Twin spacecraft that have been orbiting moon will hit surface together GRAIL mission probes ebb and flow in artists's impression. PASADENA, Calif., Dec. 14 (UPI) -- Twin lunar-orbiting spacecraft that have gathered data about the moon's internal structure and composition will be crashed into the lunar surface, NASA says. Because their low orbit and low fuel levels preclude further scientific operations, the twin Gravity Recovery and Interior Laboratory GRAIL mission probes named Ebb and Flow will make a controlled descent and impact near the moon's north pole about 5:28 EST Monday, the space agency reported. "It is going to be difficult to say goodbye," GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology said. Our little robotic twins have been exemplary members of the GRAIL family, and planetary science has advanced in a major way because of their contributions. The spacecraft, named by elementary school students in Bozeman, Mont., who won a NASA contest, have been orbiting the moon in close formation since Jan. 1. During their prime mission from March through May, the twin probes gathered data while orbiting at an average altitude of 34 miles. They were then lowered to 14 miles in August for an extended mission. Both spacecraft will hit the lunar surface at 3,760 mph in a region of the moon that will be in shadow, so no imagery of the impact is expected, NASA's Jet Propulsion Laboratory in Pasadena, Calif., said.