Stories about space exploration have strong beginnings: the suspense of launch, the big plans for exploration. Those are followed by substantive plot twists, powered by the often-unexpected data that missions send back, the new insights they provide, and questions they raise. But stories about space exploration generally don’t emphasize the ending. So what happens when the datastream dies? Some missions, such as Voyager, actually keep collecting data and journey onward toward interstellar space years longer than anyone expected. But even the most persistent robotic explorers, such as the Mars rover Spirit, eventually rest in peace.
A week ago, when the first results from GRAIL, a lunar gravity-mapping mission, were made public, the MIT scientist who heads the mission hinted that the end was near for the twin spacecraft that have been flying in tight formation around the moon, taking their primary data this spring.
“We’re still working final details,” said geoscientist Maria Zuber. “For most orbital missions, what’s usually done is after regular mapping is they are brought up to higher orbit to do longer-term observations where orbit is stable. On this particular mission, we took the opposite strategy. We wanted to make the highest-resolution global gravity map of any planet, including Earth.”
That meant the pair of washing machine-sized spacecraft, named Ebb and Flow, came closer and closer to the moon’s surface. Now, the two spacecraft are navigating as low as a commerical airliner at times, providing new challenges and experience in low-altitude navigation for scientists.
That also makes a crash-landing their only possible fate. On Thursday, NASA announced they would crash-land about 5:28 p.m. on Monday. There will be no images of their final moments, because the region where they will strike—near a crater named Goldschmidt—will be in shadow at the time. Flow will hit first, at about 3,760 miles per hour, followed by Ebb about 20 seconds afterward.
The data will live on; Zuber said her team will be analyzing their output for years to come. The first haul of measurements taken by GRAIL during its primary mission gave scientists the most detailed gravity map made of any planetary body, and gave them clues about the bombardment that the early solar system experienced. The data showed that the moon’s crust was more fractured than anyone expected.
“It’s going to be difficult to say goodbye,” Zuber said in a press release from NASA, to “our little robotic twins.”