Last December's intentional crash landing of NASA's twin GRAIL spacecraft, Ebb and Flow, was captured by an ultraviolet spectrometer aboard the Lunar Reconnaissance Orbiter. An onboard camera later revealed a pair of tiny impact craters.
On the afternoon of December 17, 2012, scientists and engineers gathered around computer monitors as a pair of identical NASA spacecraft, named Ebb and Flow, crashed into the southern face of a tall, unnamed mountain on the Moon's near side. When the craft hit the lunar surface, 30 seconds apart, cheers broke out in the control room.
And why not? The twin Gravity Recovery And Interior Laboratory (GRAIL) craft had been spectacularly successful in mapping the lumpy concentrations of mass hidden in and under the lunar topography below. To accomplish that task, the craft had flown in formation dangerously close to the lunar surface — at an average altitude of 35 miles (55 km) for the first few months, then only 15 miles (23 km) high during a six-month mission extension, and ultimately just 7 miles (11 km) up in their final month of operation.
By then the craft were doomed to strike the lunar surface. They slammed into the mountain's lower slopes in darkness, at about 4,000 miles per hour (1.7 km per second). Days later, NASA announced that the collision sites would be named to honor the late astronaut Sally Ride, who'd been a member of the GRAIL team.
When their demise inevitably came, NASA's Lunar Reconnaissance Orbiter was watching closely. Each craft's impact raised a localized cloud of debris that eventually rose some 25 miles (40 km) into sunlight. That's when LRO's ultraviolet spectrometer, known as Lyman Alpha Mapping Project (LAMP), detected distinct emissions from atoms of hydrogen and mercury. LAMP recorded two short-lived ultraviolet glows that peaked about 20 seconds apart, notes investigator Kurt Retherford (Southwest Research Institute), "very consistent with what we expected to see."
After the lunar dust settled and the mountain could be viewed in sunlight, mission scientists trained the spacecraft's powerful LROC camera on the area where Ebb and Flow struck. Then principal investigator Mark Robinson (Arizona State University) set up his computer to "blink" LROC frames taken before and after the impacts. It took a few days, but he eventually spotted two tiny smudges about 7,200 feet (2.2 km) apart. "I was skeptical that we'd even find these craters," says Robinson.
The GRAIL crash sites are surprising in two ways. First, the two tiny craters, just 15 feet (3 m) across, are round — but they should have an oval shape, given that the spacecraft came in at an angle of just 3½°. "I was expecting skid tracks myself," quips Maria Zuber, the MIT geophysicist who led the GRAIL science team. Also, the lopsided splashes outside the craters appear dark, not bright like other fresh lunar impacts. "It's something of a mystery," Robinson said during today's science update. Perhaps the dark stains resulted from a bit of residual fuel or from the craft's carbon-fiber structure.
In any case, we haven't heard the last word on the GRAIL results. As Zuber explains, once NASA's supercomputers finish chewing on all that tracking data, the resulting map should reveal variations in lunar gravity spaced only 3 miles (5 km) apart. "It's really an extraordinary thing that gravity can be resolved at this scale," she notes. Moreover, her team has yet to delve into what the GRAIL data have to say about the state of the deep lunar interior — especially its core.
"It's not every day that you applaud when a spacecraft impacts a planet," Zuber says, "but in this case it was actually quite appropriate."
I detailed many of GRAIL's initial findings three months ago, and today a special session at the annual Lunar and Planetary Science Conference highlighted more recent results. For example, GRAIL data suggest there might be only about half as many large impact basins (at least 300 km across) as had been previously suspected.