As mentioned in another recent thread, for the foreseeable future it will be easier to build better telescopes than send a payload to another star system. If you can build an interstellar probe you can also build a telescope with a mirror kilometers in diameter, linked with another telescope hundreds of millions of kilometers away to form an interferometer. But let’s say that’s been done, and we hit paydirt: an oxygen-atmosphere planet in the Alpha Centauri system has been detected and surveyed as thoroughly as possible from our solar system. So given a high-value target and the closest possible interstellar mission, how do we go about it?
Everything depends on just what propulsion technology is available. A safe bet is that if we are in a position to even consider it we have either fusion, advanced solar beamed-power, or both. A pure fusion rocket with a plausible mass ratio might be able to accellerate to 5% of lightspeed and slow to a stop again at it’s destination. So that’s approximately 90 years to A. Centauri. If some sort of lightsail or magnetic propulsion is available, that might improve either the trip time or the payload ratio. Antimatter is a much longer shot- it depends on just how efficiently it can be synthesized. Perfectly ridiculous amounts of energy would be needed to create large amounts of antimatter, although an extremely energy-rich solar civilization might be able to afford it. Smaller amounts might make it possible to build “catalyzed-fusion” designs with higher performance than a pure-fusion rocket.
Presumably by the time a space-based civilization is cabable of considering an interstellar probe, design will be mature and tested enough to function reliably for the length of the misssion. Communication back to Sol system won’t be a problem: given nuclear power and the ability to unfold a large antenna dish, it could be done with today’s microwave technology.
If the travel time is anything more than a couple of decades (very likely), then some sort of human-level intelligence will be necessary at the end of the trip. Either a very advanced AI, or if hibernation is perfected, a suicide volunteer for a one-way trip(!)
Actually the movie Avatar did a very good job of portraying the most plausible foreseeable technology capable of transporting people to a nearby star system.