Several centuries ago, manned flight through Earth's atmosphere was plausible, but technically impossible. Today, the same can be said for manned interstellar travel: it might become possible through some sort of as-yet-unimagined invention in ten years, or a hundred, but don't hold your breath. The distances involved, the demands it would put on the human body, and the cost of any hypothetical manned spacecraft are all formidable challenges. Each of these - and other problems - must be solved, or else manned interstellar travel will remain impossible.
- It's Too Far -
Distances in space are vast. So far, manned spacecraft have made it as far away as the Moon, a little under 250,000 miles. In contrast, the planet Mars - long assumed to be the next target for manned spacecraft - is 50 million miles away at its closest point, and the Sun is one Astronomical Unit (AU), equal to 93 million miles, in the other direction. In contrast, Alpha Centauri (the nearest star to our own Sun) is over 25 trillion miles away. The fastest human spacecraft yet launched would take millennia to traverse that distance. Clearly the technology we now use to meander around the solar system is simply impractical for interstellar travel.
There are a variety of techniques for accelerating to extremely high speeds, and thus crossing the interstellar medium in less time, perhaps even in several decades. However, the most plausible of these largely implausible designs works only for small and lightweight spacecraft, such as massive sails built to manipulate the solar wind coupled with some sort of large microwave transmitter in Earth orbit. Even if this were to work for a small unmanned probe, however, it could not work for a manned interstellar spacecraft.
The problem is equivalent to that faced in travelling to the Moon and Mars, except on a far grander scale: manned spacecraft must be orders of magnitude larger and more complex, since they must contain all of the apparatus and space necessary for humans to survive inside them. Adding more size and mass means that the spacecraft needs more fuel to reach the same speeds; this fuel itself has mass, which requires still more fuel simply to cancel out the increased fuel supplies; and so on. A manned interstellar spacecraft would almost certainly have to be so large that it could not even be launched into orbit from Earth: it would have to be constructed there, using supplies flown from Earth or, more likely, the moon and nearby asteroids. Technologically speaking, therefore, manned interstellar travel is simply impossible for the foreseeable future.
- How Will We Survive? -
The second set of problems are biological ones. Space travel is not kind to the human body. This is true even close to the Earth, where astronauts are at greater risk of radiation exposure as well as the effects of prolonged time in weightlessness (causing muscular atrophy). These risks are present in varying amounts whether one travels to Alpha Centauri or simply into orbit, although it's not entirely clear at the moment whether long-distance space travel would result in greater or lesser radiation problems. Both of these are solved in science fiction by technology like shielding and artificial gravity, which we do not have.
Interstellar travel, however, poses additional special problems - which is why, for the moment, manned interstellar travel is impossible. Assuming for the moment (and it's a safe assumption) that the first interstellar travellers will not have the benefit of science-fiction technology like the warp drive, they will also have to deal, in varying amounts, with two other serious problems: the effects of high acceleration, and the effects of aging. Unfortunately, these two are inversely related, meaning there's no getting around them.
Any plan to reduce the amount of time spent in space (and therefore the amount of time human travellers would age) means they will have to travel at high speeds, subjecting them to potentially dangerous g-forces as a result of high acceleration and, at the other end of the trip, deceleration. (The time in between, at the spacecraft's cruising speed, would be free of these forces.) However, if the spacecraft travels at a slower speed, then they will spend potentially decades more time in space.
Either way you look at it, there's a serious chance that manned interstellar travel would require multiple generations of humans to live on board (which leads to the space and fuel problems in the previous section). The leading alternative in science fiction is some form of stasis, or prolonged unconsciousness. Alfred C. Clarke's 2001 and 2010 both make use of this for travel within the solar system, to Jupiter. The technology to do this reliably is not currently available. Until it is, it's probably a safe bet that manned interstellar travel is impossible.
- Who Will Pay For It? -
The last and perhaps most easily resolvable reason that manned interstellar travel is impossible is that there are very few people willing to foot the bill for it. Of course, we're not exactly sure how much it would cost just yet - since most of the fuels and technologies necessary haven't even been invented. However, this is a question worth asking. Consider: right now, NASA fights desperately for the funding to send unmanned probes the comparatively paltry distances to nearby planets, and generally has difficulty sending anything of substance beyond our closest neighbour, Mars, more than once every few years.
The problems are even more severe when one considers the problem of manned spacecraft. In general, these are orders of magnitude more expensive than their unmanned counterparts, because they have to be far larger (to hold the crew), and far more rigorously tested. NASA is reluctant to lose an unmanned probe, but recognizes that accidents happen. Since the accidents of the Apollo missions, NASA has not been so willing to risk manned crews - the tragedies of the Challenger and Columbia notwithstanding - and puts far more effort into testing those designs. With all the costs averaged out, every single space shuttle flight currently costs about $1 billion, and the International Space Station has an overall budget of around $100 billion. Neither of these is designed to go beyond low Earth orbit. A manned interstellar flight project would probably require most of our current economic output to be spent on research and construction costs.
Right now, cost is one of the major reasons why the Obama administration is scaling back NASA's manned spacecraft plans. Yet compared to the challenges of manned insterstellar travel, NASA's space plans were pretty amateurish: the Orion spacecraft would be capable of returning men to the Moon, and one day taking us to Mars. If we can't afford to send manned craft to the Moon, it's a safe bet we can't afford to send them to other stars. For the most frustrating of reasons, manned interstellar travel is impossible so long as no one is willing to pay for it.