Twin NASA Spacecraft to Orbit Moon to Start “Full-Body Scan” This Weekend


NASA's twin Grail spacecraft (short for Gravity Recovery And Interior Laboratory) are slated to start circling the moon one day apart, with Grail-A arriving on Saturday and Grail-B following on Sunday –both to study our lunar satellite from crust to core. The two probes will orbit the moon in tandem, mapping the lunar gravity field in unprecedented detail and helping scientists better understand how the moon formed and evolved.

"This mission will rewrite the textbooks on the evolution of the moon," Grail principal investigator Maria Zuber, of the Massachusetts Institute of Technology, said in a statement. "I call it flying over the treetops of the moon," Zuber added. "We will take the spacecraft down as low as we can, to map, have a sensitivity to the shallowest, shallowest structures of the lunar crust."

Grail-A and Grail-B will spend two months circling lower and lower, eventually settling into orbits just 34 miles (55 kilometers) above the lunar surface, and begin taking measurements in March.

Regional differences in the moon's gravitational field will cause the two spacecraft to constantly adjust their speed –slowing down slightly, changing the distance between them, using microwave signals that they bounce back and forth to be able to determine how far apart they are from each other to within a few microns — less than the width of a human red blood cell, researchers have said.

The Grail team will use the twin probes' measurements to construct highly detailed maps of the lunar gravity field. These maps should help scientists plan out future lunar landings, of both robotic and manned spacecraft, 
"Grail is a journey to the center of the moon," Zuber told reporters Wednesday. The two probes' measurements, along with data collected by other spacecraft, she added, "will enable us to reconstruct the moon's early evolution."

Grail's primary science work should wrap up in June. But the Grail team hopes NASA grants the two spacecraft a mission extension through next December, Zuber said. She and her colleagues want to take Grail even lower, to just 15 miles (25 kilometers) or so above the lunar surface.

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