Why does a rocket launch into space?

The edge of space, commonly set at 100 km (the Kármán line), is only about an hour's drive straight up. What makes it hard to reach is not the distance but the speed needed to stay there. An object that merely climbs to 100 km falls back within minutes. To remain in orbit it must move horizontally at roughly 7.8 km/s (28,000 km/h or 17,500 mph), fast enough that as it falls toward Earth the ground curves away beneath it. Reaching that speed, plus making up the energy lost to gravity and air drag on the way up, is the entire purpose of a launch.

Aircraft engines breathe air and wings need air to generate lift; above about 30 km there is too little of either. A rocket carries its own oxidiser and works by Newton's third law: it throws hot gas backward at several kilometres per second and the vehicle is pushed forward by the same force. This works equally well in vacuum, which is why rockets are the only propulsion able to complete the climb. The price is mass: an orbital rocket is typically 85 to 90 percent propellant at liftoff, so engineers discard empty tanks in stages to avoid accelerating dead weight.

A launch begins vertically to climb through the densest air quickly, then tilts over in a gravity turn so that most of the thrust goes into building horizontal speed. After two to ten minutes the final stage cuts off at the target velocity and the payload is in orbit. Going farther, to the Moon or the planets, means adding a few more kilometres per second, which is why interplanetary probes are small compared with the rockets that carry them.