"It is difficult to go to the stars, but it is not impossible," - Robert L. Forward
All of the other propulsion systems discussed on this website are some form of rocket engine. However, it is not necessary to use a rocket to achieve propulsion... even in the depths of outer space. What follows is a discussion of a different concept of Starship propulsion... light sails.
A time tested method of getting from place to place on the Earth is by sailing ship. A sailing ship needs no engine because it gets its power directly from the wind and water. Similarly, a light sail starship carries no engine. Its source of power comes from the unlimited power of the sun. One possible design for a light sail would use the light directly from the sun, for travel within the solar system. Unfortunately, such a ship would only be capable of moving away from the sun. Tacking doesn't work in space, because there is nothing for the ship to push against to redirect its motion.
A better design for a light sail starship would utilize a microwave transmitter that stayed near the sun. The transmitter would absorb the intense sunlight and retransmit it as microwaves (which have the advantage of being able to be produced efficiently). Microwaves spread out easily, however, so any long distance starship using this propulsion method would have to do all its accelerating near the solar system. Therefore, to achieve a high velocity (to make the transit time reasonable) the ship would have to be extremely light... say, on the order of a single ounce. This is obviously impossible for a manned starship, but it would be possible to make an interstellar probe that was this light.
The design for such a lightweight, microwave driven probe is fairly simple. Figure 11 shows the general concept (dubbed "Starwisp" by Robert L. Forward):
The microwave transmitter (gray cylinder) transmits the microwaves towards a microwave mesh Fresnel zone lens (green) which focuses the microwaves on to the Starwisp (red). The Starwisp consists only of kilometer wide mesh sail weighing only sixteen grams. In addition, the Starwisp would carry 4 grams of microcircuitry. The Starwisp would be accelerated at 115 Gs for a few days by a 10 gigawatt microwave transmitter, reaching 20% light speed. When the Starwisp was nearing the system of interest, the microwave transmitter would be turned on again. Starwisp would then receive the microwaves and convert them into electrical power to run its on board microcircuits. As the probe flew through the system, it would take pictures and use its own mesh sail as a microwave antenna to send the images back to Earth.
An interstellar probe like the Starwisp isn't capable of carrying much payload, let alone humans. However, there is a light pushed sail concept that can. If high-powered lasers are used, in conjunction with a Fresnel zone lens, an energy beam can be focused at distances more than 40 light years away. In addition, Robert L. Forward has designed (and patented) a light sail starship that would be capable of carrying humans on a round-trip mission to a nearby star (such as Epsilon Eridani). The basic concept is shown in figure 12:
Both the initial light sail, and the Fresnel lens would be 1000 kilometers in diameter (approximately a twelfth of the Earth's diameter). The total weight of the Starship would be 80,000 tons, including a 3,000 ton payload. The light sail would have 3 sections, as depicted in figure 12. During the launch phase, all three sections would act as a single light sail. When the Starship got near its destination the outer-most ring would disconnect, and act as a focusing mirror to focus the laser light onto the remaining two sections, thus stopping the spacecraft. When it was time to go home, the middle section would disconnect and also perform the function of focusing mirror. When the ship got home, its small, remaining sail would be used to stop the craft at Earth.
The power required to accelerate such a craft would be 43,000 terawatts. This would allow a 1/3 G acceleration. If accelerated for 1.6 years, the craft could reach a velocity of 50% light speed.
Obviously a light sail craft such as this would be enormously expensive, and require a colossal feat of engineering. However, it is possible. The lesson to be learned here is that if we really wanted to go to another star system, we could. The technologies required for the Multi-Stage Light Sail are within our grasp. What's needed is money and willpower. However, in the long run, the Antimatter Drives will probably end up being less expensive and more versatile than the laser sail.