Can a plane travelling in space be faster than another plane travelling at the same speed on earth?
A plane in space will not be faster than a “plane traveling at the SAME SPEED on earth”. On the other question The speed will be limited by the available energy to propel the plane. The closer you get to the speed of light the more energy you need to move the vehicle. In theory I believe you would need an unlimited amount of energy to reach the speed of light.