NASA spacecraft now in orbit around moon

"Pop the bubbly & toast the moon!"

With those fitting remarks, NASA tweeted tonight that the first of two unmanned spacecraft - known as GRAIL-A - successfully entered lunar orbit.

The two spacecraft are flying NASA's $496-million Gravity Recovery and Interior Laboratory (GRAIL) mission aimed at better understanding Earth's nearest celestial neighbor.

GRAIL-A's partner, known as GRAIL-B, is set to arrive on New Year's Day, with a burn starting at 5:05 p.m. Eastern time.

Both 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 to generate new information about the moon's interior composition and evolution.

If both 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.