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With MESSENGER in the late testing stages, launch preparations will soon begin. A few days before MESSENGER is mated to the upper stage of the Delta II launch vehicle, the team will fill the spacecraft's propellant and pressurant tanks.
Like many NASA deep-space missions requiring a lot of maneuvering capability, the MESSENGER spacecraft sports a bipropellant main engine, which, like an automobile engine, needs fuel and an oxidizer to run. Automobiles use gasoline (or diesel) as fuel; the oxidizer is simply oxygen from the atmosphere. Since MESSENGER will be working in the vacuum of space, it needs to carry its own oxidizer (in this case, nitrogen tetroxide) to combine with its hydrazine fuel. Hydrazine is a clear liquid that smells like ammonia and ignites when it meets the right oxidizer, making for a rather simple motor design. Once the fuel and oxidizer are sprayed into the engine nozzle in proper proportions - you have ignition! No spark plugs or other devices are needed to keep the engine running.
Hydrazine is also the lone propellant for the 16 smaller thrusters that help steady the spacecraft. No oxidizer is used in the thrusters so there is no combustion; the fuel sprays out over a heated material (a "catalyst") that causes it to break apart into nitrogen and hydrogen gas, moving the spacecraft in the direction opposite to where the thruster is pointed. (This is a practical application of Sir Isaac Newton's Third Law of Motion: "For every action there is an equal and opposite reaction.") To simplify the spacecraft design the MESSENGER team taps off the main hydrazine tanks to run the small thrusters, thus avoiding the need for yet another tank for yet another fuel.
MESSENGER carries 595 kilograms (more than half a ton) of propellant; this includes 97 kilograms (214 pounds) of hydrazine for control and 269 kilograms (593 pounds) to burn with the 229 kilograms (505 pounds) of nitrogen tetroxide in the high-performance "biprop" mode. Helium (2.25 kilograms, or 5 pounds) is used to maintain pressure in the fuel and oxidizer lines. The hydrazine is stored in two large, custom-designed titanium tanks and the oxidizer in a third, similar tank. A smaller tank holds the helium, and an extra supply of hydrazine is carried in a small auxiliary tank.
Last updated: June 03,
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