Nasa's Ebb and Flow gravity mapping satellites are being commanded to crash into the surface of the Moon later.
The duo will use their remaining fuel supplies to put themselves on a course to collide with a 2km-high mountain in the far lunar north.
The deliberate ditching will avoid the possibility of an uncontrolled descent on to locations of historic importance, such as the Apollo landing sites.
Ebb and Flow have, by common consent, returned some remarkable data.
Their maps of the subtle variations in gravity across the Moon's surface are expected to resolve many outstanding questions in lunar science, such as how precisely the object formed.
"We have achieved everything that we could have possibly hoped for in terms of observations," said principal investigator Prof Maria Zuber from the Massachusetts Institute of Technology, US.
"Frankly, in my wildest dreams, I could not have imagined that this mission would have gone any better than it has.
"But when you orbit at very low altitudes above a body that has a very bumpy gravity field, you use a lot of fuel and so the mission is going to come to an end."
The satellites - together known as Grail (Gravity Recovery and Internal Laboratory) - are timed to hit the flank of the lunar-nearside mountain at about 22:28 GMT.
The peak, located at 75 degrees North latitude near a crater named Goldschmidt, will be in darkness at the time.
Ebb and Flow should hit within roughly 20 seconds of each other, digging out small craters.
However, being only the size of washer-driers, and having completely depleted their fuel tanks, it is not expected the pair will produce any sort of impact flash that is visible from Earth.
That said, another of Nasa's missions at the Moon, its Lunar Reconnaissance Orbiter (LRO), will be looking out for the crashes.
If it gets lucky, LRO's ultraviolet imager might see some volatile materials being driven off the surface by the heat from the impacts.
The Grail mission has produced the highest resolution, highest quality global gravity maps for any planetary body in the Solar System, including Earth.
The gravity differences the satellites have measured are the result of an uneven distribution of mass across the Moon.
Obvious examples at the surface include big mountain ranges or deep impact basins, but even inside the lunar body the rock is arranged in an irregular fashion, with some regions being denser than others.
Much of the twins' data has yet to be analysed but already scientists are getting some tantalising new insights into the Moon's structure and history.
"One of the major results that we've found is that the lunar crust is much thinner than we had believed before, and that a couple of the large impact basins probably excavated the Moon's mantle, which is very useful in terms of trying to understand the composition of the Moon as well as the Earth, because we actually think that the Earth's mantle has a similar composition to the Moon's mantle," Prof Zuber said.
The gravity data also shows the lunar body's top-most layers to be far more fractured than anyone had previously suspected. These pulverised and porous materials that coat the surface bear witness to the brutal battering the Moon received in the first few hundred million years of its existence.
In addition, Ebb and Flow found evidence for great lava-filled fissures just under all this impact debris.
These dykes, some hundreds of km long, appear to reach deep into the Moon, and may hint at an early expansion phase in its history when the hot body expanded outwards, before eventually cooling and contracting.
Grail data will be critical in tying down ideas for how the Moon came into existence. The dominant theory calls for a giant impact billions of years ago between the Earth and a Mars-sized object which threw material into space that ultimately coalesced into the familiar body we recognise in the sky today.
Some scientists have argued that Earth may once even have had two moons which later merged - although the Grail data could have sunk this idea.
"We have looked for evidence of the second moon and we have not seen any of the suggested characteristics of the internal structure of the Moon that would be consistent with the idea of a second companion," said Prof Zuber.
"That in itself does not rule out that idea at this point. We and others can look at this in more detail, but nothing jumps out in that regard."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
Source: http://www.bbc.co.uk/news/science-environment-20761903#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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