A spacecraft leaving the surface of Earth needs to be going about 11 kilometers (7 miles) per second, or over 40,000 kilometers per hour (25,000 miles per hour), to enter orbit. It is known as the orbital velocity, referred to in German as the 'first cosmic velocity' – it corresponds to more than 20 times the speed of sound. At the start of the space age, Russian scientists applied the term 'cosmic velocities' to certain velocities that are important for space exploration. The 'first cosmic velocity', known as the orbital velocity, will bring a rocket or other projectile into orbit around the Earth. A slower projectile will fall back to Earth.