Going out with a bang, two small NASA probes that flew in formation to precisely map the moon's gravity field crashed into a mile-high mountainside Monday, slamming into the lunar surface at more than a mile per second to bring a successful $500 million mission to an abrupt end.
The "targeted impacts" were intended to eliminate even a slight chance that one of the satellites might one day fall to the surface at or near a so-called "lunar heritage site," including six where manned Apollo missions landed and more than a dozen where unmanned U.S. and Russian probes touched down.
Named Ebb and Flow in a student naming contest, the two 450-pound solar-powered satellites, built by Lockheed Martin, had been flying in formation at extremely low altitudes since Jan. 1, mapping slight differences in the moon's gravitational field to gain insights into its internal structure.
Each spacecraft also carried cameras used by middle school students to photograph the lunar surface in a project sponsored by Sally Ride Science, a science education company founded by the late shuttle astronaut.
But with their fuel nearly exhausted and the mission's scientific observations complete, mission managers opted to burn the last of their propellant to target impacts on the side a mountain-like section of a partially buried crater rim near the moon's north pole.
Ebb crashed into the rim at 5:28 p.m. EST Monday, followed 32 seconds later by Flow, which hit about 1.9 miles away. The impacts were confirmed by the sudden loss of radio contact.
Flight controllers and managers at the Jet Propulsion Laboratory in Pasadena, Calif., applauded and shook hands to mark the end of the Gravity Recovery and Interior Laboratory -- GRAIL -- mission.
The spacecraft hit the moon in darkness and even though they were moving at some 3,800 mph, mission managers did not expect any observers on Earth to see a flash or any other direct evidence of impact.
But NASA's Lunar Reconnaissance Orbiter will be on the lookout for any signs of fresh craters during subsequent passes over the region. Given their relatively small size, any such craters or depressions would be expected to be just a few yards or so across.
"NASA has approved the GRAIL team's request to name the final resting place of Ebb and Flow after our teammate, Sally Ride," said Maria Zuber, GRAIL principal investigator at the Massachusetts Institute of Technology. "Sally was a visionary, the head of our MoonKAM investigation, and the team very much wanted to honor her contributions to education by naming the impact sites after her."
Ride's sister, Rev. Bear Ride, said she was "deeply appreciative."
"Speaking for the whole family, we're so grateful to Maria and the team for continuing this dream and making it such a complete success," she said.
Launched Sept. 10, 2011, Ebb and Flow reached the moon at the end of the year with the second spacecraft slipping into orbit on New Year's Day.
Flying in close formation at lower and lower altitudes, the spacecraft used timed radio signals to precisely measure the distance between them. Depending on the separation, the ranging system could detect changes as small as one micron, or the width of a red blood cell.
As they sailed over buried mass concentrations, craters, mountain ranges, basins and other geologic features, the satellites' velocity and separation changed ever so slightly.
By carefully analyzing those changes, scientists have mapped out the moon's gravity field in unprecedented detail. Among the early results is the discovery that the moon's crust is much thinner and much more fractured than previously believed.
Details about the structure of the moon's interior are expected after data analysis is complete.
"In terms of the scientific measurements, we have achieved everything we could have possibly hoped for," Zuber said last week. "Frankly, in my wildest dreams, I could not have imagined that this mission would have gone any better than it has."