Two spacecraft orbiting the moon are ending their mission by crashing into a lunar mountain at almost 4,000mph.
The twin Gravity Recovery and Interior Laboratory (GRAIL) probes, known as Ebb and Flow, have spent almost a year mapping the moon's gravity in unprecedented detail.
With their fuel running low, US space agency NASA took the decision to stage a controlled descent and impact.
The alternative would have been to let the probes crash randomly which may have damaged places of historic interest, such as the Apollo landing sites.
The spacecraft will be flown into a mountain on the edge of a crater near the moon's north pole.
Both will hit the surface at 10.28pm UK time at a speed of 3,760mph.
There will be no spectacular images of the impact because the region will be in shadow.
Grail principal investigator Maria Zuber, from Massachusetts Institute of Technology in Cambridge, US, said: "It is going to be difficult to say goodbye.
"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."
In one final experiment, the probes will fire their main engines until they have run out of fuel to determine the precise amount of propellant left in their tanks.
This will help NASA engineers come up with more accurate predictions of the fuel needs of future missions.