The Gravity Recovery and Interior Laboratory-- or "GRAIL"-- probes, Ebb and Flow, will crash into a mountain on the moon Monday afternoon.
It will end a fruitful mission to study the surface and composition of the moon.
NASA is crashing the probes, about the size of an apartment washer and dryer, because they're running out of fuel.
It's all according to plan.
NASA says thanks to GRAIL, scientists now have the highest-resolution gravity field map of any celestial body.
And the probes have given scientists unprecedented insight into what's below the surface and how the moon may have formed.
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