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The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) is a NASA mission, launched August 3, 2004, 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. The mission will be the first time to return to Mercury in over 35 years - the last probe to visit Mercury was Mariner 10, which completed its mission in March, 1975. MESSENGER will offer several vast improvements in scanning ability, with cameras that can resolve surface features down to just 60 feet across (18 meters) compared to the one mile (1.61 kilometer) resolution offered by Mariner 10. MESSENGER will also be the first probe to image the entire planet; Mariner 10 was only able to observe one hemisphere that was lit during its flybys.
Launch and journey to Mercury
The spacecraft was scheduled to launch during a 12-day window that opened May 11, 2004, aboard a Boeing Delta II rocket from Cape Canaveral Air Force Station, Florida, but on March 26, 2004, NASA announced that a later launch window starting at July 30, 2004 with a length of 15 days would be used. This significantly changed the trajectory of the mission and will delay the arrival at Mercury by two years.
The new plan called for MESSENGER to make an Earth flyby a year after launch, on 1 August 2005, at an altitude of 2842 km, to receive a gravity-assisted boost, two swings past Venus on 24 October, 2006 and June 6, 2007. Then there will be three flybys of Mercury on 15 January and October 6, 2008 and September 30, 2009. Mercury orbit insertion will be on March 18, 2011, beginning a year-long orbital mission.
The Delta rocket carrying MESSENGER lifted off from Cape Canaveral at 02:15:56 EDT on August 3, 2004. An hour later, NASA confirmed that MESSENGER had successfully separated from the third stage booster and commenced its roundabout route to Mercury.
Mercury observation plan
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. 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.
Once there scientists hope to test is a theory that the planet is shrinking, contracting in on itself as its core slowly freezes. The probe will look for signs of surface buckling on Mercury's unobserved hemisphere, as well as collect surface composition data on material that may have once spewed out of the planet's interior. The idea that Mercury's surface was somehow shrinking arose when Mariner 10 returned images of great scarps biting deep into the planet's surface. One such scarp, Discovery Rupes, cuts a one mile (1.61 kilometer) into Mercury's crust as it cuts across the surface.
Spacecraft and subsystems
The MESSENGER spacecraft is a squat box (1.27 m × 0.71 m × 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 metal hydride 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.
MESSENGER's onboard computer system is based on the Integrated Electronics Module (IEM), a device that combines core avionics in a single box. The spacecraft carries a pair of identical IEMs for backup purposes; both house a 25 megahertz main processor and 10 MHz fault protection processor. All four are radiation hardened RAD6000 processors, based on IBM POWER CPUs (predecessor of the PowerPC chip found in today's Macintoshes). The RAD computers, slow by current PC/Mac standards, are state of the art for the radiation tolerance required on the MESSENGER mission. For data, the spacecraft carries two solid-state recorders (one backup) able to store up to one gigabyte each. Its main processor collects, compresses, and stores on the recorder images and other data from MESSENGER's instruments.
- MESSENGER Web Site - official site.
- MESSENGER Mission Page - official information regarding the mission on the nasa.gov website.
- NSSDC. NSSDC Master Catalog: Spacecraft: MESSENGER. Greenbelt, Maryland: National Space Science Data Center. July 14, 2004.
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