Earth From Space

Space versus orbit
To perform an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. A spacecraft has not entered orbit until it is traveling with a sufficiently great horizontal velocity such that the acceleration due to gravity on the spacecraft is less than or equal to the centripetal acceleration being caused by its horizontal velocity (circular motion). So to enter orbit, a spacecraft must not only reach space, but must also achieve a sufficient orbital speed (angular velocity). For a low-Earth orbit, this is about 7,900 m/s (28,400 km/h; 17,700 mph); by contrast, the fastest airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.
Konstantin Tsiolkovsky was the first person to realize that, given the energy available from any available chemical fuel, a several-stage rocket would be required. The escape velocity to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,000 m/s (39,600 km/h; 24,600 mph) The energy required to reach velocity for low Earth orbit (32 MJ/kg) is about twenty times the energy required simply to climb to the corresponding altitude (10 kJ/(km·kg)).