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GRAIL Probes Set to Begin Lunar Orbit Saturday, Sunday

This undated artist rendering provided by NASA shows the twin Grail spacecraft mapping the lunar gravity field. The two probes are scheduled to enter orbit around the moon over New Year's weekend.

U.S. space agency NASA says its twin lunar spacecraft that are on a mission to study the Moon’s gravitational field and internal structure are set to begin orbiting Earth’s satellite on Saturday and Sunday.

NASA launched the two unmanned Gravity Recovery and Interior Laboratory, or GRAIL, probes in September. It says the first probe, GRAIL A, is scheduled to be placed into its lunar orbit on Saturday, beginning at 2121 Universal time [4:21 p.m. EST Saturday). The second probe, GRAIL B, will start its orbit placement on Sunday at 2205 Universal time [5:05 p.m. EST Sunday].

The orbiting GRAIL spacecraft are scheduled to spend about 90 days gathering data to create the first complete high-resolution map of the Moon’s curiously uneven gravitational field. Scientists say they will use the new gravity data to find out what goes on beneath the lunar surface, “from crust to core.”

Mission principal researcher, Maria Zuber of the Massachusetts Institute of Technology, said the GRAIL project will “rewrite the textbooks on the evolution of the Moon.”

NASA says it expects insights from the GRAIL mission to lead to a better understanding of how the Earth and the three other rocky planets [Mercury, Venus and Mars] in the inner solar system developed into the diverse worlds seen today.

The GRAIL A and GRAIL B orbital maneuvers on Saturday, December 31 and on Sunday, January 1 happen to coincide with New Year’s Eve 2011 and New Year’s Day 2012.

NASA anticipates that GRAIL’s comprehensive map of the Moon’s gravitational field also will provide an important navigational tool for future lunar spacecraft. The space agency says it plans to plunge both GRAIL spacecraft into the lunar surface several weeks after the orbital mapping mission is complete.

The Moon orbits the Earth from an average distance of about 385,000 kilometers. It has a diameter of nearly 3,500 kilometers. By contrast, the Earth’s is 12,756 kilometers wide. Because the Moon is smaller and has less mass, the force of gravity on the lunar surface is only 17 percent of that on the Earth. That means the same object that weighs 100 kilograms here on Earth will weigh only 17 kilograms on the Moon.

Astronomers believe the Moon formed only 30 to 50 million years after the rest of our solar system first coalesced from left over stellar dust and gas some 4.6 billion years ago. The dominant theory proposes that the Moon is made of debris that circled the still-molten Earth in the wake of a cataclysmic collision with a smaller Mars-sized protoplanet.