This paper looks at some of the constraints and drivers that I think apply to interstellar exploration using technologies which are currently available in a basic form. The ideas are based on developing those technologies to what I consider are reasonable levels. While you may disagree with how far they can be taken, such differences should only affect the quantitative results rather than the basic conclusions.
Obviously the problem going beyond the Solar System is the distances involved and any project must maximise the speed. That in turn means that launch energy requirements are high and in most approaches the cost of launch energy is a limiting factor. The big problem with using rocket-style launch systems is that the fuel must be accelerated as well as the payload. A much better approach is to keep the fuel at the launch site and only accelerate the payload. Probably the best known of such techniques are based on 'solar sails', either powered by sunlight or some artificial energy source such as a laser or a microwave beam. For this proposal, I assume use of solar sail propulsion.
Another advantage of a sail based system is that there is no engine to carry which helps minimise the weight of the craft. This however results in the next problem - how to stop once the craft reaches its destination. It is possible to devise systems based on very powerful (and very expensive) arrays of lasers, microwave transmitters, etc. that can launch the craft at high speed. Unfortunately there is nothing equivalent at the other end of the journey to stop the craft. It may be possible to use some form of magnetic drag using plasma-based technology but this is in its infancy and gets back to the problem of providing fuel to run the braking system and relies on the magnetic field characteristics of the target star being well known. Even if suitable technology can be developed and these risks are acceptable, the mass of the braking system must still be accelerated by the launch system, further adding to the costs. Using only direct solar radiation on the sail for the launch has a major advantage: assuming the target star is of comparable brightness, the same radiation pressure on the sail will also stop the craft. This means that no braking system is needed other than the sail itself, which has to be there anyway for the launch, and I will therefore assume this method of stopping.
Other combinations of launch system and braking techniques could allow higher launch forces, but this would be partly offset by the increased mass of the braking system and the possible overall gain in speed is therefore questionable (IMHO). I leave it to others to consider whether such systems would give a worthwhile improvement.
The sunlight hitting a simple sail will fall as the craft moves away from the Sun as the inverse square of the distance. At the same time, the craft is accelerating and these effects combine to give a rapid decline in acceleration. Effectively, for the very high speeds we will consider, all the acceleration takes place within the first few hours after launch. The critical factor is therefore to achieve a high initial acceleration. Ignoring the payload, changing the size of the sail has no effect on the initial acceleration because the total force and mass are both proportional to area, We can therefore always achieve the maximum performance, that of the raw sail material, by making the mass of the payload much less than that of the sail. In reality however, there are practical engineering limits on the sail size and that limits the payload mass.
On the other hand, it is very easy to increase the initial acceleration simply by starting closer to the Sun. Starting at half the distance gives four times the acceleration so we should start as close as possible. The limiting factor is temperature at which the sail melts or at least loses its strength to the point that it will break up during launch. The sail will heat up because the material is not a perfect reflector. For example, if 97.5% of the incident radiation is reflected, 2.5% will be absorbed. That power heats up the sail to the point where thermal radiation from the back surface balances the energy absorbed. The best we can achieve is to match a perfect 'black-body' radiator by applying a suitable coating on the dark side of the sail. Incidentally, this will also absorb any radiation which might have passed through the sail due to partial transparency, an effect that can therefore be neglected.
The total radiated energy for a black-body scales as the fourth power of the temperature so one approach is to choose the highest temperature material we can find. For example, tungsten has a melting point of 3640K so it could in theory withstand 400MW per square metre assuming 97.5% reflectivity. However, that level of radiation is not possible using sunlight, and building power stations to achieve even illumination at that level over say 3 square km would be prohibitively expensive. The initial acceleration would be 14g and the final velocity 0.1%c reaching 95% of that value within 2 hours.
Tungsten is a very dense material at 19.3g/cc, or 19.3 gsm (gm per square metre) for a 1m thick sheet, so if the power source is to be sunlight, the final speed may not be as good as using a lower temperature, low density material if launched from a reasonable distance. Wright has suggested that, by manufacturing the sail material in space, areal density as low as 1.17 gsm could be achieved. Assuming a maximum temperature of 450 K and the same reflectivity of 97.5% suggests a launch from 0.13 AU, still well inside the orbit of Mercury. The initial acceleration would be only 0.05gm/s^2 but the final speed would still be 0.05%c because the force would be applied for longer. In fact the probe would take over 250 hours to reach 95% of its final speed.
A possible launch system might use a carrier vehicle consisting of a large, heavy, reflective metal plate shielding the sail, with louvres which could be opened to let the light through. The combination would be set on an elliptical orbit with the desired aphelion and the louvres opened at that point. The acceleration is such that the sail would be forced clear of the carrier vehicle within seconds and would move almost radially out from the Sun while the launch vehicle continued its orbit back out to Earth (or wherever it started).
Given that a speed of 0.05%c or better can be achieved with no fuel cost, no engine or braking system payload and using materials which could arguably be produced with current technology, the question then is whether it is worth spending large sums on possible alternatives. At this speed, we can send a probe to the nearest stars in less than 10,000 years. The question then is "How much is it worth spending to reduce the time by half to only 5,000 years?" I know of no current technologies that hold out any hope of reducing the time to less than 500 years even with the most optimistic assumptions about limits of development but I leave it to the reader to consider the benefits of other approaches.
Assuming the materials suggested by Wright, what payload mass could we reasonable launch? Taking as an example a circular sail of 400m diameter which is perhaps impractical today but is credible as a development target, the mass would then be 368kg. Adding a payload of 100kg would reduce the speed by about 12% and increase the travel time to say Alpha Centauri by 1300 years.
A possible Low-mass Payload.
A Java Solar Sail Calculator.
Another overview of solar sail physics.
Carbon fibre as a sail material.