MESSENGER’s dual-mode propulsion system includes a 660-newton (150-pound) bipropellant thruster for large maneuvers and 16 hydrazine-propellant thrusters for smaller trajectory adjustments and attitude control. The Large Velocity Adjust (LVA) thruster requires a combination of hydrazine fuel and an oxidizer, nitrogen tetroxide. Fuel and oxidizer are stored in custom-designed, lightweight titanium tanks integrated into the spacecraft’s composite frame. Helium pressurizes the system and pushes the fuel and oxidizer through to the engines.
At launch the spacecraft carried just under 600 kilograms (about 1,320 pounds) of propellant, and it will use nearly 30 percent of it during the maneuver that inserts the spacecraft into orbit about Mercury. The small hydrazine thrusters play several important roles: four 22-newton (5-pound) thrusters are used for small course corrections and help steady MESSENGER during large engine burns. The dozen 4.4-newton (1-pound) thrusters are also used for small course corrections and serve as a backup for the reaction wheels that maintain the spacecraft’s orientation during normal cruise and orbital operations.