Rockets are the only way humans have been able to launch objects from Earth into space so far. A rocket is a vehicle system that carries all of its own propellant. The propellant is accelerated to a high speed—usually by combustion, converting it into gas and heating it up—and pushed out the back of the rocket as exhaust. The rocket, following Newton’s third law of motion, is pushed forward by the motion of the exhaust.

Most launch vehicles consist of a series of successively smaller rockets placed one on top of the other. The largest rockets provide the most thrust, but are also heaviest; so once their fuel is expended, they are released away from the other, smaller rockets, which then have much less mass to push. With this successive downscaling of the launch vehicle’s mass, the payload—usually a spacecraft or satellite—can reach the speeds high enough to get into space, reach orbit, or escape Earth’s gravity toward other locations or objects in the universe.