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 was supported by the National Aeronautics and Space Administration under Contract No. NASW 99037. Any opinions, findings, 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.

Available in limited supply from: Aeronautics and Space Engineering Board, HA-292, 2101 Constitution Avenue, N.W., Washington, DC 20418, (202) 334-2855.

Cover artwork by Nila Rusnell Oakes. Stratus Composition, 1999, oil on canvas, 48” × 48”.

Internet: http://www.nilaoakes.com