The second Gravity Recovery And Interior Laboratory, or GRAIL-B, reached its lunar orbit at 5:43 p.m. EST (2243 GMT) on Sunday, according to the National Aeronautics and Space Administration (NASA).

　　The GRAIL-A started orbiting the moon at 5 p.m. (2200 GMT) Saturday.

　　The insertion maneuvers placed the spacecraft into a near-polar, elliptical orbit with an orbital period of approximately 11.5 hours, NASA said.

　　Over the coming weeks, the GRAIL team will execute a series of burns with each spacecraft to reduce their orbital period to just under two hours. At the start of the science phase in March 2012, the twin GRAILs will be in a near-polar, near-circular orbit with an altitude of about 55 km.

　　"NASA greets the new year with a new mission of exploration," Administrator Charles Bolden said in a statement. "The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet."

　　During GRAIL's science mission, the two probes 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 twin spacecraft will change slightly.

　　Scientists will translate this information into a high-resolution map of the moon's gravitational field.

　　The data will allow scientists to understand what goes on below the lunar surface and increase knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.