Everybody knows what a rocket is, right? Rockets are the big white cylinders strapped to the sides of the shuttle gas tank, and they're the black nozzles on the back of the shuttle itself, right? Quite correct, but do you remember Deep Space 1? This long-range probe used an engine that looked and behaved quite a bit differently than you might expect.
After all, what makes a rocket? Is it the shape? The propellant? The use we put it to? Very simply, and perhaps boringly, a rocket is defined by the way we do the math. Let's look at Newton's second law in its general form F=d(m*v)/dt. If that doesn't mean anything to you, don't panic. All it says is that the Force on an object is equal to the time rate of change of the quantity (Mass times Velocity). So, force is related to how we change two things, mass and velocity, ok?
Many people (freshman engineering students) who use the second law just jump a few steps and say F=ma, where they've shortcutted by saying the mass is constant and we only change velocity (and change of velocity is acceleration). Simple and easy, and it works great for billiard balls, pulleys and ropes, and lots of other things we see often and have a good intuition about. But they've just skipped right over rocketry.
Rockets change their mass to make them go. That's all there is to it. A rocket is a rocket because it doesn't shortcut Newton's law.
OK, great, but what does that mean, and how does it relate to Deep Space 1?
To be a rocket, you throw mass out the back. It's as simple as that. Do we care what kind of mass we throw out the back? Not really. So here's where DS1 comes in. It threw ions. That's right. Little bitty charged atomic-size particles. The engine on that probe, called NSTAR, looked like a flashlight when it was on. It glowed bluish-white, the color of excited xenon gas, which you may have guessed by now, is the propellant.
Atom by atom, DS1 shot xenon gas out the back at speeds of ~40556 m/s (compare to the unclassified SR-71 Blackbird top speed of 1062 m/s). Each Xe atom has a mass of 2.2x10^-19g, and each atom is leaving the spacecraft, flying backwards (here's where our dm/dt comes from). So, each atom "pushes" the spacecraft with a force of 8.9x10^-18N. This is teeny-tiny, of course, but it's a good thing that there are so many atoms out there. 1g of xenon generates 5.4x10^6N of thrust! Think about that. If you could hold up 15 fully-loaded tractor-trailers, you could push DS1 with as much force as one gram of xenon out of NSTAR.
That's the common ground on rockets, folks. They shoot stuff out the back. The faster they can shoot it, the more efficient they will be. The more they can shoot, the harder they can push. So the next time you see a satellite launch, or watch Discovery or Endeavour's engines blasting away, think about all the little gas particles bouncing out of the back of the engine and how there's an equal and opposite reaction pushing the vessel forward. That's all there is to it.