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COPYRIGHT 2005 Natural History Magazine, Inc.
In daily life you rarely need to think about propulsion, at least the kind that gets you off the ground and keeps you aloft. You can get around just fine without booster rockets--simply by walking, running, rollerblading, taking a bus, or driving a car. All those activities depend on friction between you (or your vehicle) and Earth's surface.
When you walk or run, friction between your feet and the ground enables you to push forward. When you drive, friction between the rubber wheels and the pavement enables the car to move forward. But try to run or drive on slick ice, where there's hardly any friction, and you'll slip and slide and generally embarrass yourself as you go nowhere fast.
For motion that doesn't engage Earth's surface, you'll need a vehicle equipped with an engine stoked with massive quantities of fuel. Within the atmosphere, you could use a propeller-driven engine or a jet engine, both fed by fuel that burns the free supply of oxygen provided by the air. But if you're hankering to cross the airless vacuum of space, leave the props and jets at home and look for a propulsion mechanism that requires no friction and no chemical help from the air.
One way to get a vehicle to leave our planet is to point its nose upward, aim its engine nozzles downward, and swiftly sacrifice a goodly amount of the vehicle's total mass. Release that mass in one direction, and the vehicle recoils in the other. Therein lies the soul of propulsion. The mass released by a spacecraft is hot, spent fuel, which produces fiery, high-pressure gusts of exhaust that channel out the vehicle's hindquarters, enabling the spacecraft to ascend.
Propulsion exploits Isaac Newton's third law of motion, one of the universal laws of physics: for every action, there is an equal and opposite reaction. Hollywood, you may have noticed, rarely obeys that law. In classic Westerns, the gunslinger stands flat-footed, barely moving a muscle as he shoots his rifle. Meanwhile, the ornery outlaw that he hits sails backward off his feet, landing butt first in the feeding trough--clearly a mismatch between action and reaction. Superman exhibits the opposite effect: he doesn't recoil even slightly as bullets bounce off his chest. Arnold Schwarzenegger's character the Terminator was truer to Newton than most: every time a shotgun blast hit the cybernetic menace, he recoiled--a bit....
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