The propulsion system designed to carry MESSENGER through a six-year, nearly 4-billion mile trip to and around Mercury is complete, marking a major step in the mission's development.

GenCorp Aerojet designed, built and installed the propulsion system. Several members of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) management and engineering teams visited Aerojet's Sacramento plant Jan. 28 for the system's official rollout, which included a salute to the staffers who integrated the system with the spacecraft's composite structure.

MESSENGER engineering leads and Aerojet staffers stand before the spacecraft during the official rollout of MESSENGER's propulsion system Jan. 28 at Aerojet's Sacramento plant.

"It is one thing to view design drawings and computer-drawn renderings, but it is another to see the realization of those designs," says MESSENGER Principal Investigator Sean C. Solomon, of the Carnegie Institution of Washington (D.C.). "The MESSENGER science team members are deeply grateful that the project and its industrial partners have found the technical solutions - low-mass materials, innovative integration of spacecraft and propulsion, and mission design - that will enable the first mission to Mercury since 1975 and the first spacecraft ever to orbit that planet."

MESSENGER's lightweight, high-performance propulsion system includes custom titanium tanks, a main bipropellant thruster and 16 small monopropellant thrusters positioned around the spacecraft. Propellant will account for more than 55 percent of MESSENGER's projected launch weight of 2,410 pounds (1,093 kilograms) - a lot for a spacecraft that size, but necessary for the tricky task of placing it into orbit around Mercury.

"It takes a lot of energy to slow the spacecraft down once it reaches Mercury," says MESSENGER Mission System Engineer Andrew G. Santo, of The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "MESSENGER's structure and propulsion system had to be very light to accommodate a large amount of fuel, and the team has done a very nice job of meeting those requirements."

MESSENGER will be loaded onto a truck today for a three-day, cross-country trip to APL, which is building the remainder of the spacecraft and manages the mission for NASA. After vibration tests and a thermal "bake out" to clean the structure at APL, the MESSENGER team will start integrating components and science instruments on the craft in mid-February.

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