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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.

This study is supported by contracts NNH05CC16C and NNH11CD58D between the National Academy of Sciences and the National Aeronautics and Space Administration.

Cover: Design by Tim Warchocki. Images courtesy of NASA.

International Standard Book Number-13: 978-0-309-22159-7
International Standard Book Number-10: 0-309-22159-5

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Copyright 2011 by the National Academy of Sciences. All rights reserved.

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