They've been dutifully circling the moon all this year, but the craft that make up NASA's GRAIL mission end with a flash today. Ebb and Flow, twin spacecraft the size of a washer and dryer, have been gathering data about gravity for a high-resolution map of the inside of the celestial body. The plan is for the robotic twins to hit the edge of a lunar crater and disintegrate as a separate orbiter records any data kicked up by the impact.