Twin satellites are due to go into orbit around the Moon this weekend with the intention of mapping its gravity.
Nasa's Grail spacecraft are expected to give scientists remarkable new insights into the internal structure of the lunar body.
This new data should clarify ideas about the Moon's formation and resolve many mysteries, such as why its near and far sides look so different.
Lead scientist Dr Maria Zuber is expecting some dramatic discoveries.
"Grail will improve our knowledge of the Moon's nearside gravity by more than 100 times over what was previously known, and by more than 1,000 times over what was known on the far-side," the Massachusetts Institute of Technology (MIT) researcher said.
"In science, when we can improve measurements by a factor of two, we usually learn an awful lot; but when we improve measurements by orders of magnitude, the kind of science that we do is actually transformative."
The duo were launched from Cape Canaveral, Florida, last September, and have taken a long spiral out to their destination.
This weekend's approach to the Moon will bring them in over the south pole. Each satellite will execute a roughly 40-minute engine burn to slow itself and take up an elliptical orbit around the 3,500km-wide sphere.
The Grail-A spacecraft will initiate the manoeuvre at 2121 GMT on Saturday, 31 December; Grail-B will do a very similar burn a day later, starting 2205 GMT. Nasa should have confirmation near midnight on New Year's Day that both satellites are in the positions they should be.
"Following the lunar orbit insertion, the spacecraft will perform a series on intricate burns that take about two months, and these are required to get both spacecraft down to a [55km; 34-mile] altitude; and once that's done, that's when the science for Grail can begin," explained David Lehman, the mission's project manager at the Jet Propulsion Laboratory (JPL) in California.
Grail will map the small variations in gravity across the Moon.
These differences are the result of an uneven distribution of mass. Obvious examples at the Moon's surface include big mountain ranges or deep impact basins, but even inside the lunar body the rock will be arranged in an irregular fashion, with some regions being denser than others.
Near and far-side of the Moon
- It takes the Moon about the same amount of time to rotate on its axis as it does to complete an orbit of the Earth
- This is known as "synchronous rotation" and explains why the Moon always presents its familiar nearside to Earth (top left)
- The nearside is covered in smooth, dark lunarmaria (Latin for "seas") created by magma flooding into crater depressions
- The far-side is more rugged, with a thicker crust pock-marked by impact craters; the highest elevations are on the far-side (top right; bottom)
- In 1959, the USSR's unmanned spacecraft Luna 3 became the first to image the far-side; many of its features have old Soviet names
As the lead spacecraft flies through the uneven gravity field, it will experience small accelerations or decelerations. The second spacecraft, following some 100-200km behind, will detect these disturbances as very slight changes in the separation between the pair - deviations that are not much more than the width of a human red blood cell.
When the gravity map is combined with comparable-resolution topographical information showing the surface highs and lows, scientists should be able to deduce the Moon's probable internal structure and composition. This is fundamental knowledge that will play into theories of how the lunar body formed and how it has evolved over time.
"We believe the Moon formed from the impact of a Mars-sized object into Earth, but we understand little really of how this happened and how the [lunar body] cooled off after the violent event," said Dr Zuber. And she described as "shocking", the continued inability of science to explain why the rugged far-side of the Moon looks so different from that of the nearside with its great swathe of dark volcanic plains, or maria.
"Given that we've sent so many missions that have studied the outside of the Moon, it seems that the answer is not on the surface. The answer is locked in the interior," she said.
Grail's mapping phase will last for 82 days until early June. The Moon then goes into shadow, into eclipse, behind the Earth.
If the satellites can survive the hours of darkness on their batteries, it is likely they will be tasked with a second mapping cycle in the second half of 2012.
This would be at a much reduced altitude, perhaps as low as 25km from the surface. Getting lower would improve the resolution of the gravity maps yet again, and enable scientists to study even the structure of relatively small, shallow craters.
Grail is an acronym for Gravity Recovery and Internal Laboratory. The satellites will be given more engaging names than just "A" and "B" once the weekend's orbit insertion is confirmed. The names are being chosen via a public competition.
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