NASA plans to crash a pair of small robotic science probes into the moon next week.
The Gravity Recovery and Interior Laboratory spacecraft will plunge on Monday into a mountain near the north pole on the moon.
The impacts, which are not expected to be visible from Earth, will take place about 20 seconds apart at 5.28pm EST on Monday.
The probes made a final steering maneuver on Friday and shut down their science instruments in preparation for the crash.
The two spacecraft, each about the size of a small washing machine, have been flying in close formation around the moon 11km above its surface for nearly a year to map lunar gravity.
Scientists precisely measure the distance between the two, a figure that slightly changes as they fly over denser regions of the moon. Gravitational pull causes first the leading probe and then the following one to speed up, altering the gap between them.
NASA said on Thursday that the spacecraft will hit the surface at about 6120kph. No pictures are expected because the region will be dark at the time of impact, but another spacecraft circling the moonwill attempt to survey the crash site.