NASA Launches Pair of Moon-Mapping Spacecraft
Known as Gravity Recovery and Interior Laboratory, or GRAIL, the spacecraft will circle the moon to measure its gravity field. The mission is expected to also widen understanding of the moon's entire structure, cooling and heating, history of collisions with asteroids, as well as determine desirable landing sites.
GRAIL-A and GRAIL-B, both due to reach the moon by January, will reshape and merge their orbits until one spacecraft is following the other. The science collection phase is expected to last 82 days.