(UPDATED BELOW)

NASA is to launch Thursday a $500 million pair of unmanned spacecraft that will chase each other around the Moon as they use gravity tools to map its inner core for the first time.

Scientists hope the satellites will help solve mysteries about how the Moon formed, how the unexplored far side differs from the near side which humans have walked on, and whether there was once another Moon that melded with ours.

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The GRAIL (Gravity Recovery And Interior Laboratory) mission, aims to launch on a single Delta II rocket as early as 8:37 am Thursday (1237 GMT). A second launch window opens at 9:16 am (1316 GMT).

If weather delays the attempt — and it might with forecasters predicting just a 40 percent chance of favorable conditions for launch — other opportunities are available over the next 42 days.

Despite 109 past missions to study the Moon, and the fact that 12 humans have walked on its surface since the dawn of the space age in 1959, GRAIL program scientist Bobby Fogel said there has never been a serious attempt to peer inside.

“GRAIL will be the first mission to determine the internal structure of the Moon,” he told reporters.

“We have used gravity science before to try to gain some insight as to what is going on inside the Moon, however these have been very primitive attempts.

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“If those previous attempts could be likened to a magnifying glass, GRAIL by contrast would be a high powered microscope.”

Scientists believe that the Moon was formed when a planet-sized object crashed into the Earth, throwing off a load of material that eventually became what we now recognize as our planet’s airless, desolate satellite.

How it heated up over time, creating a magma ocean that later crystallized, remains a mystery.

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A recent hypothesis that there may have been two Moons that slowly merged into each other, creating mountains instead of craters, can also be tested with this mission, said principal investigator Maria Zuber.

“If we want to reconstruct the evolution of the Moon over time, we certainly need to reconstruct the temperature structure of the Moon right now,” she said.

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Little is known for certain about what lies inside the Moon. The widely held belief that there is a small solid iron core surrounded by a liquid iron core is unproven, said Zuber.

“It is actually quite possible that deep inside the Moon the core could be titanium oxide which is a material that would have fallen out or would have crystallized out of the magma ocean and sunk to the deep interior of the Moon,” she said.

“And we certainly don’t have a good idea about the thermal state,” she added. “It is not out of the question that parts of the deep lunar mantle are actually molten.”

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The GRAIL twins will journey for more than three months, with one spacecraft entering Moon’s orbit on New Year’s Eve and the second on New Year’s Day.

Once there, they will line up with each other and “essentially chase each other around in a polar orbit as the Moon rotates slowly underneath them,” said Zuber.

They will hover about 34 miles (55 kilometers) above the lunar surface, with the distance between them ranging from 37 to 140 miles (60 to 225 kilometers), collecting measurements of the terrain beneath.

The duo will accomplish the mission’s primary aim of understanding the Moon’s inner character by performing a series of low-altitude gravity field measurements using what is known as a Ka-band ranging instrument.

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The mission itself is relatively short in duration, just 90 days once the two spacecraft reach orbit.

About 40 days after their work is done, the pair will plunge into the lunar surface, NASA said. Scientific analysis of their data is expected to continue for a year.

UPDATE: (AFP) High winds forced NASA to delay Thursday its planned launch of a $500 million duo of unmanned spacecraft that will use gravity tools to map the inner core of the Moon for the first time.

The next launch opportunities at Florida’s Cape Canaveral will open Friday at 8:33 am (1233 GMT) or 9:12 am (1312 GMT), the US space agency said.

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Scientists hope the satellites will help solve mysteries about how the Moon formed, how the unexplored far side differs from the near side which humans have walked on, and whether there was once another Moon that melded with ours.

Known as GRAIL (Gravity Recovery And Interior Laboratory), the twin satellites would arrive in the Moon’s polar orbit around the start of the New Year and circle it for about three months, using gravity mapping to reveal what lies inside.