 miles," Lehman said.
When the science activities are concluded on May 29 the GRAIL craft will have mapped the gravity field of the moon three times, NASA said.