One of the most exciting and compelling possibilities in space science is the opportunity to compare and contrast different but analogous systems. Within the Sun-Earth Connection theme, we have the clearest and best understanding of the terrestrial environment. However, terrestrial studies are limited to certain ranges of physical parameters that are relevant in atmospheric and magnetospheric studies. By exploring and characterizing other planetary environments, we extend and generalize the knowledge gained from terrestrial studies. Such comparative research has already demonstrated that the Solar System is a vast laboratory for studying the "plasma universe."

The Comparative Environments campaign is intrinsically exploratory and is filled with opportunities for discovery. It is intimately linked to the concerns of OSS's other thematic areas and thus contributes substantially to NASA's entire space science program. Comparative studies of the terrestrial and planetary magnetosphere-atmosphere systems and of the fundamental physical processes occurring therein bear directly on our efforts to understand the structure and evolution of the Universe. In our solar system laboratory, we learn about the general role of magnetic fields, and we are able to study magnetospheres on many physical scales. We acquire direct knowledge of phenomena, such as collisionless shock formation and boundary region processes, that are of fundamental importance in all astrophysical settings.

The Comparative Environments campaign is keyed to understanding the origin and evolution of planetary systems, both around the Sun and around other stars. For example, the neutral gas and plasma interactions and dusty plasma phenomena that are one focus of this campaign play an essential role in planetary formation and in protostellar collapse. Moreover, an understanding of the space physical processes subsumed under the SEC theme is also critical to answering questions about the origins and evolution of planetary and satellite magnetic fields, about the modification of planetary and satellite regoliths and ices through surface-plasma interactions, about the formation and evolution of the neutral atmospheres and ionospheres, and, finally, about the conditions for the existence of life.