It has been described as a cosmic ballet -- two spaceships in a delicate, silent dance 230,000 miles from Earth, correcting their course in tandem with air thrusts softer than a human breath, their instruments so fine-tuned they can detect a shift in gravity that pulls them no farther than the width of a strand of hair.
As soon as Thursday, NASA expects to launch its Gravity Recovery and Interior Laboratory, or GRAIL, from Cape Canaveral, Fla. Shortly after launch, two spacecraft will peel away from NASA's rocket for a meandering journey to the moon. GRAIL-A and GRAIL-B are scheduled to arrive on New Year's Eve and New Year's Day, respectively, then spend three months making 12 polar orbits of the moon each day.