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For LISA, the name of the game is stable positioning. Each spacecraft must keep its position extremely stable relative to the other two, despite being millions of miles apart. If an internal or external disturbance causes a spacecraft to move slightly off course, it will have to maneuver back into position, but without overcompensating for the disturbance. That's where the Micronewton Thrusters enter the picture.
Each LISA spacecraft will carry three pairs of thrusters distributed equally around the spacecraft. These miniaturized rockets exert a very weak force of 5 to 30 micronewtons—roughly the force of a falling grain of sand. They will provide the thrust that keeps each spacecraft centered on its test mass.
LISA's Micronewton Thrusters will use a technology known as Field Emission Electric Propulsion. The propellant consists of atoms made of the element cesium. The atoms are heated to a temperature hot enough that they become liquid. Next, they flow through filaments to a narrow slit. Strong electric fields strip electrons from the cesium atoms (a process known as ionization) and accelerate the ions through the slit into space with a velocity around 135,000 miles per hour (60 kilometers per second). The ions are ejected in minuscule quantities, so the amount of thrust is very small.
A key challenge is to keep the thrusters operational for the entire five-year mission. Both American and European engineers are working to solve this problem.
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