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 jointly by the National Science Foundation and the National Aeronautics and Space Administration under Grant No. NASW-96013. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors.

Space Studies Board, National Research Council, HA 584, 2101 Constitution Avenue, N.W., Washington, DC 20418; Internet <http://www.national-academies.org/ssb>; and

Board on Physics and Astronomy, National Research Council, HA 562, 2101 Constitution Avenue, N.W., Washington, DC 20418; Internet <http://www.national-academies.org/bpa>.