About the Instrument
Flying aboard NASA’s New Horizons spacecraft, the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) searched for neutral atoms that escape Pluto’s atmosphere and become charged — either through solar ultraviolet radiation or through charge exchange with solar wind particles — and then stream away from Pluto as “pickup” ions carried by the solar wind. However, both before and since the 2015 Pluto flyby, PEPSSI has been capturing and recording valuable data about how energetic particles are accelerated by solar wind structures as they flow outward, in addition to serving as a critical monitor of galactic cosmic rays in the outer heliosphere.
PEPSSI is a classic “time-of-flight” instrument: particles enter the detector and knock other particles (electrons) from a thin foil; they zip toward another foil before hitting a solid-state detector. The instrument clocks the time between the foil collisions to determine the particle’s speed and figures its total energy when it collides with the solid-state detector. The combination allows scientists to determine the mass, and from this they can determine the composition of each particle.