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Some vehicles can get low thrust with high specific impulse because they use tiny amounts of fuel and still generate thrust. Thrust is related to the mass flow through the engine, while specific impulse is a measure of how efficiently the thrust is made. Specific impulse is the amount of thrust divided by the rate at which fuel is used, essentially fuel efficiency. One type of engine, the “ion thruster,” generates some of the highest specific impulses, but produces a small thrust. An ion thruster produces thrust by speeding up ions, which are very tiny charged particles. Since it is using ions as fuel, the amount of fuel is tiny compared to other rockets. So, even at low thrusts, it can still have a high fuel efficiency and specific impulse. For example, NASA’s Evolutionary Xenon Thruster (NEXT) only uses .000012 pounds of fuel per second. Even with a high specific impulse of 4,190 seconds, it only creates .053 pounds of thrust. The space shuttle, on the other hand, uses over 1,850 pounds of fuel per second and produces 1.5 million pounds of thrust. That’s a specific impulse of only 300 seconds.