Interstellar flight became possible with the invention of the dual-drive system. Starships use sublight drives to leave a planet's gravity well, using the hyperdrive to travel between stars at faster than light speed. Plotting a course to a star system was a highly complex operation. In order to achieve success, calculations to travel at light speed were left up to a navigation computer or astromech droid. A common sublight drive was the ION engine, which had a high power to weight ratio, and were used in Imperial TIE fighters and Star Destroyers.