The article ('How Humans Will Travel to Alpha Centauri') in the current issue of ASTRONOMY magazine enticed the unwary reader with powerful overtones of "possibility" but mostly to the exclusion of feasibility-practicality. Basically, yeah, we can currently get to Alpha Centauri at about 4.3 light years away, if you are prepared to take 76,000 years to get there! (This is assuming a constant speed close to Voyager I's or about 37,100 mph). As the author of the piece dryly observes:
"So we already know that a decades-old probe could, technically, reach the Alpha Centauri system if we're willing to wait long enough and if it continues functioning."
A couple of very big IFs there! And, for reference, 76,000 years surpasses the life phase of all human civilizations, and is about half as long as modern Homo Sapiens has existed. In other words, it's a serious length of time but about what one could expect using an energy density of 100 megajoules per kilogram or 100 MJ/kg. At that low energy density, according to one space propulsion specialist, Andrea Tziolas, to get to Alpha in 100 yrs. would "require fuel tanks 55 time larger than the mass of the observable universe."
Nuclear power which gins up the energy density by tenfold (to 100 Terajoules/ kg) would be much better- likely relying on the fission of uranium and plutonium nuclei. Trouble is, the putative vehicle would require hundreds of thousands of tons of mostly hydrogen gas to fuel the reactions. Also, since one would therefore see extremely high temperatures on the order of hundreds of thousands of degrees Celsius, the design would need to incorporate more advanced materials to support ingenious cooling systems beyond anything we now have.
Nuclear fusion power trumps fission, and was at the basis of Project Daedulus - the 1970s concept proposal by which an unmanned probe could have reached Alpha Centauri. In this case, energy densities of 300 TJ/ kg could be reached via deuterium- helium 3 fusion, e.g.
D2 + He3 -> He4 + H1 + 18.4 MeV
The key problem in supporting this fusion reaction is the extreme scarcity of He 3 on Earth, which would require mining it from the atmospheres of the gas giant planets: Jupiter, Uranus, Saturn. Assorted estimates for such a fusion-propelled craft indicate it could very well attain 10% of the speed of light. If so, the trip to Alpha Centauri could be attained within 50 years, as opposed to 76,000 yrs.
But at what cost? The author of the ASTRONOMY piece estimates $3.45 trillion for a Project Daedulus-type project in 2011 dollars. I say this is too low, maybe by a factor ten. Bear in mind, the central problem remains of having to first mount mining missions to the gas giants to collect the helium -3 in sufficient quantities to power the fusion star ship.
One observer commenting, has perhaps the most realistic take and I give it here:
"The energy requirements for a starship to travel to a nearby star would be 100 times the current energy output of our entire planet."
In other words, from all practical - as opposed to pie in the sky perspectives- it's impossible and we will not be seeing it any time in our lifetimes, and I doubt over the human lifetime as a species. Others in the piece have appealed to "new forms of propulsion" but let's get real! We can't even get congress critters to approve the sums needed for sustaining new forms of energy once the oil runs out! SO, how the hell are we going to get them to approve new energy propulsion systems to travel to the nearest star system when that effort lacks one hundredth the practicality of the alternate energy sources?
Having said that, it appears humanity elected the course it would take many decades ago, and that was to build ever more powerful weapons and weapon-support technology rather than travel to the stars. Adding up the total cost of all nuclear warheads ever created, and the missile systems to mount them, the added technology (e.g. radar tracking etc) to support them, and all other conventional weapons systems - one gets nearly $14 trillion. I submit that this amount if applied uniformly, consistently and rigorously might have funded an ion propulsion system that might well have taken us to the stars....at least to Alpha Centauri. Alas, our species has too much been beset with weapons myopia to look beyond to stellar travel.
Maybe we don't deserve to get to the stars.