Pluto Energetic Particle Spectrometer Science Investigation
Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), is an instrument on the New Horizons space probe to Pluto and beyond, it is designed to measure ions and electrons. Specifically, it is focused on measuring ions escaping from the atmosphere of Pluto during the 2015 flyby.
Source: Wikipedia — Pluto Energetic Particle Spectrometer Science Investigation (CC BY-SA 4.0)