The maximum speed for a space rocket depends on many factors, including how much fuel it has, its current weight and its position relative to objects in space at the time of measurement. Currently, the fastest rocket is Voyager 1, traveling in space at over 38,500 miles per hour.
As a rocket flies through space, it burns fuel, becoming lighter; the decrease in weight allows it to move faster. All the points of reference for determining how fast an object travels in space are themselves in space, so they are also moving at a very high rate of speed. The rocket equation used by NASA applies laws of physics to the ever-changing conditions in and around a space rocket to determine its speed.