Book excerpt: Solar-Terrestrial Physics: The Study of Mankind's Newest Frontier Solar-Terrestrial Physics (STP) has been around for 100 years. However, it only became known as a scientific discipline under that name when the physical domain studied by STP became accessible to in situ observation and measurement by man or man-made instruments. Indeed, it was STP that provided the initial scientific driving force for the launching of man-made devices into extra-terrestrial space during the International Geophysical Year - aided of course by the genetically engrained drive of humans to expand their frontiers of knowledge, influence and dominance. We may define STP as the discipline dealing with the variable components of solar corpuscular and electromagnetic emissions, the physical processes governing their sources and their propagation through interplanetary space, and the physical-chemical processes related to their interaction with the Earth and other bodies in interplanetary space. Much of STP deals with fully-or partially-ionized gas flows and related energy, momentum and mass transfer in what now appears as one single system made up of distinct but strongly interacting parts, reaching from the photosphere out to the confines of the heliopause, engulfing planets and other solar system bodies, and dipping deep into 6 the Earth's atmosphere.