Saturday, 19 March 2011

Mission background


In 1973 Mariner 10 was launched to make multiple flyby encounters of Venus and Mercury. Mariner 10 provided the first detailed data of Mercury, but only mapped 40-45% of the surface.[5][6] The final flyby of Mercury by Mariner 10 occurred on March 16, 1975, ending close-range observations of the planet for thirty years. As Mercury was the least explored terrestrial planet with no future planned mission, a study published in 1998 detailed a potential mission to send an orbiter to the planet. Since the Mariner 10 mission, subsequent mission proposals to revisit Mercury had appeared too costly, requiring large quantities of propellant and a heavy lift launch vehicle. However, using a trajectory designed by Chen-wan Yen in 1985, the study showed it was possible to seek a Discovery-class mission by using multiple, consecutive gravity assist, 'swingby' maneuvers around Venus and Mercury, in combination with minor propulsive trajectory corrections, to gradually slow the spacecraft thereby minimizing propellant needs.

The primary science objectives of the mission include:

    * determine accurately the surface composition of Mercury
    * characterize the geological history of the planet
    * determine the precise strength of the magnetic field and its variation with position and altitude
    * investigate the presence of a liquid outer core by measuring Mercury's libration
    * determine the nature of the radar reflective materials at Mercury’s poles
    * investigate the important volatile species and their sources and sinks on and near Mercury.

The contrived acronym MESSENGER was chosen because Mercury was the messenger of the gods according to Roman mythology.
Spacecraft design

The MESSENGER bus measures 1.85 meters (73 in) tall, 1.42 m (56 in) wide and 1.27 m (50 in) deep. The bus is primarily constructed with four graphite fiber / cyanate ester composite panels which support the propellant tanks, the LVA (large velocity adjust) thruster, attitude monitors and correction thrusters, antennas, the instrument pallet, and a large ceramic-cloth sunshade, measuring 2.5 m (8.2 ft) tall and 2 m (6.6 ft) wide, for passive thermal control.[8]
Attitude control and propulsion

    Main propulsion is via the 645 N, 317 sec. Isp bipropellant LVA thruster. Four 22 N (4.9 lbf) monopropellant thrusters provide spacecraft steering during main thruster burns, and ten 4 N (0.9 lbf) monopropellant thrusters are used for attitude control. For precision attitude control, a reaction wheel attitude control system was also included. Information for attitude control is provided by star trackers, an inertial measurement unit, and six sun sensors.

    The spacecraft is designed to carry 607.8 kilograms (1,340 lb) of propellant (hydrazine and nitrogen tetroxide) and pressurizer (helium).


    The probe includes two small deep space transponders for communications with the Deep Space Network and three kinds of antennas: a high gain phased array whose main beam can be electronically steered in one plane, a medium-gain “fan-beam” antenna and a low gain horn with a broad pattern. The high gain antenna is used as transmit-only at 8.4 GHz, the medium-gain and low gain antennas transmit at 8.4 GHz and receive at 7.2 GHz, and all three antennas operate with right-hand circularly polarized (RHCP) radiation. One of each of these antennas is mounted on the front of the probe facing the sun, and one of each is mounted to the back of the probe facing away from the sun.


    The space probe is powered by a two-panel, Gallium Arsenide/Germanium (GaAs/Ge) solar array providing an average of 450 watts at Mercury. Each panel is rotatable and includes optical solar reflectors to balance the temperature of the array. Power is stored in a common-pressure-vessel, 23-ampere-hour nickel hydrogen battery, with 11 vessels and two cells per vessel.

    The computer system is based on the Integrated Electronics Module (IEM), a device which combines core avionics into a single box. The computer features two radiation-hardened IBM RAD6000, a 25 megahertz main processor and 10 MHz fault protection processor. For redundancy, the spacecraft carries a pair of identical IEM computers. For data storage, the spacecraft carries two solid-state recorders able to store up to one gigabyte each. The IBM RAD6000 main processor collects, compresses, and stores data from the MESSENGER instruments for later playback to Earth

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