Can this be done feasibly (with current/near term emerging technologies and sub-trillion dollar budget)? Assuming that the human race survives the 21st century, I think we'll have the resources for a multi-decade unmanned flyby mission to Alpha Centauri even if technology shows little progress in these 100 years. To get information back to earth, we need a very powerful transceiver so that the signal/noise ratio would be high enough to pick up at interstellar distances. The Cassini probe had a 20 W transmitter and 4 m parabolic antenna and could do 166 kbps from Saturn at 9 AU. At Alpha Centauri, we'd be at 4*63*10^3 AU. This is clearly not feasible. Let's do lasers instead. A lightyear is about 10^12 km. With a 300 nm laser, the divergence of this laser, using the rough estimate new radius = distance*wavelength/original diameter, assuming a 1m across transmitter, would mean a 2.e+10 m radius beam by the time it got to earth from 1 lightyear. Quadruple that for 4 lightyears. Let's say we E=nhf with a 1000 kW laser. 1.51 e+24 photons emitted. We have roughly 10^22 square meters of the beam. There's going to be 151 photons per square meter. This is detectable! We can communicate with the probe and if we engineer the mission such that a gravitational swingaround can be achieved, we'll be able to pick up the signals much easier on the return voyage; if we do a pure flyby, the signals will get weaker and by the time we can communicate with the probe it would be even further away. To prevent the satellite from destroying the receiver, within the solar system we need a backup radio system, and only after we're close to the lightyear level will the high powered laser be used. To get to Alpha Centauri within 5 decades we need 0.1c propulsion. Let's set a 10 ton mass budget. All instruments, sensors, flight actuators, communications and mission computers can be thought of as roughly 1 ton. The power source can be a small 10 MW pebblebed reactor+heat engine weighing in at 5 tons. To change direction, the probe can expel used pebbles in a selected direction. The rest of the tonnage should be a very large light sail. I'll use a Wikipedia estimate http://en.wikipedia.org/wiki/Interstellar_travel and say that we'll hit 0.11c at 0.17 lightyears with a 10 ton probe for a 650 GW laser on the moon (to keep maximum line of sight at all times). To get the probe to a distance where the 650 GW laser doesn't melt the sails, we'll assemble the whole thing in space and use booster rockets and gravity assists at first. The funding for the whole mission would be 1 trillion 2012 USD equivalents in 2100, paid with a global tax on the investment banks, with the moon based laser propulsion/receiver taking up the bulk of the 1 trillion. The actual probe itself would probably be only a few hundred billion dollars. What do you guys think? Alpha Centauri mission doable or not?