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Grail-A and Grail-B  (Source:
The probes will search the moon "from crust to core" in an effort to understand the moon's origins

Two NASA probes sent to orbit the moon last year have finally entered lunar orbit on New Year's Eve and New Year's Day.

The twin probes, named Grail-A and Grail-B (Grail stands for Gravity Recovery And Interior Laboratory), are part of a $496 million mission that launched September 10, 2011. They are two washing machine-sized probes that were sent to orbit the moon in an effort to better understand the moon's origins.

At this point, it is believed that the moon came to be after a Mars-sized object hit the Earth 4.5 billion years ago and the blast sent large materials out into space that eventually came together to create the moon.

There are many aspects of the moon that remain unknown, however. One question that NASA hopes to uncover is how the moon has evolved since that crash millions of years ago. Also, researchers have wondered why the near side of the moon, which is the side we see from Earth, is so different looking from the far side of the moon. The far side is much more mountainous while the near side consists of plains of volcanic rock.

"We don't actually know why the near side and far side of the moon are different," said Maria Zuber, head Grail investigator from the Massachusetts Institute of Technology. "We think that the answer is locked in the interior."

Grail-A and Grail-B were set out to answer such questions, and after launching back in September, the two have finally entered lunar orbit to begin searching for clues. Grail-A entered orbit December 31, 2011 while Grail-B made its way to the moon January 1, 2012.

Now, the probes will spend two months orbiting the moon while gradually moving closer to the moon's surface. They'll eventually orbit 34 miles above the surface, and then begin an 82-day journey to find clues about the moon's past. They will search the moon "from crust to core" while remaining 75 to 225 miles apart depending on the differences in the lunar gravity field.

When the probes return their findings back to NASA, researchers will use the information to create descriptive maps of the moon's gravitational field, which will provide a better understanding of the moon's formation.

Sources:, Fox News

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Apollo Missions.
By drycrust3 on 1/2/2012 4:10:42 PM , Rating: 2
I recall reading somewhere that when the Astronauts had trouble keeping the Command Module in orbit around the moon because of its uneven gravitational field, which is interesting, because one would expect the gravitational field to be more of an average for the entire mass of an object, and not fluctuate a lot, as does actually happen.
This could have an interesting effect on the laws of physics.

RE: Apollo Missions.
By HoosierEngineer5 on 1/2/2012 5:17:34 PM , Rating: 2
If I recall correctly, the gravity field of a solid mass behaves exactly as if all it's mass were located at it's center (or more accurately, the center of gravity). In order to have an 'uneven field' you would need to be below the surface, or the mass would need to be unstable. I suppose this could be a result of tidal effects from the earth. Perhaps this is what they are analyzing?

RE: Apollo Missions.
By tng on 1/3/2012 10:26:57 AM , Rating: 3
If I recall correctly, the gravity field of a solid mass behaves exactly as if all it's mass were located at it's center (or more accurately, the center of gravity).
The moon is thought to have very little iron in it, since it was formed from crust material thrown off 4.5 billion years ago. Most of Earths heavy elements have sunk to the core.

My thought is that there is an uneven distribution of heavier elements due to the way it was formed that causes the gravity variations.

RE: Apollo Missions.
By JediJeb on 1/3/2012 2:05:29 PM , Rating: 2
The center of gravity of an object would be where the averages of its mass is centered. If there is a large dense portion near the surface on one side that would skew the average to near that point. So if the most dense part of the core is off-center relative to the surface then the gravitational field will not be centered around its surface.

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