| Summary: | As the stepping stone, cornerstone, capstone, and keystone of planetary science, exploration of the Moon has provided the foundation, inspiration, and bridge to understanding the planets of our solar system. These concepts are highlighted with countless examples throughout various chapters of this book. Much of our detailed geochemi cal understanding of this extraterrestrial neighbor comes, of course, from the samples returned during the Apollo era. The more recent remote measurements of Clementine and Lunar Prospector nevertheless jolted a complacent community. The decades-long neglect of lunar exploration after Apollo stunted what had been an enormous growth of knowledge in the years immediately following the return of the initial samples. The small pulse of new data in the early 1990's made an enormous impact. The "New View" of possible accumulation of water ice at the lunar poles, the unique character of the enormous South Pole-Aitken basin on the farside, and the asymmetric concentration of heat-producing elements are fundamental aspects of lunar science whose importance was unrecognized a decade ago. All three of these paradigm shifts in our understanding beg explanations. They remind us in no uncertain terms, that our knowledge is terribly incomplete. Clearly, remote observations, in situ measurements, and sample return all play a significant role in exploration of planetary bodies. There is ample evidence that to extend exploration beyond simple reconnaissance in fact requires both in situ and orbital measurements as well as - and integrated with - analysis of returned samples. There are arguments (and examples) about which sequence is the most efficient. On one hand, detailed remote analyses coupled with targeted in situ measurements allow assessment of regional properties and selection of sites that will optimize the value of samples returned from limited targets. On the other hand, the enormous amount of information inherent in the detailed analyses performed on samples in Earth-based laboratories has repeatedly identified broad-scale issues that were unimagined with the more limited remote measurements. A useful strategy could involve two stages. Firstly a simple (and less costly) grab sample from the surface (regolith) that would enable an interpretation of orbital data. The second, more costly phase would target specific localities identified from this combination of orbital and in situ measurements. Like most areas of science, discovery and progress in understanding how planets work is cyclical in nature. As valuable data are first acquired in one area, the information derived lays the foundation for all subsequent questions and the definition of new steps. The danger, of course, is that our ignorance can never tell us what we have missed. We must make the best use of lessons learned.
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