The Energetic Particles Spectrometers (EPS) on MESSENGER and New Horizons

In the course of this decade, two NASA deep space mission to the inner and outer heliosphere, MESSENGER to the planet Mercury and New Horizons to the planet Pluto, will carry onboard energetic particle spectrometers. The combination of measurements near the Sun (0.3 AU), and from the outer heliosphere (up to almost 40 AU), will ideally complement the information available from Ulysses and from near‐Earth orbiting spacecraft, yielding boundary conditions on the processes that accelerate energetic particles. EPS is a hockey‐puck‐size Time‐of‐Flight (ToF) spectrometer that measures ions and electrons over a broad range of energies and pitch angles. Particle composition and energy spectra will be measured for H to Fe from 15 keV/nucleon to 3 MeV/nucleon and for electrons from 15 keV to 1 MeV. The ion section of EPS is a compact ToF telescope with two main components: a ToF section and a Solid State Detector (SSD) array to measure separately velocity and total energy of the incoming particles. Electrons are identified in EPS by the presence of an energy signal and by the absence of start or stop pulses, since energetic electrons have low efficiency for production of secondary electrons when passing through thin foils. For both ions and electrons the angle of arrival is determined by the position of the solid‐state detector that collects the particle.