NASA's twin GRAIL spacecraft will be placed into orbit around the moon over the New Year weekend, in preparation for their mission to study its composition and gravity field.
GRAIL-A will be placed into orbit at 1:21 pm PST on December 31, with GRAIL-B following the next day at 2:05 pm PST.
Both orbiters will approach the moon from the south, flying near the lunar south pole. A roughly 40-minute insertion burn will place each orbiter into a near-polar, elliptical orbit with a period of 11.5 hours.
"Our team may not get to partake in a traditional New Year's celebration, but I expect seeing our two spacecraft safely in lunar orbit should give us all the excitement and feeling of euphoria anyone in this line of work would ever need," says David Lehman, project manager for GRAIL at NASA's Jet Propulsion Laboratory (JPL).
Over the next few weeks, a series of burns will cut this orbital period down to just under two hours.
"This mission will rewrite the textbooks on the evolution of the moon," says Maria Zuber, GRAIL principal investigator from MIT.
"Our two spacecraft are operating so well during their journey that we have performed a full test of our science instrument and confirmed the performance required to meet our science objectives."
The probes have taken the long way round to reach the moon. Rather than the three-day trip the Apollo crews experienced, the GRAIL ships have taken some three months and covered more than 2.5 million miles.
When the science phase of the mission begins in March 2012, the two GRAILs will be in a near-polar, near-circular orbit about 34 miles above the surface.
The two spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. As they fly over areas of greater and lesser gravity, caused both by visible features such as mountains and craters and by masses hidden beneath the lunar surface, they will move slightly toward and away from each other.
An instrument aboard each spacecraft will measure the changes in their relative velocity very precisely, and the data will be used to produce a high-resolution map of the moon's gravitational field.