Since the invention of gunpowder in China more than seven centuries ago, humans have sent cylinders soaring into the skies with the help of controlled explosions. These craft and their engines, called rockets, have taken on many roles as fireworks, signal flares, and weapons of war.

But since the 1950s, rockets also have let us send robots, animals, and people into orbit around Earth—and even beyond.

As tempting as the logic may be, rockets don't work by “pushing against the air,” since they also function in the vacuum of space. Instead, rockets take advantage of momentum, or how much power a moving object has.

If no outside forces act on a group of objects, the group's combined momentum must stay constant