The 667-pound (303-kg) craft of the U.S. space agency's Gravity Recovery and Interior Laboratory, or GRAIL, mission have been flying to the moon since their joint launch in September.
GRAIL A is due to begin a 40-minute braking maneuver to put itself into orbit around the moon at 4:21 p.m. ET on Saturday, with GRAIL B following suit 25 hours later. Both are needed for the intricate gravity-mapping mission scheduled to begin in March.
"We won't be celebrating a lot until after we get GRAIL B into orbit late afternoon on New Year's Day," said project manager David Lehman with NASA's Jet Propulsion Laboratory in Pasadena, California.