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updated 12/29/2011 4:48:24 PM ET 2011-12-29T21:48:24

Two identical NASA space probes are due to arrive at the moon this weekend to learn what is inside Earth's companion and how it formed.

Among the most interesting questions scientists will attempt to answer is if our moon holds the wrecked body of a lost sibling body.

Evidence of the crash, if it occurred, should be buried inside the moon, in the form of remnant radioactive materials, like uranium and thorium, which would have been heated in the smash-up.

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According to a recently published paper, scientists suspect a second moon once circled Earth in the same orbit and at roughly the same speed as our moon. It eventually bumped into its companion, but instead of causing an impact crater, the second moon stuck and made a mountain. That feature today would be the lunar highlands located on the side of the moon that permanently faces away from Earth.

"One prediction of this model is that the whole exterior of the moon was once molten, and it started to cool off -- actually cooled from the outside in -- so you were left with a molten channel in the base of the moon's crust," said planetary scientist Maria Zuber, with the Massachusetts Institute of Technology.

Simulations show that when the second moon hit our moon the molten material was pushed around to the near side, traces of which should remain today.

"We're looking for layering in the lower crust," said Zuber, who is the lead researcher on NASA's Gravity Gravity Recovery and Interior Laboratory, or GRAIL, mission.

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Flying in formation 34 miles above the lunar surface, the two GRAIL spacecraft will map the moon's gravity down to fractions of a micron. A micron is about the width of a red blood cell.


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Scientists can use the precise measurements to model the moon's interior, a key piece of data missing despite more than 100 previous missions to the moon, including six human excursions by NASA astronauts under the 1969-1972 Apollo program.

"We believe the moon formed from the impact of a Mars-sized object with Earth, but we understand little really of how this formation happened and how it cooled off after the violent event," Zuber said.

"One fundamental thing that we don't know about the moon -- shockingly after all these missions that have gone to the moon -- is why the near side of the moon is different than the far side," she added.

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Grail A is due to begin a 40-minute braking maneuver to put itself into orbit around the moon at 4:21 p.m. EST on Saturday. Grail B arrives 25 hours later on New Year's Day.

Both spacecraft are needed to complete the mapping mission, which is scheduled to last 82 days.

"We won't be celebrating a lot until after we get Grail B into orbit," said project manager David Lehman, with NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

It will take two months for the spacecraft to move themselves from their initial lunar orbits into position to begin mapping. If the battery-powered probes survive the next lunar eclipse, scientists want to move them even closer to the moon for a follow-on mission that would provide even more precise measurements.

© 2012 Discovery Channel

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