Nasa's twin GRAIL (Gravity Recovery And Interior Laboratory) spacecraft are now in orbit around the Moon. Having achieved lunar orbit at 2 p.m. PST on New Year's Eve, GRAIL-A was joined by GRAIL-B at 2:43 p.m. PST on New Year's Day. The twin spacecraft are now in a near-polar, elliptical orbit with an orbital period of approximately 11.5 hours. In readiness for the science phase of the mission which is due to start in March 2012, both spacecraft will undergo a series of burns in coming months to place them in a near-polar, near-circular orbit with an altitude of about 34 miles (55 km) and orbital period of just under two hours.
GRAIL's science mission will see the twin spacecraft orbiting the Moon in formation and transmitting radio signals that will allow the distance between them to be precisely measured. By measuring the changes in distance between the spacecraft caused by the changes in gravity as they pass over visible features such as mountains and craters, and masses hidden beneath the lunar surface, scientists will be able to create a high-resolution map of the Moon's gravitational field. This will allow scientists to determine the inner structure of the Moon and aid in the understanding of the evolution of Earth and its rocky neighbors in the inner solar system.
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As part of the MoonKAM (Moon Knowledge Acquired by Middle school students) program, each GRAIL spacecraft also carries a small camera that will allow thousands of fifth- to eighth-grade students to select target areas of the lunar surface for MoonKAM snapshots that will be sent back by the GRAIL satellites for the students to study. By then, the GRAIL spacecraft will also have new names as the result of a student contest - the results of which are due to be announced some time this month.