Named the Gravity Recovery And Interior Laboratory, the spacecraft are scheduled to enter their orbits beginning at 4:21 p.m. EST for GRAIL-A Dec. 31, and 5:05 p.m. EST for GRAIL-B the next day, NASA said in a release Thursday.
"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," David Lehman, project manager for GRAIL at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said.
From their orbits around the moon, the spacecraft will transmit radio signals precisely defining the distance between them 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.
The data will be translated into a high-resolution map of the moon's gravitational field.
"This mission will rewrite the textbooks on the evolution of the moon," Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology, said.
Researchers say the mission will increase knowledge of how Earth and its rocky neighbors in the inner solar system developed.