The first of NASA's two Gravity Recovery And Interior Laboratory (GRAIL) spacecraft, GRAIL-A, entered lunar orbit on New Year's Eve (Dec. 31, 2011), a day ahead of its twin.
"Pop the bubbly and toast the moon! NASA's GRAIL-A spacecraft is in lunar orbit," the agency's Jet Propulsion Laboratory (JPL) wrote on Twitter.
GRAIL-A fired its main thruster at 3:21 p.m. CST (2121 GMT) for 40 minutes, slowing its approach by 427 miles per hour (687 kilometers per hour). Engine cutoff and lunar orbit insertion was confirmed at 4:01 p.m. CST (2201 GMT).
"Burn complete! GRAIL-A is now orbiting the moon and awaiting the arrival of its twin GRAIL-B on New Year's Day," wrote JPL.
Launched aboard the same rocket on Sept. 10, 2011, GRAIL-A's mirror twin, GRAIL-B, is also closing the gap between itself and the moon. GRAIL-B is scheduled to perform its lunar orbit insertion burn on New Year's Day (Jan. 1, 2012) at 4:05 p.m. CST (2205 GMT).
As it will for GRAIL-B, GRAIL-A's insertion maneuver placed it into a near-polar, elliptical orbit with an orbital period of 11.5 hours. Over the following weeks, flight controllers will execute a series of burns with each spacecraft to reduce their period down to just under two hours.
At the start of the science phase in March 2012, the two GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles (55 kilometers).
When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon in formation. Regional gravitational differences on the moon are expected to expand and contract that distance. Scientists will use these accurate measurements to define the moon's gravity field.
The data will allow mission scientists to understand what goes on below the surface of the moon, providing more information about how it, the Earth and other terrestrial planets formed.