NASA is ready to launch a pair of twin spacecraft on a mission to map the moon’s gravity in greater detail than ever. Such insight will allow scientists to deduce the moon’s interior structure, composition, and its history. The $496 million Gravity Recovery And Interior Laboratory (GRAIL) mission is scheduled to launch September 8; its launch window extends through October 19.

The GRAIL spacecrafts, GRAIL-A and GRAIL-B, will reach their destined orbit, a short 55 kilometers above the lunar surface, by January 2012. They will chase each other around the moon measuring the distance to each other with great precision. The distance will range between 121 to 262 kilometers depending on the moon’s gravitational field. The technique–twin spacecraft flying in formation–utilizes radio links between the two spacecraft as well as radio links to a station back on Earth.

“What we’re trying to measure is the width of less than a human hair,” said John Henk, Grail program manager at Lockheed Martin Space Systems in Denver, according to Space.com. Measuring these precise distance changes allows researchers to map the lunar gravity field more accurately, 100 to 1,000 times better than previously possible, according to GRAIL scientists.

While scientists expect that the mission will improve their knowledge of the moon, they believe it should also provide information on the formation and evolution of other bodies in the inner solar system, such as Mercury, Venus, Earth, and Mars.

See a photo gallery of GRAIL preparing for launch.