NASA's Gravity  Recovery And Interior Laboratory (GRAIL) spacecraft orbiting the moon  officially have begun their science collection phase. During the next 84  days, scientists will obtain a high-resolution map of the lunar  gravitational field to learn about the moon's internal structure and  composition in unprecedented detail. The data also will provide a better  understanding of how Earth and other rocky planets in the solar system  formed and evolved.

"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," said Maria Zuber, principal investigator for the GRAIL mission at  the Massachusetts Institute of Technology in Cambridge. "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 mission's twin, washing-machine-sized spacecraft, named Ebb and  Flow, entered lunar orbit on New Year's Eve and New Years Day. GRAIL's  science phase began yesterday at 8:15 p.m. EST (5:15 p.m. PST). During  this mission phase, the spacecraft will transmit radio signals precisely  defining the distance between them. As they fly over areas of greater  and lesser gravity caused by visible features such as mountains, craters  and masses hidden beneath the lunar surface, the distance between the  two spacecraft will change slightly. Science activities are expected to  conclude on May 29, after GRAIL maps the gravity field of the moon three  times.

"We are  in a near-polar, near-circular orbit with an average altitude of about  34 miles (55 kilometers) right now," said David Lehman, GRAIL project  manager from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.  "During the science phase, our spacecraft will orbit the moon as high as  31 miles (51 kilometers) and as low as 10 miles (16 kilometers). They  will get as close to each other as 40 miles (65 kilometers) and as far  apart as 140 miles (225 kilometers)."

Previously  named GRAIL A and B, the names Ebb and Flow were the result of a  nation-wide student contest to choose new names for the spacecraft. The  winning entry was submitted by fourth graders from the Emily Dickinson  Elementary School in Bozeman, Mont. Nearly 900 classrooms with more than  11,000 students from 45 states, Puerto Rico and the District of  Columbia, participated in the contest.

JPL  manages the GRAIL mission for NASA's Science Mission Directorate in  Washington. The GRAIL mission is part of the Discovery Program managed  at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed  Martin Space Systems in Denver built the spacecraft.

See the webside for more details:http://www.nasa.gov/mission_pages/grail/multimedia/pia13956.html（SY）