NASA will crash two moon probes Monday

Friday, December 14, 2012 - 1:00am

A pair of robotic twins that have been diligently mapping the moon this year will go out with a bang Monday.

The Gravity Recovery and Interior Laboratory (GRAIL) probes Ebb and Flow will crash into a mountain on the moon Monday afternoon, ending a fruitful mission to study the surface and composition of the moon.

"Scientifically we are learning a great deal about not only the moon but about the early evolution of terrestrial planets," said principal investigator Maria Zuber of the Massachusetts Institute of Technology at a press conference Thursday.

Thanks to GRAIL, scientists now have the highest-resolution gravity field map of any celestial body, NASA said. That means the probes have been making a high-quality map of the gravitational field of the moon, which give scientists unprecedented insight into what's below the surface and how the moon may have formed.

Results from this mission have delivered several other important findings, Zuber said. These include:

--Evidence that the crust of the moon is much thinner than scientists believed.

--Some of the large-impact basins dug into the moon's mantle, which is useful because scientists want to understand the moon's composition. It is believed that the composition of the Earth's mantle is similar to that of the moon.

--The moon's crust is much more fractured than scientists thought.

--It appears the crusts of planets have been bombarded in a similar way, including Mars. If there were water on Mars, as scientists believe there may have once been, where would it have gone? GRAIL, looking at the moon, provides a clue:

"These fractures provide a pathway deep inside the planet, and it's very easy to envision now how a possible ocean at the surface could have found its way deep into the crust of a planet," Zuber said.

Another major finding: Identifying large, lava-filled cracks (dykes) which are hundreds of miles long and exist about 6 miles below the surface. There's no record of them at the surface of the moon.

"These dykes actually provide evidence for early expansion of the moon shortly after it formed," Zuber said. "This had been predicted by models, but no evidence had been found."

The aluminum content in the moon's crust is similar to what's found on Earth, which supports the theory that the moon was born when a Mars-sized body slammed into Earth. That body and Earth mixed so well that material thrown off into the moon is indistinguishable from Earth material.

A side benefit of the GRAIL mission, Zuber said, is that scientists have improved their knowledge of the latitude-longitude grid of the moon.

"If there is a place where a future robotic or human mission wanted to land, you would be able to do that extremely precisely," Zuber said.

The spacecraft have performed "flawlessly," Zuber said, and it's all according to plan that they are running low on fuel.

After Friday, the science instruments on board will turn off. The probes are about the size of a washer and dryer that would be found in an apartment, scientists said.

The Ebb probe will crash at 2:28:40 p.m. PST Monday, followed by Flow about 20 seconds later. These impacts will happen in the dark.

"We are not expecting a big flash or a big explosion," Zuber said, adding that the impacts will likely not be visible from Earth.

Data collected by GRAIL between March and May has already been released; the rest will become available at the end of May 2013, scientists said.

Scientists took care to not crash the probes into historic sites where humans have previously landed probes on the moon. Right now, the likelihood of hitting one of those sites is eight in 1,000,000, said GRAIL project manager David Lehman of NASA's Jet Propulsion Laboratory.

As for exactly what will happen to the successful twins on Monday, Lehman describes it like this:

"It's going to make a crater on the moon, and they're going to be completely blown apart at the time."


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