3. What is the top speed of a VASIMR® engine?
VASIMR® does not have a maximum speed. Any rocket engine will continue to accelerate its spacecraft until it runs out of fuel. On Earth, an airplane has a top speed because at some velocity the drag from air friction matches the maximum force that the airplane engines can apply. Space is a vacuum and hence there is no air friction to slow spacecraft down. VASIMR® thruster is very efficient with its fuel, so it can attain very high velocities before it runs out of fuel. For example, a 200 kW VASIMR® spacecraft designed to sling-shot a payload to Jupiter will attain a velocity relative to the Sun of ~50 km/s before releasing its payload and returning to Earth orbit.