As New Year's Eve revelers toasted 2012 at various parties throughout the world last night, one of two NASA probes was entering lunar orbit high above them.

The first Gravity Recovery And Interior Laboratory spacecraft (GRAIL-A) entered the Moon's orbit at 5pm Eastern on Saturday; by 6pm GRAIL-A was in an orbit 56 miles by 5,197 miles around the moon, which takes about 11.5 hours to complete, NASA said.

"My resolution for the new year is to unlock lunar mysteries and understand how the moon, Earth and other rocky planets evolved," Maria Zuber, GRAIL principal investigator at the Massachusetts Institute of Technology in Cambridge, said in a statement. "Now, with GRAIL-A successfully placed in orbit around the moon, we are one step closer to achieving that goal."

The second probe, GRAIL-B, is scheduled to do the same at 5:05pm Eastern time on Sunday. As of 6pm last night, GRAIL-B was 30,018 miles from the moon, closing in at 896 mph.

"With GRAIL-A in lunar orbit we are halfway home," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory. "[Sunday] may be New Year's everywhere else, but it's another work day around the moon and here at JPL for the GRAIL team."

Once in orbit, the twin probes will spend the next few weeks moving closer to the Moon, reducing the orbit time from 11.5 hours to about two hours. By March 2012, when the 82-day science phase of the mission begins, both GRAILs "will be in a near-polar, near-circular orbit with an altitude of about 34 miles," NASA said.

Technology aboard both spacecraft will measure changes in relative velocity, which scientists will use to create a high-resolution map of the Moon's gravitational surface.

"This information will increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today," NASA said.

The GRAIL mission to the Moon launched successfully on Sept. 10 from Florida's Cape Canaveral. Both GRAIL spacecraft are about the size of a washing machine, NASA said, and include a Lunar Gravity Ranging System that can measure the distance between them down to a few microns. They also house cameras for the MoonKAM, the first time a NASA planetary mission has included instruments specifically for the purpose of providing the public with a glimpse of its activities.

The Apollo mission to the Moon took about three days, but GRAIL is traveling via a low-energy, long-duration trajectory in order to give "mission planners and controllers more time to assess the spacecraft's health," NASA said. "The path also allowed a vital component of the spacecraft's single science instrument, the Ultra Stable Oscillator, to be continuously powered for several months. That allowed it to reach a stable operating temperature long before science measurements from lunar orbit are to begin."

The mission will place two spacecraft into the same orbit around the Moon, which will be used to determine the structure of the lunar interior from crust to core and advance our understanding of the thermal evolution of the moon.

From now until Aug. 12, the Natural History in Manhattan has a special exhibit on the future of space flight, including what it might take to colonize the Moon and Mars. For more, see the slideshow below.

Update: GRAIL-B achieved lunar orbit at 5:43pm Eastern on Sunday.

For more from Chloe, follow her on Twitter @ChloeAlbanesius.
