SAMPEX was a NASA satellite designed to study the energy, composition, and charge states of four classes of charged particles that come from beyond the Earth: galactic cosmic rays (from supernova explosions in our galaxy), anomalous cosmic rays (from interstellar gas surrounding our solar system), solar energetic particles (from explosions in the Sun's atmosphere), and magnetospheric electrons (particles from the solar wind trapped by Earth's magnetic field). SAMPEX was the first of NASA's SMEX (Small Explorer) missions.