What are space rockets?

A rocket works thanks to Newton's third law. It burns fuel and oxidizer in a chamber, and the gases exit through the nozzle at several kilometers per second; the vehicle receives an equal and opposite thrust. Unlike an aircraft jet engine, it carries its own oxidizer on board, so it doesn't depend on the atmosphere and keeps thrusting in space. In fact it performs better in a vacuum, where the nozzle can expand the gases without atmospheric pressure pushing back.

Reaching orbit requires about 7.8 km/s, and Tsiolkovsky's rocket equation dictates that to gain that much speed, propellant must make up most of the mass: in a typical launcher it's around 90%. To avoid hauling empty tanks, the vehicle is divided into stages that are jettisoned once spent. Each successive stage is smaller and starts out with the speed given to it by the previous one. This scheme, proposed in the early 20th century, remains the one used by every launcher in service today.

Depending on the propellant, there are solid rockets, simple and powerful but not throttleable, and liquid rockets, more complex but controllable and able to be shut down; many launchers combine both. By mission, suborbital rockets, which go up and come back down without reaching orbit, are distinguished from orbital ones, capable of placing satellites, capsules, or probes into a stable orbit or on an escape trajectory. From Sputnik in 1957 to today's constellations, everything humanity has sent beyond the atmosphere has traveled on one of them.