THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW 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 study is based on work supported by Contract NNH06CE15B between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the agency that provided support for the project.

Cover: Cover design by Penny E. Margolskee. The false-color mosaic image of the “tiger stripe” fractures in the southern polar region of Saturn’s moon Enceladus was constructed from data collected at ultraviolet, green, and near-infrared wavelengths by the Cassini spacecraft during its close flyby of Enceladus on August 11, 2008. Enceladus’s striking plumes emanate from discrete locations along the “tiger stripes.” Courtesy of NASA/Jet Propulsion Laboratory (JPL)/Space Science Institute. Inset, left: An artist’s impression of the New Horizons spacecraft during its flyby of the Pluto-Charon system in July 2015. Courtesy of Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Inset, center: Enigmatic plumes of ice particles, water vapor, and trace organic compounds emanate from the southern polar region of Saturn’s moon Enceladus. This false-color image was created from data collected by the Cassini spacecraft on November 27, 2005, when it was 148,000 km from Enceladus. Courtesy of NASA/JPL/Space Science Institute. Inset, right: An artist’s impression of the proposed Europa Clipper spacecraft. This Jupiter-orbiting spacecraft is designed to undertake intensive observations of Europa during a series of close flybys. Courtesy of NASA/JPL.

International Standard Book Number-13: 978-0-309-25675-9

International Standard Book Number-10: 0-309-25675-5

Copies of this report are available free of charge while supplies last from:

Space Studies Board

National Research Council

500 Fifth Street, NW

Washington, DC 20001

Additional copies of this report are available from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800) 624-6242 or (202) 334-3313; http://www.nap.edu.

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

Printed in the United States of America.