HOW COME? Hot gases, high pressure propel rockets

How can a rocket move in the vacuum of space? asks a reader.When you blow up a balloon and pinch the opening shut, you're actually holding a homemade rocket. Let go, and it jets wildly around the room, finally landing in a deflated, wrinkled heap.

But balloons aren't the only things that exhibit rocketlike behavior. Has a crazily forceful sneeze ever caused your head to snap back, cartoon-character style? If the answer is yes, consider yourself a human rocket, at least for a moment.

The balloon flew around the room because its air escaped in a steady stream, forcing it forward. A rocket's streaming gases work the same way. (Think of the space shuttle, as its boosters lifted it off its launchpad.)

But it seems like those escaping gases should need something to push against -- like the launch platform, or the air -- to shove a vehicle away. Sort of like how a swimmer might thrust herself away from the side of the pool, using her feet.

But a rocket acts less like a swimmer and more like an old-fashioned cannon, "kicking back" each time a cannonball is fired out.

Like your head during a hard sneeze, an object moves when something is thrown away from it, with great force. When matter rushes out in one direction, the object it's ejected from is pushed in the opposite direction.

The way rockets work is actually explained by some of the basic principles of motion in our universe. These "laws," first described by British scientist Sir Isaac Newton in the late 1600s, apply to moving objects (and the forces that affect them) everywhere.

Newton's second law noted that the force of a moving object depends on its mass (the amount of matter it contains) and its acceleration. So to make a good rocket, make sure it's spewing out a large mass of speeding matter.

Newton's famous third law says that for every forceful action, there's an equal, opposite reaction. In a rocket, hot gases, under high pressure, jet out of the exhaust nozzle like air from a balloon. The forceful reaction: The rocket is shoved forward, just like a firing cannon is kicked back.

And escaping gases don't need a launchpad or air to push against. In fact, when the gases encounter other gases in air (let alone a solid object), their acceleration drops -- slowing the rocket- powered vehicle, too.

In the emptiness of space, with no air to stand in the way, gases rushing out of a rocket accelerate effortlessly. With nearly no friction to slow a vehicle as it moves, even a small thrust from the engines can send it gliding off into the great beyond.