THE NATIONAL ACADEMIES PRESS
500 Fifth Street, N.W. Washington, DC 20001

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 project was supported by Grant No. DE-FG02-00ER41141 between the National Academy of Sciences and the Department of Energy, Grant No. NAG5-9268 between the National Academy of Sciences and the National Aeronautics and Space Administration, and Grant No. PHY-0079915 between the National Academy of Sciences and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.

International Standard Book Number 0-309-07406-1

Library of Congress Control Number 2003100888

Additional copies of this report are available from the
National Academies Press,
500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu

and

Board on Physics and Astronomy, National Research Council, NA 922, 500 Fifth Street, N.W., Washington, DC 20001; Internet, http://www.national-academies.org/bpa.

Cover: Artwork ©2002 by Don Dixon/cosmographica.com

Copyright 2003 by the National Academy of Sciences. All rights reserved.

Printed in the United States of America



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement