NASA's Twin Lunar-Orbiting Probe to Crash into Moon Dec.17

NASA's twin spacecraft that orbited the moon for nearly a year will crash into the moon's surface at about 5:28 p.m. EST (2:28 p.m. PST) Dec. 17, mission officials announced Thursday.

The twin probes, named Ebb and Flow, were circling the moon as part of the Gravity Recovery and Interior Laboratory (GRAIL) mission since Jan. 1, 2012, to learn the internal structure and composition of the lunar surface. The spacecraft are sent crashing into the lunar surface purposefully, as the probes have low fuel levels and low orbit, preventing them from performing further scientific operations.

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

The twin probes will make contact with a mountain located near a crater named Goldschmidt on the moon. They will hit the surface at 3,760 mph (1.7 kilometers per second). Ebb, the first probe which reached the moon, will be the first one to crash down at 5:28 p.m. EST, whereas Flow will hit the surface 20 seconds later.

Before the end of their mission, both the probes will conduct a final experiment. They will fire their main engines until their propellant tanks are empty, in order to estimate the amount of fuel remaining in their tanks. This will help NASA scientists make a better prediction 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 NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Even during the last half of their last orbit, we are going to do an engineering experiment that could help future missions operate more efficiently."

On Friday morning (Dec. 14), scientific instruments of the probes will be turned off and they will make their final journey. The depletion burn designed by NASA engineers will allow the probes to descend gradually for several hours and glide along the surface of the moon until they hit the mountain's terrain Dec. 17. 

No images of the probes' final movements will be available, as the region will be in shadow at that time. But the Lunar Reconnaissance Orbiter will later try to locate the crash site, The Associated Press reported.