Space physics is, to a great extent, the study of naturally occurring plasmas. Approximately 99% of all the matter in the Universe exists in the form of plasmas of many different types; including the partially-ionized, relatively cool plasmas of planetary ionospheres, the million degree plasmas found in the solar corona, the solar-wind plasmas, the planetary magnetospheres, and the highly relativistic galactic cosmic-ray plasma. Not only do these plasmas have very different physical scales, but each has phenomena occurring on a wide range of scales within it. The challenge for space physics is to arrive at an integrated view that relates large-scale and small-scale plasma phenomena by drawing upon concepts from the frontier of modem scientific research. In particular the Sun and the heliosphere--the nearest star and its sphere of influence-harbor a large number of fundamental questions that are of consequence not only for the solar system, but also for astrophysics as a whole. Mankind now has the intellectual curiosity and technical capability to investigate the many basic and interconnected questions regarding the internal structure of the Sun, the heating of the corona, and coronal expansion into the fast and slow streams of the solar wind. Exploration of the Earth's nearby space environment has revealed a dynamic and complex system of plasmas interacting with the magnetic fields and electric currents surrounding our planet. This region, comprising the magnetized solar-wind plasma plus the perturbation in the heliosphere caused by the presence of the magnetic Earth, is the region defined as Geospace. Solar influence shapes and links the three major regions of geospace: the magnetosphere, the ionosphere, and the Earth's neutral atmosphere, where life exists. Thus space physics can be characterized as the study of the heliosphere as one system; that is, of the Sun and solar wind, and their interactions with the upper atmospheres, ionospheres, and magnetospheres of the planets and comets, with energetic particles, and with the interstellar medium. Over the next twenty years space physicists will continue to explore the space environment of the Earth and Sun to achieve a new kind of scientific understanding. The new frontiers will be the inner and the outer limits of the solar system; with the Sun as a variable star at its center and the interstellar medium--the frontier of the galaxy--on the outside. Once the Sun and interstellar medium have been studied in situ, it will be possible to start the integrated study of the entire heliosphere as one interacting system. It is a subject that is breathtaking both in the scope of the physics involved, and in its potential application to routine human activities on Earth, including the operation of near-Earth spacecraft. The knowledge gained by this research program will be critically important to understanding the effects of energetic particles and solar variability upon the Earth's environment and the human exploration of space. The purpose of this handbook is to provide background data on current, approved, and planned missions, including a summary of the recommended candidate future missions that emerged from the two workshops. As the priorities of science questions to be addressed are agreed upon, and technical assessments are developed, this report will be updated to reflect the developing implementation plan.
