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How did the Moon form? Is the lunar core still molten? And why is the Moon such an uneven lump?
Over the decades, more than 100 spacecraft have gone to the Moon, “and yet we still don’t know why the front side of the Moon looks different than the back side of the Moon,” said Maria T. Zuber, the mission’s principal investigator.
The side of the Moon that faces Earth is flat and mostly smooth. The other side is mountainous.
“So, to me,” said Dr. Zuber, a professor of geophysics at the Massachusetts Institute of Technology, “if you’ve taken a hundred spacecrafts’ worth of measurements and you still don’t know the answer to something fundamental like that, then it’s likely the answer isn’t on the surface, it’s somewhere else. We’re making a bet here, by really learning the internal structure very well, that we’ll be able to answer those questions.”
This $496 million NASA mission, called the Gravity Recovery and Interior Laboratory, or Grail, will conduct a single measurement: a map of the Moon’s gravitational field. But it will do it with such precision that scientists will get a clear picture of the interior.
Grail consists of two identical spacecraft that were launched side by side on a rocket on Sept. 10. Instead of taking a direct three-day trip to the Moon, they followed a longer, lower-energy trajectory that headed toward the Sun before looping back to cross paths with the Moon.
Grail-A entered an elliptical orbit of the Moon Saturday afternoon. Grail-B is to follow on Sunday at 5:05 p.m., when it will be time for a belated New Year’s party for the mission team of 60 people.
“We’ll be celebrating late Sunday afternoon, hopefully,” said David Lehman, Grail’s project manager. “There is some nonalcoholic Champagne the team has ready.”
Over the next couple of months, a series of shorter engine firings will nudge the spacecraft into circular orbits passing over the Moon’s poles, 34 miles above its surface.
In March, the spacecraft will begin collecting the gravity data. Variations in density — for example, a mountain range or a clump of heavier metals below the surface — change the Moon’s gravitational pull and create wobbles in its orbits, and the gravity map will be calculated from the changes in distance and speed between the two spacecraft.
For the near side, the data will be 100 times better than what scientists have now, Dr. Zuber said. For the far side, the improvement will be a factor of 1,000. That precision will allow the scientists to test, for example, predictions of a hypothesis that the Moon is actually an amalgamation of two moons that collided long ago, with the remnants of the smaller moon forming the far-side highlands.
Just as the Moon’s gravity creates the rise and fall of ocean tides on Earth, so does the Earth’s gravity slightly deform the shape of the Moon. Measurement of this slight deformation will tell whether the Moon’s core has cooled and turned solid. If it has, the deformation will be greatest when the Moon is closest to the Earth, the scientists said. (The Moon’s orbit is almost, but not exactly, circular.)
But if part of the core is still molten, as is currently thought, the sloshing will delay the deformation to slightly after the closest approach. The effect is so tiny that the scientists will first have to account for effects like the pressure of light from the Sun pushing on the Moon.
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