PEPSSI

PEPSSI
Name	PEPSSI
Mission	New Horizons
Operator	NASA / Applied Physics Laboratory
Type	Time-of-flight spectrometer / solid-state detector
Launch	January 19, 2006
Spacecraft	New Horizons (spacecraft)
Principal investigator	Alan Stern
Mass	1.5 kg (approx.)
Power	2.5 W (nominal)

PEPSSI

The Pluto Energetic Particle Spectrometer Science Investigation flew aboard New Horizons (spacecraft) to measure energetic ions and electrons in the outer Solar System environment during encounters with Pluto, Arrokoth, and in the heliosphere. Developed at the Johns Hopkins University Applied Physics Laboratory under the leadership of Alan Stern, it provided in situ measurements of charged-particle fluxes, composition, and energy spectra that complemented remote-sensing instruments such as Ralph (New Horizons) and Alice (spacecraft).

Overview

PEPSSI is a compact energetic particle spectrometer designed to detect electrons, protons, and heavy ions across a broad energy range while meeting the mass, power, and telemetry constraints of the New Horizons (spacecraft) mission profile to Pluto system and beyond. The instrument was conceived in the context of earlier missions with particle instruments such as Voyager 1, Voyager 2, Ulysses, Galileo (spacecraft), Cassini–Huygens, and STEREO (spacecraft), adopting time-of-flight and solid-state detection techniques pioneered on experiments like EPAM, SIS, and CRRES. Its data contributed to studies involving solar wind interaction with planetary atmospheres, magnetospheric-like structures, and interplanetary energetic particle populations characterized by missions such as ACE (spacecraft), Wind (spacecraft), and SOHO.

Instrument Design and Components

The instrument combines a time-of-flight (TOF) subsystem and solid-state detector (SSD) stack to derive particle mass, energy, and arrival direction. The entrance aperture and start/stop foils, electrostatic optics, and microchannel plate elements are reminiscent of designs used on MMS (spacecraft), Cluster (spacecraft), and Geotail. Charged particles entering the aperture produce secondary electrons at a start foil, yielding TOF signals recorded between microchannel plate arrays; the SSDs measure residual energy to enable mass discrimination between species like helium, oxygen, and heavier ions analogous to analyses from ACE (spacecraft) SWICS and Ulysses SWOOPS. Electronics include charge-sensitive amplifiers, pulse-height analyzers, and onboard processing firmware developed in coordination with engineering teams at Johns Hopkins University Applied Physics Laboratory and commercial contractors with heritage from Ball Aerospace and other instrument builders.

Science Objectives and Measurements

PEPSSI aimed to quantify energetic particle populations near Pluto, characterize solar wind pickup ions from escaping atmospheres, and monitor suprathermal electrons during flybys and cruise phases. Specific objectives paralleled investigations performed by Voyager 2 at the heliosheath and by Cassini–Huygens at Saturn: measure ion composition (H+, He++, O+), energy spectra from a few keV to several MeV, and angular distributions to infer flow anisotropies and boundary layers. The instrument provided data on interactions between escaping neutral atmospheres and the incident plasma, informing comparative studies with Mars Atmosphere and Volatile Evolution (MAVEN), Rosetta (spacecraft), and Venus Express observations of atmospheric escape and pickup-ion processes.

Calibration and Data Processing

Preflight calibration campaigns used particle beam facilities and vacuum chambers at institutions including Johns Hopkins University Applied Physics Laboratory and partner laboratories with beamlines similar to those used by Los Alamos National Laboratory and NASA Goddard Space Flight Center for detector characterization. Calibration established response matrices for species, energy-to-pulse-height conversions, time resolution, and geometric factors, following methods developed for ACE, Ulysses, and Voyager particle instruments. Onboard processing applied event selection, pulse-height discrimination, and accumulation into telemetry bins; ground processing pipelines at Southwest Research Institute and APL implemented deconvolution, background subtraction, and conversion to physical units, producing data products archived within NASA Planetary Data System standards and cross-referenced for multi-instrument studies with datasets from Ralph (New Horizons), Alice (spacecraft), and radio science experiments.

Mission Operations and Results

During the 2015 Pluto encounter, PEPSSI measured enhancements in energetic particle fluxes consistent with solar wind interaction and pickup-ion signatures from the escaping Pluto atmosphere. The instrument detected anisotropic distributions and temporal variations tied to solar wind conditions monitored by heliospheric assets such as ACE (spacecraft), STEREO (spacecraft), and SOHO. Post-Pluto, PEPSSI operated through the Kuiper Belt encounter with 2014 MU69 (Arrokoth), contributing to the characterization of the local particle environment and interplanetary energetic particle background during cruise operations studied by missions including New Horizons (spacecraft) teammates and Earth-based facilities like Arecibo Observatory and Goldstone Solar System Radar. The dataset supported joint analyses with magnetospheric and heliospheric communities referencing results from Voyager 1 and Voyager 2 in the outer heliosphere.

Legacy and Impact on Planetary Science

PEPSSI's compact, low-resource design demonstrated the viability of energetic-particle instrumentation for fast, long-range flyby missions, informing instrument concepts for future outer-planet and small-body missions influenced by heritage from New Horizons (spacecraft), Europa Clipper, and proposed Kuiper Belt explorers. Its measurements of pickup ions and particle-environment interactions at extreme heliocentric distances bridged gaps between inner heliosphere studies by ACE (spacecraft) and outer heliosphere observations by Voyager probes, aiding models developed by groups at Southwest Research Institute, NASA Goddard Space Flight Center, and university teams. Data from PEPSSI continue to be used for comparative planetology, atmospheric escape quantification, and planning of instrument suites on missions proposed to Uranus, Neptune, and additional Kuiper Belt Object explorers.

Category:Spacecraft instruments