Twin NASA spacecraft nearing arrival at moon

Twin spacecraft launched in September from Cape Canaveral have completed final maneuvers to line up staggered entries into lunar orbit this weekend.

The first of the two unmanned spacecraft flying NASA's $496-million Gravity Recovery And Interior Laboratory (GRAIL) mission is scheduled to fire its main engine at 4:21 p.m. EST Saturday, New Year's Eve, to begin dropping into orbit around the moon.

Following the spacecraft known as GRAIL-A, the nearly identical GRAIL-B will perform the same maneuver New Year's Day, with a burn starting at 5:05 p.m. Eastern time on Jan. 1.

The engine firings will last about 40 minutes, initiating an elliptical, near-polar orbit that will be circularized and lowered over a period of weeks.

NASA plans to release the results after each orbit insertion maneuver, but there will be no live TV coverage.

The small spacecraft launched Sept. 10 from Cape Canaveral Air Force Station atop a United Launch Alliance Delta II Heavy rocket.

Their goal is to map the moon's gravity field in the greatest detail yet, information that will provide new information about the moon's interior composition and evolution.

If the lunar orbits are achieved as planned, a science mission lasting at least 82 days would begin in March, with the spacecraft chasing each other about 34 miles above the moon's surface. The mission could be extended another several months.