A pair of NASA spacecraft are successfully circling the Moon after journeying through space for more than three months.

The Grail-B probe entered lunar orbit Jan. 1 after firing its main engine at 5:05 p.m. EST in a 40-minute-long orbital insertion burn, NASA officials said. It joined its twin spacecraft, Grail-A, which arrived at the Moon Dec. 31 after completing a similar maneuver.

The two probes are on a mission to study Earth’s nearest neighbor from crust to core. After gradually circling down to low-altitude orbits, the pair will circle the Moon in tandem, working together to map the lunar gravity field in unprecedented detail, researchers said.

Scientists expect the probes’ measurements to help unlock some longstanding mysteries about the Moon’s composition and evolution — mysteries that have withstood more than 100 Moon missions over the years.

“You might think that, given all of these observations, we would know what there is to know about the Moon,” Grail principal investigator Maria Zuber of the Massachusetts Institute of Technology told reporters Dec. 28. “Of course that’s not the case.”

Scientists think the Moon formed after a Mars-size body struck Earth about 4.5 billion years ago. This titanic impact blasted huge amounts of material into space, and they eventually coalesced.

While this basic outline of the Moon’s origin is pretty well established, many mysteries remain about how the rocky body has evolved since then.

Chief among these puzzles, perhaps, is why the near side of the Moon — the face always visible from Earth — is so different from the far side. For example, plains of volcanic rock (called maria) are much more widespread on the near side, and the far side is higher and more mountainous, with the lunar surface 1.9 kilometers higher in elevation, on average, than the near side.

“We don’t actually know why the near side and far side of the Moon are different,” Zuber said. “We think that the answer is locked in the interior.”

One possible explanation, proposed by researchers this August, is that the gigantic collision 4.5 billion years ago actually created two Moons. The second moon, which was much smaller, later slammed into our Moon’s far side, the idea goes, spreading itself over the surface rather than creating a crater.

The Grail probes’ observations could help determine whether that scenario is accurate, researchers said.

“We believe that we can obtain the information that we need to test this hypothesis,” Zuber said.

The $496 million Grail (short for Gravity Recovery and Interior Laboratory) mission launched Sept. 10 aboard a United Launch Alliance Delta 2 rocket. The two washing-machine-size probes then spent more than three months making their slow, circuitous way to the Moon.

By comparison, NASA’s manned Apollo 11 mission in 1969 reached the Moon in three days. But Apollo 11 prioritized speed, while Grail’s path is energy-efficient and has given engineers ample time to assess the probes’ health and charge up their scientific gear, researchers said.

Grail-A and Grail-B still have some work to do from their initial lunar orbits to get into position. They will spend two months circling lower and lower, eventually settling into orbits just 55 kilometers above the lunar surface by March.

Only then will the twin spacecraft begin their science campaign. They will chase each other around the Moon for 82 days, staying 121 to 362 kilometers apart.

Regional differences in the lunar gravity field will cause the two spacecraft to speed up or slow down slightly, changing the distance between them as they fly. Bouncing microwave signals back and forth off each other, Grail-A and Grail-B will gauge these tiny distance variations constantly.



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