By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - The launch of a pair of robotic probes designed to map the moon's gravity was delayed on Thursday by poor weather at Cape Canaveral Air Force Station in central Florida.
The next opportunity for launch of the unmanned Delta 2 rocket carrying NASA's Gravity Recovery and Interior Laboratory, or GRAIL, spacecraft is 8:33 a.m. EDT on Friday.
The weather, however, could be an issue again. Air Force meteorologists on Thursday predicted a 60 percent chance of another delay due to rain, thunderstorms or high winds.
The twin GRAIL probes are designed to precisely map the moon's gravity so scientists can learn what lies beneath the lunar crust and whether the moon's core is solid, liquid or some combination of the two.
Combined with high-resolution imagery, ongoing analysis of rock and soil samples returned by the 1969-1972 Apollo missions and computer models, the gravity maps are expected to fill in the biggest missing piece in the puzzle of how Earth's natural satellite formed and evolved.
Linked by radio waves, the spacecraft will be able to detect changes in the tug of lunar gravity as small as one micron -- about the width of a red blood cell.
Pockets of terrain with more mass will cause first one and then the second satellite to speed up slightly as they fly over, changing the distance between the two probes in minute, but measurable amounts. Less dense regions, similarly, will cause the probes to slow slightly.
Scientists say knowing the locations of the moon's gravity lumps will give them a picture of what is inside the moon, from which they can piece together the story of how the body -- as well as other rocky planets like Earth, Venus, Mars and Mercury -- formed.
United Launch Alliance, a joint venture of Lockheed Martin Corp and Boeing Co, manufacture and provide launch services for the Delta 2 rocket. Lockheed Martin also is the prime contractor on the GRAIL satellites.
The $496 million mission is managed by lead scientist Maria Zuber, with the Massachusetts Institute of Technology.
(Editing by Tom Brown and Doina Chiacu)