Ion engines have been a designed item since at least the '80s, but until just recently, they had not been tested or used. They work by way of a complicated engine, with a simple idea behind them. The idea comes partly from a Newtonian equation, and partly from the capability of adapting its meaning. The formula allows you to put in the exhaust velocity, mass, and other such things, and come out with specific impulse. The equation says that the higher the velocity and mass of the exhaust expelled from a rocket, the higher the specific impulse. Now, why can't you shoot a lead ball out of the back of your space craft and have it go? You can, and it would work relatively well, because you've got the mass part of the equation down.

Here's how the ion engine uses this principle: You send an electric charge through some sort of matter (usually Xenon gas), the particles of the matter become highly excited, or ionized, and bounce around with a great amount of speed. Next, you direct the ions through some sort of a nozzle (usually a magnetic field, or something else not able to be touched). The ions are excelerated further by this nozzle, and are turned into an exhaust the width of a single ion. The effect of the ions is slow, but if you keep it up for a year, a speed gain of around 7000 mph can be had. Think of it like floating in your space suit out in space, and throwing pebbles. Believe it or not, physics would make it so that you would accelerate in the opposite direction of the pebbles you throw. Obviously, you've got more mass being thrown than an ion, but the ion also has 99,970 mph on your velocity. Anyway, if you throw pebbles all day, or even week, you might reach 60 mph. The advantage is that it only takes 5,000 pebbles, when you might carry 1,000,000 in a container the size of you! That means that you could continue throwing your pebbles (assuming you never get tired, like the engine) for 200 weeks, that means that you could get going 12,000 mph with only as much fuel as will fit in a you sized container!

Obviously, if you want to go to pluto, and your payload is light and non-human, this propulsion is ideal. If you can carry 1,000,000,000,000 or more xenon atoms, which you turn into 1,000,000,000,000 xenon ions, you can get going fast. Say that you gain 100 mph for every 1,000,000 ions, than you can reach 1,000,000 mph before your fuel runs out. However, once you figure in inertia, and the time it takes to expell 1,000,000,000,000 ions one at a time, your ending speed to Pluto will be considerably less. The most promising thing, though, is that 1,000,000,000,000 xenon atoms is a considerable understatement of a craft's capacity, just as is the 1,000,000 ions per 100 mph.

The practical applications of this space drive have already been realized. Besides numerous experiments, the ion drive is riding aboard, or driving, the Stardust Spacecraft, one of NASA's newest data collectors. The applications of this peculiar, but amazing, technology are just being realized. But don't expect to ride on a ion powered 777, because the thrust-to-weight ration is to low for atmospheric travel (in rockets the ratio must be more than 1:1, and on planes, it must be enough to counter-act air friction, and reach 140 mph).