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NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft will turn their attention away from Earth's star as they pass through a pair of Lagrangian points this fall to look for asteroids that may hold clues to the origins of Earth's moon.Orbiting the sun as solar-weather observatories since they were launched on Oct. 25, 2006, the two probes will pass through the L4 and L5 in September and October. Lagrangian points are gravity wells where the gravity of celestial bodies balance out, creating spaces where material can collect in space.


Stereo A will pass through the sun-Earth L4 Lagrangian point in September, and Stereo B will transit L5 in October. During their passages as they separate from each other along their orbits around the sun, program scientists will use the wide-view telescopes in the probes' Earth Connection Coronal and Heliospheric Instruments to look for asteroids in the region.


Subsequent analysis of the compositions of any asteroids found their could reveal more about how the moon was formed, according to Michael Kaiser, Stereo project scientist at NASA's Goddard Space Flight Center.


"These points may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago," Kaiser said.


"According to Edward Belbruno and Richard Gott at Princeton University, about 4.5 billion years ago when the planets were still growing, a hypothetical world called Theia may have been nudged out of L4 or L5 by the increasing gravity of other developing planets like Venus, sending it on a collision course with Earth. The resulting impact blasted the outer layers of Theia and Earth into orbit, which eventually coalesced under their own gravity to form the moon."


The moon contains little of the iron and other heavy elements that would have fallen to the centers of the molten Earth and Theia at the time they collided. Analysis of the material in asteroids found at the two Lagrangian points will give scientists more data to support or reject the variation of the so-called "giant impact" theory suggested by the Princeton scientists, Kaiser said.


"Computer models by Belbruno and Gott indicate that Theia could have grown large enough to produce the moon if it formed in the L4 or L5 regions, where the balance of forces allowed enough material to accumulate," he said.











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