The main thrust -- to use an apt word -- of Heppenheimer's sequel to his Colonies in Space involves the needs which could be served by establishing extraterrestrial frontiers. An aerospace engineer with a practical bent, he posits a ""sixfold way"" to proceed. Step one begins with the NASA space shuttle scheduled to take off in the coming year, which -- according to his scheme -- would be instrumental in setting up orbiting construction platforms. These in turn would be the base for the building of communications spacecraft -- but their main purpose would be the development of hugh Manhattan-Island-sized energy generators using solar power (""powersats"") to beam energy back to earth. At first, earth materials would be used in construction, but eventually the moon would be mined, bringing us to stage four. By this time communities of a thousand or more would be needed and true space colonies would be built -- step five. These, with experience and time, would lay the groundwork for space cities to be colonized by all peoples, with the prospect that future generations would carry colonization further into space. Many of these ideas have been voiced before, especially in the works of Gerard O'Neill (who supplies an introduction here). That they are taken seriously is evident from NASA studies, and others, exploring what could be accomplished and at what cost. Heppenheimer alludes to such studies and also exhibits an awareness of the self-serving interests of the military and the aerospace industry. Still, he is essentially an enthusiast making the point that communications needs alone -- to say nothing of energy needs -- demand a better technology. (Presently we run the risk of interference between neighboring, proliferating communications satellites.) Rounding out the text are interesting chapters on current theories of the solar system's origin and the chances of finding advanced cultures elsewhere; a skeptical view of UFOs; and some good historical background. The one ominous note is Heppenheimer's analogy between early space settlements and the Canal Zone, with all the attendant company-town smugness, bureaucracy, and intellectual stifling. On the other hand, the practical neccessities are persuasively presented and the challenge is exciting.