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NASA’s twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft are on track to enter lunar orbit. GRAIL-A is scheduled to be placed in orbit at 1:21 p.m. PST on Dec. 31 and GRAIL-B will enter orbit at 2:05 p.m. PST on Jan. 1. "This mission will rewrite the textbooks on the evolution of the Moon," said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology. "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." When data collection begins, instruments aboard each spacecraft will measure changes in their relative velocity. Scientists will translate this information into a high-resolution map of the Moon's gravitational field, allowing mission scientists to understand what goes on below the surface of the Moon and increasing our knowledge of how Earth and other rocky planets developed.

SOURCE: NASA Press Release