In the course of millions of years the human species has adapted itself to the narrow ranges of temperature and air pressure, the availability of food and water, the chemical and physical components of our earthly environment. Now that the means are at hand for mankind to transcend this environment, the questions arise: Where else in the universe can such physical conditions be found? Does our medium-sized planet, circling a medium-sized star in the outer reaches of a typical spiral galaxy, have a counterpart among the countless heavenly bodies which surround it? What will men find as they gradually extend the range of their explorations?

In speculating on some future consequences of manned space flight, this book looks forward to a time when human beings will be able to travel the vast distances to the other stars. It then attempts to determine—on the basis of our present biological and cosmological knowledge—whether there are other worlds where man can survive or where human life may even now be flourishing.

This book is absolutely indispensable for anybody who really wants to write “hard sf,” and has a considerable amount of interest for the general science fiction fan or astronomy buff.

Asimov here is distilling a report prepared by Stephen H. Dole for the RAND Corporation (Dole himself had little to do directly with the present volume). It represents a careful, considered examination at just exactly what makes a planet “habitable” and what the odds are for such a beast existing anywhere outside our solar system.

The subject is examined from a careful, scientific perspective and is well-done. There is no doubt but that it has become somewhat behind-the-times over the past forty-five years, as astronomy has marched on and new information has become available—but the enumeration of the various factors involved in habitability (if nothing else) remains interesting and thought-provoking.