Twin NASA probes to crash into the Moon

NASA's twin lunar-orbiting spacecraft, that have allowed scientists to learn more about the internal structure and composition of the Moon, will end their gravity-mapping mission in a spectacular way next week.

Ebb and Flow, the Gravity Recovery and Interior Laboratory (GRAIL) mission probes, are being sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations.

The duo's successful prime and extended science missions generated the highest resolution gravity field map of any celestial body. The map will provide a better understanding of how Earth and other rocky planets in the solar system formed

and evolved, NASA said in a statement.

"It is going to be difficult to say goodbye," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge.

"Our little robotic twins have been exemplary members of the GRAIL family, and planetary science has advanced in a major way because of their contributions," Zuber said.

The mountain where the two spacecraft will make contact is located near a crater named Goldschmidt. Both spacecraft have been flying in formation around the Moon since January, 2012.

The first probe to reach the Moon, Ebb, also will be the first to go down. Flow will follow Ebb about 20 seconds later.

Both spacecraft will hit the surface at 1.7 kilometers per second. Ebb and Flow will conduct one final experiment before their mission ends. They will fire their main engines until their propellant tanks are empty to determine precisely the amount of fuel remaining in their tanks.

This will help NASA engineers validate fuel consumption computer models to improve predictions of fuel needs for future missions.

"Our lunar twins may be in the twilight of their operational lives, but one thing is for sure, they are going down swinging," said GRAIL project manager David Lehman of

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