Two spacecraft are due to begin orbiting the Moon on New Year's Eve and New Year's Day.  The mission will enable scientists to better understand the Moon's gravitational field and the lunar interior.  

NASA scientists say our moon has the most uneven gravitational field they know of in the solar system.  And scientists want to learn more about the field because it will yield clues about what is going on beneath the lunar surface.  

David Lehman of NASA's Jet Propulsion Laboratory is the project manager for GRAIL, the U.S. space agency's Gravity Recovery And Interior Laboratory mission. "The purpose of the GRAIL mission is to obtain gravity data on the Moon.  And with that data, the scientists are able to determine the structure of the lunar interior, from crust to core," he said.

The twin GRAIL spacecraft were launched from Florida in September.  At that time, Ed Weiler of NASA's Science Mission Directorate described the nearly half-a-billion dollar mission. "GRAIL, simply put, is a journey to the center of the Moon.  It will probe the interior of the Moon and map its gravity field 100 to 1,000 times better than ever before," he said.

Weiler explained that the two satellites, named "GRAIL A" and GRAIL B," will essentially chase each other around the moon and use radio signals to determine the distance between them. "As the first satellite goes over a higher mass concentration or higher gravity, it will speed up slightly, and that will increase the distance.  And then as the second satellite goes over, that distance will close again," he said.

In this tandem formation, the spacecraft will spend three months mapping the Moon's gravitational field.  

NASA's David Lehman says the insights gained from the GRAIL mission will help scientists better understand how rocky planets, including our own, were formed.  

"Five billion years ago, the Earth was formed, and all the other terrestrial planets were formed, and the Moon was formed.  But since that time, the Earth evolved.  There're earthquakes, there's erosion, there's lots of rain.  And so, when we study the Earth now, we really don't have good feeling for what happened because it changed so much," he said.

Plus, scientists say the new gravitational map of the Moon will be an important navigational tool for future lunar spacecraft.  

"GRAIL A" will enter lunar orbit on New Year's Eve and "GRAIL B" will do so on New Year's Day.