NASA is kicking off the new year by getting twin spacecraft into lunar orbit this weekend.

GRAIL, which stands for Gravity Recovery And Interior Laboratory, is a set of two orbiters that launched on September 10. GRAIL-A will get to its destination of circling the moon on the afternoon of December 31, with GRAIL-B following the next day at 5:05 p.m. ET.

They will arrive about 25 hours apart, giving breathing room in between these important milestones, says GRAIL project manager David Lehman.

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