NASA this week detailed its plans to crash two lunar-orbiting probes into the moon next week. On Monday, December 17 at 5:28 p.m., the probes, named Ebb and Flow, are part the Gravity Recovery and Interior Laboratory (GRAIL) project. They will descend and impact a lunar mountain near the moon’s north pole.

The crash-landing is purposeful, as the probes’ low orbit and low fuel levels make it impossible for them to be of any further use for scientific studies. The probes have successfully completed their primary and extended science missions.

Just last week, NASA revealed a gravity map of the moon that was based on GRAIL data taken by the probes. The map is the highest-resolution gravity field map of any celestial body to date, and will provide researchers with a better understanding of how Earth and other rocky planets formed.

“It is going to be difficult to say goodbye,” said Maria Zuber, GRAIL principal investigator at the Massachusetts Institute of Technology. “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.”

Ebb will be the fist probe to hit the moon’s surface, with Flow following 20 seconds later. The spacecrafts will be travelling at 1.7 kilometers per second (3,760 miles per hour) when they hit the surface. The area of the moon where the probes are expected to impact will be in shadow at the time of impact, so no images of the event are expected.

Before their final flight, the probes will both empty their propellant tanks by firing their main engines. This will show exactly how much fuel each probe has remaining, helping NASA engineers improve computer models that predict fuel consumption for future missions. After the burn, the probes will skim the surface of the moon for several hours until the lunar mountain rises in front of them.

“Our lunar twins may be in the twilight of their operational lives, but one thing is for sure, they are going down swinging,” said David Lehman, GRAIL project manager at NASA’s Jet Propulsion Laboratory. “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.

“Such a unique end-of-mission scenario requires extensive and detailed mission planning and navigation. We’ve had our share of challenges during this mission and always come through in flying colors, but nobody I know around here has ever flown into a moon mountain before. It’ll be a first for us, that’s for sure.”