Measuring the Moon's gravity The Grail satellites measure the distance to each other as they circle the Moon, pole to pole

Flying over gravity variations will induce tiny acceleration changes in the spacecraft twins

Making the measurements requires the satellites to carry very precise timing devices

The survey will probably continue until the end of 2012 if the pair survive an eclipse in June

The US space agency (Nasa) has succeeded in placing two new satellites in orbit around the Moon.

Both spacecraft were put in elliptical paths around the lunar body over the weekend after performing braking manoeuvres following their more than 100-day journey from Earth.

The identical Grail twins are to map gravity variations across the lunar body in unprecedented detail.

This will help scientists refine our theories for how the Moon formed.

It will also enable them to test new ideas, such as the provocative suggestion made earlier this year that there were probably two moons in the sky above Earth billions of years ago.

Lead scientist Dr Maria Zuber is certainly hoping for some dramatic discoveries.

"Grail is a journey to the centre of the Moon and it will use exceedingly precise measurements of gravity to reveal what the inside of the Moon is like," the Massachusetts Institute of Technology (MIT) researcher said.

"This information will be combined with the plethora of remarkable observations of the Moon that have been taken by other satellites before, and together they will enable us to reconstruct the Moon's early evolution."

The 300kg Grail spacecraft were launched from Cape Canaveral, Florida, last September, and took a long spiral out to their destination.

This weekend, they approached the Moon over the south pole, 25 hours apart. Each satellite in turn fired its main engine to slow it and put it in an elliptical orbit around the lunar sphere.

This orbit has a period of 11.5 hours and must now gradually be reduced in size and circularised before any science can begin.

A series of further burns on each spacecraft should achieve this goal by March.

The twins will then map the small variations in gravity across the lunar surface from an altitude of 55km.

Image caption Existing gravity maps are very coarse compared with what Grail should be able to produce

These gravity 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.

All this will have a subtle influence on the pull of gravity sensed by the over-flying spacecraft.

The Grail twins will make their measurements by carrying out a carefully calibrated pursuit of each other.

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 changed through time.

Near and far-side of the Moon Image caption The lunar far side is much more rugged than the familiar near side 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 lunar maria (Latin for "seas") created by magma flooding into crater depressions

(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

"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.

One fascinating idea to emerge recently that will come under scrutiny from Grail is the suggestion that the highlands on the far-side were formed as a result of a low-velocity impact by a second, much smaller moon. A research team published a paper in the journal Nature last August that showed how such an impact could have added material to the crust on the far hemisphere.

It made very clear predictions that the Grail data will be able to test.

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.

"Simple bowl-shaped craters, which on the Moon have diameters up to about 15km or so, are the most common landforms on the surfaces of the terrestrial planets, and this is a whole new area of science that will open up to us if we're able to do the extended mission," Dr Zuber explained.

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.

Image caption The Grail twins are very compact spacecraft. They each weighed little more than 300kg at launch

Jonathan.Amos-INTERNET@bbc.co.uk

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