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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. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study is based on work supported by Contract NNH06CE15B between the National Academy of Sciences and the National Aeronautics and Space Administration and Grant No. AGS-1106426 between the National Academy of Sciences and the National Science Foundation. Any views or observations expressed in this publication are those of the authors and do not necessarily reflect the views of the agencies that provided support for the project.

International Standard Book Number-13: 978-0-309-26564-5
International Standard Book Number-10: 0-309-26564-9

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Copyright 2012 by the National Academy of Sciences. All rights reserved.

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