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Energetic Particle and Plasma Spectrometer (EPPS)

NSSDCA ID: 2004-030A-06

Mission Name: MESSENGER


The Energetic Particle and Plasma Spectrometer (EPPS) on MESSENGER is designed to measure ions from about 10 keV to 5 MeV and electrons from about 20 to 700 keV. Scientific objectives of the instrument are to study the exosphere and magnetosphere of Mercury. the EPPS is mounted on the side of the spacecraft near the top (zenith-pointing) deck. The instrument consists of a FIPS (Fast Imaging Plasma Spectrometer) head for thermal plasmas and an EPS (Energetic Particle Spectrometer) head for energetic ions and electrons. The instrument is expected to return roughly 4.4 Gb over the one year nominal mission.

The EPS head, a flattened 10 cm diameter cylinder, is designed to measure the energy spectra, atomic composition, and pitch-angle distributions of energetic ions from 10 keV/nuc to about 5 MeV (H, He, CNO, Fe) and electrons from about 20 to 700 keV. Energetic ions enter through a collimator with a field of view of 160 x 12 degrees divided into 6 active segments of 25 degrees each and pass through two aluminized polyimide foils, each 9 micrograms per square cm. Time of flight is measured using the secondary electrons generated as the ion passes through the foils as start and stop signals detected by a microchannel plate electron multiplier. A pixellated silicon solid state detector measures particle total energy. Timing, energy, and event classification chips produce an eight point energy spectrum for each of the four species in all six pixel directions. Electrons are measured in two of the 25 degree segments, which have both a bare silicon detector and a foil covered detector to provide electron-ion separation. Spectra are read out on average every 36 seconds.

The FIPS head is designed to detect H, 3He, 4He, O, Ne, Na, K, S, Ar, and Fe. It consists of an entrance aperture which will allow particles in over a 360 degree azimuth and down to 70 degree elevation. Particles with the selected narrow E/q range and polar angle are allowed to pass through the deflection system. After passing through the electrostatic analyzer, which consists of two sets of collimator plates, the ions are post-accelerated by a fixed voltage and pass through a thin (1 microgram/square cm) carbon foil into the time of flight analyzer. After travelling a known distance, the ions hit a stop microchannel plate assembly, which measures the time of flight and position, forward scattered electrons from the foil are focused onto start microchannel plates on the side. The mass per charge of the particles can be determined from the E/q and time of flight. The deflection voltage is stepped from 0 to 8 kV over 1 minute and covers a range of roughly 0 to 10.0 keV/q. The time of flight ranges from 50 - 500 ns.

Alternate Names

  • EPPS

Facts in Brief

Mass: 3.1 kg
Power (avg): 2 W
Bit rate (avg): 0.14 kbps

Funding Agency

  • NASA-Office of Space Science Applications United States


  • Planetary Science: Fields and Particles

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Barry H. MaukExperiment ScientistApplied Physics Laboratory
Dr. Barry H. MaukGeneral ContactApplied Physics Laboratory
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