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Launch Date: August 2, 2004

The Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission is designed to study the characteristics and environment of Mercury from orbit. Specifically, the scientific objectives of the mission are to characterize the chemical composition of Mercury's surface, the geologic history, the nature of the magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere over a nominal orbital mission of one Earth year.

Mission Profile

The first launch opportunity for MESSENGER is at 6:17:44 UT 30 July 2004 (2:17:44 a.m. EDT). Launch will take place on a Delta 7925H (a Delta II Heavy launch vehicle with nine strap-on solid-rocket boosters). The dates in the description below are based on the original May launch date and have presumably changed and will be updated as soon as the new dates are available. There will be three Venus flybys, on 2 November 2004 at an altitude of 12,666 km, on 28 August 2005 at an altitude of 6416 km and finally on 22 October 2006 at 14,292 km altitude. The first of two Mercury flybys, both at 200 km altitude, will be on 16 October 2007 and the second will be on 7 July 2008. There will also be two deep space manuevers. Data collected during the Mercury flybys will be used to help plan the scientific campaign during the orbital phase. Mercury orbit insertion will take place on 2 July 2009, requiring a delta-V burn of 1.64 km/s. The nominal orbit is planned to have a periapsis of 200 km at 60 degrees N latitude, an apoapsis of 15,193 km, a period of 12 hours and an inclination of 80 degrees. The periapsis will slowly rise due to solar perturbations to over 400 km at the end of 88 days (one Mercury year) at which point it will be readjusted to a 200 km, 12 hour orbit via a two burn sequence. Data will be collected from orbit for one Earth year, the nominal end of the primary mission will be on 1 July 2010. Global stereo image coverage at 250 m/pixel resolution is expected. The mission should also yield global composition maps, a 3-D model of Mercury's magnetosphere, topographic profiles of the northern hemisphere, gravity field to degree and order 16, altitude profiles of elemental species, and a characterization of the volatiles in permanently shadowed craters at the poles.

Important Dates

Spacecraft and Subsystems

The MESSENGER spacecraft is a squat box (1.27 m x 0.71 m x 1.05 m) with a semi-cylindrical thermal shade for protection from the Sun and two solar panel wings extending radially. A 3.6 m magnetometer boom also extends from the craft. The total mass of the spacecraft is 987.7 kg, 607.8 kg of this is propellant and helium. The structure is primarily graphite-cyanate-ester (GrCE) composite and consists of two vertical panels which support two large fuel tanks and two vertical panels which support the oxidizer tank and plumbing panel. The four vertical panels make up the center column and are bolted at their aft ends to an aluminum adapter. A single top deck panel mounts the LVA (large velocity adjust) thruster, small thrusters, helium and auxiliary fuel tanks, star trackers and battery.

Main propulsion is via the 645-N, 317-s bipropellant LVA thruster, four 22-N monopropellant thrusters provide spacecraft steering during main thruster burns, and ten 4-N monopropellant thrusters are used for attitude control. There is also a reaction-wheel attitude control system. Knowledge for attitude control is provided by star tracking cameras, an inertial measurement unit, and six solar sensors. Power is provided by the solar panels, which extend beyond the sunshade and are rotatable to balance panel temperature and power generation, which provides a nominal 450 W in Mercury orbit. The panels are 70% optical solar reflectors and 30% GaAs/Ge cells. The power is stored in a common-pressure-vessel nickel-hydrogen battery, with 11 vessels and 2 cells per vessel.

Communications are in X-band with downlink through two fixed phased-array antenna clusters and uplink and downlink through medium- and low-gain antennas on the forward and aft sides of the spacecraft. Passive thermal control, primarily a fixed opaque ceramic cloth sunshade, is utilized to maintain operating temperatures near the Sun. Radiators are built into the structure and the orbit is optimized to minimize infrared and visible light heating of the spacecraft from the surface of Mercury. Multilayer insulation, low conductivity couplings, and heaters are also used to maintain temperatures within operating limits.

Five science instruments are mounted externally on the bottom deck of the main body: the Mercury Dual Imaging System (MDIS), Gamma-Ray and Neutron Spectrometer (GRNS), X-ray Spectrometer (XRS), Mercury Laser Altimeter (MLA), and Atmospheric and Surface Composition Spectrometer (MASCS). The Energetic Particle and Plasma Spectrometer (EPPS) is mounted on the side and top deck and the magnetometer (MAG) is at the end of the 3.6 m boom. Radio Science (RS) experiments will use the existing communications system.

Source: NASA