Sailing Through the Solar System

Say you want to get around, but there is no ground to push off of, and no fluid to swim through. What's a weary traveler to do? Well, you could always bring some stuff with you to throw away-- conservation of momentum says that if you throw mass in one direction with a given speed, you gain speed in the opposite direction. This is the very key and core of the so-called Rocket Principle. Rockets are, right now, the only game--but you might have noticed that there aren't many PetroCan stations on Mars or the moon with which to refuel our rockets, and to drag all that fuel along, you're going to need a bigger rocket. Which is going to need more fuel, and to drag all that fuel along, you're going to need a bigger rocket, which is... well, you get the idea. This has lead to a number of schemes being proposed for in situ (on site) propellant production, using local materials and possibly some ingredients you dragged along-- the trouble is that you usually have better things to do with any of these resources to throw them into the inky blackness of space. Hydrogen and oxygen are the best rocket propellents, and it is convenient enough to make them out of water-- but at the same time, you could also drink that water, and oxygen is pretty nice stuff if you happen to be the kind of space traveler who enjoys breathing. Another minus is that chemical rockets are never going to get you going around the solar system very fast, anyway. Six months to Mars, a couple years out to Jupiter. This is because you cannot possibly carry enough rocket fuel to cover your whole way under power: you fire your rockets at the beginning, to get up to speed, again at the end to slow down (no friction in space: nothing to rub against, so you do not slow down) and the rest is up to god and Sir Issac.
If you want to get around the solar system faster than the scale of years you get with rockets, and with more control over your trajectory, you need to find someway to give yourself a constant push-- even a feather-light touch will add up to enormous speeds, if you keep it up over time. To do this without expending mass, you have to go sailing.
Yes, sailing. Clipper ships in the sky, and all that.
There are three types of sails: the most commonly known, and the oldest (dating way, way back to Johannes Keplar himself) is the photon sail, A.K.A the 'solar sail,' which relies on the pressure of sunlight pushing against a (very) large, (very) thin and light area of very shiny sail. Once again, the sail relies on conservation of momentum: light can be imagined to consist of a great many tiny particles, called photons, and each of these posesses a tiny momentum related to its colour (not velocity*mass like normal particles, since light always travels at the same speed, ~300 000km/s). When the photon hits the very shiny sail, it either bounces off, or it is absorbed. If it bounces off, a tiny amount of its momentum is aquired by the sail (and the light bouncing off the sail is thus shifted ever so slightly towards the red end of the rainbow)-- it isn't much, since the sail and attatched spacecraft is infinately more massive, but there are billions of photons in the average sunbeam. This adds up to a small but measurable pressure along the entire area of the sail, enough to make things move. Of course, the more massive an object the harder it is to make it move, so there is a maximum weight/area ratio for a workable sail, and it is very, very small. This necessitates the use of exotic materials: thin films of alumanized plastics, like a space blanket only a few atoms thick. And they have to be big: acres or kilometers in area, depending on how big of a payload you want to move. Solar sails see limited use in Earth Orbit to help orient satellites, but have not been tested for large scale propulsion due to the problem of assembling or unfolding such a large, thin and delicate structure in orbit. (Perhaps more importantly, when a prototype was launched, the rocket blew up.)
Solar sails, ironically, have absolutely nothing to do with the solar wind, which is a stream of charged particles (mostly protons, electrons, helium and other light nuclii)--these particles penetrate the thin sail without depositing any momentum that might aid in propulsion (but fortunately without damaging the sail: the particles in the solar wind are small enough to fit between the atoms). There are two forces that can interact with a charged particle: the electric force and magnetism, and there are designs for sails which rely on each.
Both are quite simple, and work in the same general way: instead of a big, expensive and delicate sail stretching out to catch the sun's light, we can create an even larger electric or magnetic field to interact with the solar wind, instead. The magnetic sail would see a hoop of superconducting wire cast out around the craft, supported by its own magnetic field. The main downsides are that the superconductors that would work are nearly as expensive as exotic solar sail materials, and it requires an investment of power to set current running through the superconducting loop (but nonethereafter: that is why we picked a superconductor). The upshots tend to outweigh the downsides: a magnetic sail will tend to weigh less and thus be much more efficient than a solar sail for the same area and trust, and also will tend to deflect the particles in the solar wind that it interacts with. This is a good thing, as another word for "charged particles" is "radiation," and we all know that too much radiation is never a good thing. For crewed ships (that don't want to end up crewed by the X-men), that benefit alone justifies the cost of dragging along superconductors; when the superconductors drag you along, and faster than a rocket could, well. That's an orgasm made of fair-trade chocolate, is what it is.
After that stunning endorsement, you might not be suprized to guess that the electric sail does not quite measure up, at least not when people are involved. The sail works like this: start your spaceship spinning, and play out a bunch of fine wires like spokes on a bicycle wheel: the centrifugal force from the spinning will keep them straight. Using an electron gun, give the works a strong positive charge: this tends to deflect protons and alpha rays (helium nuclii), which gives the sail momentum. It also attracts negatively charged electrons, which are captured and contribute even more to thrust--but high energy electrons are radiation, too. While the bonus is that electron guns can be made quite light and power-thrifty, and that the wires in the electric sail can be made of cheap, lightweight aluminum, the downside is that you've become a beta-ray magnet. Unless someone wants to start a Reed Richards Experience Tour, the electric sail will have to be relegated to use in transporting cargo and automated probes. That said, for cargo and probes the electric sail is the best bet: it is cheaper and possibly lighter than either electric or magnetic sails, per swept area, though the higher power requirements may dwarf that advantage.
That power source can easily be solar as well, at least in the inner solar system, but once you get out past the orbit of Mars, you hit double jeprody from the inverse square law. You see, both the strength of the solar wind pushing you along, and the solar sails you want power from fall off as the square of the distance to the sun: (intensity ~ 1/r^2) and soon you have neither enough power or enough wind to get anywhere. Using a solar sail, or a power source not reliant on the sun, will allow you to get out further (perhaps as far as the orbit of Saturn)-- but in the outer dark, sailing definately loses its appeal.
Of course, since mankind has yet to leave the kiddy pool of the Earth-Moon system, worrying about what happens once you get past Saturn is a bit accademic. There's plenty of room to grow in the Astroid Belt and Jovian Space before anyone except scientists and the most intrepid of explorers have to worry about the limitations of sails, and for those applications they can be conqured: a photon sail works on any light, not just the sun, and will work just as happily under the influence of a laser beam shone out from more sunward environs. Without having to carry fuel, this means you can fly your probes as far out as you want, for missions as long as you want: even to the stars, if patient enough you be.
Those who love the romance of creaking ropes and whistiling rigging can take heart: the Age of Sail has not even yet begun. Arthur C. Clarke once said that if the human race is to survive, for all but a tiny period in its history the word 'ship' is going to have to mean 'space ship'-- those ships may very well have sails and masts, and those words are going to change as well.