Historically, most rockets have needed to use all of their available fuel in order to get their payload into space. SpaceX rockets were built from the beginning with reusability in mind—they have enough built-in fuel margin to deliver a Dragon to the space station and return the first-stage to Earth. That extra fuel is needed to reignite the engines a few times to slow the rocket down and ultimately land the first stage after it has sent the spacecraft on its way. In addition to extra fuel the rocket has small, foldable heat-resistant wings called grid fins needed for steering the first-stage as it plummets from the edge of space through Earth’s atmosphere, cold-gas thrusters on the top of the first-stage that are used to flip the rocket around as it begins its journey back to Earth, and strong but lightweight carbon fiber landing legs are deployed as it approaches touchdown. All of these systems, while built and programmed by humans, are totally automated once the rocket is launched—and are reacting and adjusting their behavior based on incoming, real-time data.