NASA will try again Friday to launch a $500 million pair of unmanned spacecraft that will use gravity tools to map the Moon's inner core for the first time, after high winds delayed a first attempt.

The GRAIL (Gravity Recovery And Interior Laboratory) mission, aims to launch on a single Delta II rocket as early as 8:33 am (1237 GMT). A second launch window opens at 9:12 am (1316 GMT).

Weather conditions for Friday are the same as Thursday -- 40 percent favorable -- though upper level winds were deemed too strong to try it on the first day of the launch window.

If Friday's bid is also pushed back, plenty more opportunities will arise over the next 41 days.

Scientists hope the satellites will shed light on how the Moon formed and whether there was once another Moon that melded with it, forming lunar mountains.

Despite 109 past missions to study the Moon since 1959, and the fact that 12 humans have walked on its surface, 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 this week.

"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.

"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.

A recent hypothesis that there may have been two Moons that slowly merged into each other 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.

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.

The GRAIL twins will journey for more than three months, both entering orbit around 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.

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.

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