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 Mercury 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.  In addition, data will be collected
    during the flybys of Earth and Venus. 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
    Mercury's crust, tectonic history, the structure of the atmosphere
    and magnetosphere, and the nature of the polar caps.
 
    The Mercury Atmospheric and Surface Composition Spectrometer (MASCS)
    is comprised of a small Cassegrain telescope with a 257-mm effective
    focal length and a 50-mm aperture that simultaneously feeds both an
    UltraViolet and Visible Spectrometer (UVVS) and a Visible and
    InfraRed Spectrograph (VIRS). MASCS will investigate Mercury's
    exosphere by measuring altitude profiles of known species as well as
    searching for previously undetected species. MASCS will investigate
    the mineralogical composition of the surface of Mercury by obtaining
    maps of surface reflectance spectra on spatial scales of 5 km.
 
    UVVS is a scanning grating, Ebert-Fastie monochromator with a focal
    length of 125-mm and is equipped with three photomultiplier tube
    detectors. The three detectors cover the wavelength ranges of the
    far ultraviolet (115-180 nm), middle ultraviolet (160-320 nm), and
    visible (250-600 nm) with an average spectral resolution of 0.6 nm.
    Two of the three photomultiplier tubes can be operated simultaneously;
    any combination of two is possible.
 
    The UVVS instrument will help determine the composition of Mercury's
    exosphere by measuring the spatial and vertical distribution of known
    species (H, O, Na, K, and Ca) as well as previously undetected but
    predicted species (e.g. S, Si, Al, Mg, Fe, and OH). In addition to
    determining the composition and structure of the exosphere, these
    data will provide the basis for determining exospheric processes,
    studying the relationship between surface and exospheric composition,
    and studying surface-exosphere-magnetosphere interactions.
 
    VIRS is a fixed concave grating spectrograph with a 210-mm focal
    length. A beam splitter simultaneously disperses the spectrum onto
    two solid-state array detectors: a 512-element silicon photodiode
    array, with a sensitivity to visible wavelengths (300-1050 nm), and
    256-element indium-gallium-arsenide photodiode array, to measure
    near infrared wavelengths (850-1450 nm). It is optimized to measure
    visible and near infrared surface reflectance, and VIRS will obtain
    data with a resolution of 5 nm. Together, the VIRS and UVVS
    detectors will measure surface reflectance at middle ultraviolet to
    visible to near infrared wavelengths to search for ferrous bearing
    minerals, Fe- Ti bearing phases, and ferrous iron. These
    measurements will be made with a spatial resolution of 5 km or
    better.
 
    The MASCS instrument is described in full detail in
    [MCCLINTOCK&LANK2007].