The MErcury Surface, Space ENvironment, GEochemistry and Ranging
    (MESSENGER) mission is designed to orbit Mercury following one Earth
    flyby, two flybys of Venus and three of Mercury.  It launched in
    August 2004 and will use these flybys to achieve an orbit insertion
    around Mercury in March 2011.  Initial data collection will begin
    during the three flybys of Mercury, and will primarily consist of
    global mapping and measurements of the surface, atmosphere and
    magnetosphere composition.  MESSENGER will remain in orbit for the
    rest of the nominal mission, which is scheduled to end in March
    2012. Once in orbit around Mercury it will begin a series of
    observations using multiple instruments. These observations will
    provide data to answer questions about the nature and composition of
    the crust, tectonic history, the structure of the atmosphere and
    magnetosphere, and the nature of the polar caps.
 
    The Energetic Particle and Plasma Spectrometer (EPPS) system
    encompasses 2 instrument subsystems - the Energetic Particle
    Spectrometer (EPS) and the Fast Imaging Plasma Spectrometer (FIPS).
    EPS covers the energy range of 25 to > 500 keV for electrons, and 10
    keV/nucleon to ~3 MeV total energy for ions.  FIPS covers the
    energy/ charge range of < 50 eV/q to 20 keV/q.  The desired
    throughput for FIPS charged particle and EPS event processing is 5
    kHz.  The Johns Hopkins University/Applied Physics Laboratory
    constructed the EPS instrument, while the FIPS instrument was
    constructed by the University of Michigan Space Physics Research
    Laboratory.
 
 
    FIPS Overview
    =============
    The FIPS sensor is designed to measure the distributions and
    composition of magnetosphere ions, as well as to characterize the
    nature of the planetary magnetic field of Mercury.  It will do this
    by measuring the mass per charge, the energy per charge, and
    incident angles for particles entering the sensor.  The particle
    intensity is also calculated from the event rate information.  FIPS
    generates a single 48-bit raw event packet format, which includes a
    1-bit header that identifies the event as a proton event or a
    non-proton event; an 11-bit time-of-flight (TOF) value; as well as
    Wedge, Strip, and ZigZag values (each 12 bits in size). In addition,
    the FIPS system generates counter and housekeeping information that
    the EPPS software can access via the I2C bus interface.
 
    The FIPS consists of an electrostatic analyzer (ESA), located at the
    entrance to the sensor, a post-acceleration chamber between the
    output of the ESA and the carbon foil, and a time-of-flight
    telescope.  The ESA at the entrance to the FIPS acts as a wide-angle
    lens for ions.  It only allows ions with a specific energy/charge
    band to enter through its output plane.  This band is stepped once
    per second by changing the deflection voltage of the ESA.  A
    measurement cycle consists of 64 deflection voltage steps in nominal
    mode or 8 in burst mode.  Associated with each step in a scan is a
    voltage setting, a threshold, a settling time, and a duration or
    time interval after which the next voltage step is performed.  Ions
    exit the output plane of the ESA and are then accelerated in the
    post- acceleration chamber.  This acceleration is done to boost
    low-energy ions to penetrate the carbon foil.  The acceleration also
    helps to reduce energy straggling and angular scattering - effects
    that cause degradation in mass resolution and imaging.  The carbon
    foil serves as the source for secondary electrons, which are
    scattered out by the penetration ions.  After penetrating the foil,
    the particle resides within the TOF chamber where velocity and
    incoming angle are computed.  Velocity is determined by the time
    difference between the generation of secondary electrons in the
    start foil and a stop surface, and angle is determined by spatially
    imaging the position of the generation of the start secondary
    electrons.  From the velocity, energy per charge, and the
    post-acceleration potential, it is then possible to calculate the
    mass per charge.  The measured species for the FIPS range from H to
    Fe.
 
    The FIPS instrument provides a single serial stream of event data to
    the EPPS system at rates of up to 50000 events per second.  The EPPS
    software maintains a mass distribution spectrum for the FIPS
    instrument. This spectrum consists of a collection of 256 bins (each
    24 bits wide) that count the number of events corresponding to mass-
    per-charge values. In addition, the software maintains a set of 5
    element energy spectra.  Each FIPS spectrum corresponds to a
    specified mass-per-charge range and consists of 64 24-bit bins.  For
    events whose mass-per-charge values fall within one of the selected
    ranges, an energy value is computed and used to determine which bin
    within the corresponding spectrum to increment. The spectra are
    accumulated over an integral number of voltage scans, after which
    they are compressed and output in telemetry.  FIPS also records 5
    heavy-ion energy-summed images (called velocity distributions) for
    each of the same 5 mass-per-charge values plus one for protons.  A
    commanded number of raw events will be recorded at each scan level.
 
 
    EPS Overview
    ============
    The EPS determines the distributions of the higher-energy
    magnetospheric ion and electrons, including the composition of the
    ions, to characterize the nature of the planetary field of Mercury.
    It does this by measuring the energy and velocity of the particles
    and then using a look-up table to determine the mass and therefore
    the species of particle.  The measured species for the EPS include
    H, He, CNO, Fe, and electrons.  Electrons are measured by
    solid-state detectors behind absorbing aluminum flashing.
 
    The EPS sensor consists of a 60-mm diameter, tuna-can-like cylinder,
    in which a start foil and stop foil, wrapped around opposite curved
    sides of the cylinder, constitute the TOF chamber.  An incoming
    particle hits the start foil and scatters one or more electron,
    which is attracted to the start-anode ground.  The particle
    continues and hits the stop foil, scattering other electrons, which
    are then attracted to the stop-anode ground. The solid-state
    detectors outside of, but wrapped around the curved face of, the
    stop foil, then detect the particle and measure the energy state.
 
    The detectors are arranged so that each detector senses the events
    within a given range of incidence angles.  Each of the 6 detector
    modules is composed of 4 pixels: large and small ion and large and
    small electron. This provides 24 detector elements.  At any one
    time, 12 of the 24 elements are used (6 ion and 6 electron
    detectors). Each of the 6 EPS detector modules also maintains its
    own spectrum via 64 16-bit bins. 63 bins will count the
    particle/energy combinations of interest, and 1 will count the
    remaining background events that do not fall in the particle/energy
    combinations of interest.  The spectra are accumulated over a time
    set by ground command, after which they are compressed and reported
    in telemetry.
 
    The EPS system also includes 32 16-bit rate counters and 3 24-bit
    rate counters that are read by the EPPS software every n seconds (n
    specified by command).  EPS status and housekeeping data such as
    voltages, currents, and temperatures are also periodically sampled.
 
    The EPPS instrument is described in full detail in
    [ANDREWSETAL2007].