The instruments on NASA's Living With a Star (LWS) Program's Radiation Belt Storm Probes (RBSP) mission will provide the measurements needed to characterize, quantify, and understand the plasma processes that produce very energetic ions and relativistic electrons. The RBSP mission is part of the broader LWS Geospace program whose missions were conceived to explore fundamental processes that operate throughout the solar system and in particular those that generate hazardous space weather effects in the vicinity of Earth and that can impact solar system exploration. RBSP instruments will measure the properties of charged particles that comprise the Earth's radiation belts, the plasma waves which energize and scatter them, the large-scale electric fields which transport them, and the particle-guiding magnetic field. There are two RBSP spacecraft in nearly identical eccentric orbits. The orbits cover the entire radiation belt region and the two spacecraft will lap each other several times over the course of the mission, achieving various spatial separations that will permit tests of fundamental processes over a range of spatial scales. Using two identical spacecraft, the RBSP in situ measurements will discriminate between spatial and temporal effects, and determine the contributions of various proposed mechanisms to the acceleration, transport, and loss of energetic particles. The mission will contribute the measurements needed to improve specification models for predicting the radiation belt hazards to spacecraft and human explorers. The mission intends to transmit space weather data products via a low-power real time beacon. The Space Weather Beacon will operate in a near-continuous broadcast mode from each RBSP observatory. The beacon data stream will be sent at a rate of about 250 bps, and will contain real-time information about the state of the radiation belts, including differential energy spectra for ions from a few keV to several hundred MeV, and for electrons from a few tens of keV to a few MeV.