The closest star is a staggering 4.2 light years away, so is traveling to the stars ever going to be realistic given our current knowledge of physics and technology?
The answer is yes, but it will by far be not as easy, cheap or fast as most science fiction stories portray it. This page intends to give you a feel on how scientists have proposed to solve the problem.
Scientists have already made several conceptual designs of propulsion systems capable of interstellar travel capable of up to 10% of light speed based on technology available now or in the near future. The most common category uses nuclear fission or fusion to build rockets with a very high exhaust velocity ve (as you know from the rocket equation article, the maximum speed any rocket can go is a few times the exhaust velocity). Such a starship would be about 1000 times faster than current chemical rockets can go, and be able to reach any one of the few hundred nearest stars within two centuries of travel time. This might still seem too slow to you, but it is what it is. The laws of physics are not going to change just because we want to - unlike SciFi authors, we're stuck with the universe we live in... If people like us would launch a starship, at best our children would see it arrive.
In our universe, we're stuck with slower-than-light travel. We have to learn to accept this. Travel times of over a century seem unpractical. However, it seems unavoidable that civilization with starfaring ambitions must learn to think in such time spans. It is feasible to develop technology able to function that long. And even if it is once decided to send settlers, a technology such as hibernation, allowing them to bridge the large time span, can be developed. Of course, other scenarios can be thought of: humans might live much longer in the future, or artificial intelligence could be realized allowing intelligence to reside almost forever in an computer...
A few of the more renown starship propulsion concepts that have already been proposed are summarized here. These have been published in peer-reviewed journals such as the Journal of the British Interplanetary Society, Acta Astronautica and the journals of American Institute of Aeronautics and Astronautics. Admittedly, none have been put into practice because of lack of financing (i.e. all the costly engineering work still remains to be done) but starship construction seems certainly difficult, but feasible.
In contrast, sadly the 'pull the lever and you're there' technology from popular science fiction (warp drive, hyperspace etc.) is probably never going to be realized: it requires unimaginably high energy, exotic (never observed) kinds of matter and would even violate basic causality.
Most pictures shown are made by the talented artist and aerospace illustrator Adrian Mann.
More propulsoin methods can be found elsewhere. The once shown here are those we believe to be most feasible, based on our knowledge of science and technology.
The Orion drive is one of the first credible (though arguably somewhat crude) interstellar-capable propulsion methods ever proposed. It is based on nuclear pulse propulsion: a starship propelled by the explosions of nuclear bombs (relatively) close to a pusher plate. This is of course a brutal method, but has the great advantage it can by built with technology available since the 1950s. That's right, that's over 60 years ago now!
When using high yield thermonuclear bombs, speeds of up to a few percent of light speed are possible as estimated by the renowned physicist Freeman Dyson who worked on this concept. The disadvantage is that typical starship designs are quite large as there is a practical minimum to the explosive yield of nuclear charges, necessitating enough mass for radiation protection, shock absorbers etc. Manned startships of this kind could be the size of a small city.
Orion was long a secret project as it was based on sensitive nuclear bomb technology. Many original files, dating from the 1950s, have now been released to the public. For more information, you can also look up the BBC documentary 'To Mars by A Bomb" (It's on Youtube) about the people who were actually paid to develop Orion ships and dreamt of embarking on a trip around the solar system without much restrictions, seing Jupiter and Saturn's rings from nearby and being back in just a few months. The project was canceled by president Kennedy in the beginning of the 1960s.
Basic lesson here is that if humanity would want to at any cost, we could build a starship now. Seen the thousands of nuclear explosions needed to propel it to high speed, launching it from the ground as the concept originally proposed does not seem the most sane idea now. With considerably higher cost, an Orion ship could be build in Earth orbit, reducing the radioactive pollution generated.
A design for an interstellar robotic probe made by scientists of the British Interplanetary Society in the 1970s. The idea is to induce nuclear fusion in small frozen deuterium/helium-3 pellets by powerful particle beams and direct the high-energy reaction products in a single direction by powerful magnets, creating a rocket. As every single explosion would be much smaller, many of the problems of the Orion designs are avoided. The designers proposed a 54000 ton unmanned probe, holding 50000 tonnes of fuel and 500 tons of scientific equipment. Seen the current progress in miniaturization the mass could probably be reduced. The original design should be able to read a top speed of around 12% of light speed.
Inertial confinement fusion by particle or laser beams is not available yet. However, nuclear fusion including laser driven inertial confinement fusion is being invested in as it is a promising source of clean energy on Earth. (see for example: the French Laser Mégajoule, the proposed European HiPER project or the National Ignition Facility in Livermore, California). Thus, great progress in this direction is expected rather soon.
Project Icarus is a detailed update of the now 30 year old Daedalus study going on right now. Dozens of scientists and engineers from around the world are working on project Icarus right now. The resulting design will take latest results from nuclear fusion and other scientific research into account. The ambitious main goal of the project is to design an interstellar probe that can arrive at a nearby target star within 80 years. One of the improvements made to the orginal Daedalus design in the swithch to readily availbe deuterium as fuel, instead of the helium-3 fuel that has to be mined somewhere else in the solar system, e.g. in the atmosphere of Jupiter. For more information, please visit www.icarusinterstellar.org.
Project Longshot is a conceptual design for an interstellar probe developed by the US Naval Academy and NASA from 1987 to 1988. It is somewhat similar to the Daedalus project, albeit using a standard fission reactor as a power source instead of fusion power. The fission power would be used to ignite intertial confinement fusion in pellets.
The probe's weight would be 396 metric tonnes at the start of the mission, including 264 tonnes of fuel. Its top speed would be 4.5% of light speed in a trajectory that would include slowing down and going into orbit around the target star.
Fission fragment rocket
This is a non-explosive, fission-only design based on magnetic separation of the reaction products of a fission reaction from the rest of the fuel material. It could yield a specific impulse of over 1 million which translates up to 7.5% of light speed when using a 10:1 fuel: rest mass ratio.
Clark, R.; Sheldon, R. Dusty Plasma Based Fission Fragment Nuclear Reactor American Institute of Aeronautics and Astronautics. 15 April 2007.
At this moment, members of the Fourth Millennium Foundation are designing their own concept of an interstellar drive based on a sail covered with a thin layer of fissile material. More info will follow soon!
Light sail based approaches
All concepts mentioned so far are basically rockets: they take mass with them and expel it backwards, thus accelerating forwards. There are several non-rocket based approaches described in scientific literature. Many of them propose a system in which a spacecraft is propelled by a sail, pushed by light shining on the sail.
In fact, spacecraft propelled by a sail have already flown in space! An example is the Japanese IKAROS-1 probe, sent to the moon. This spacecraft was propelled by light from the sun. This accelerates the spacecraft, but only slowly. Now, instead of using sunlight directly, you could place very powerful lasers somewhere in the solar system, and beam light directly on the sail of the spacecraft. Such designs typically reach even better speeds, although the propulsion lasers would require even more resources and energy to build than rocket-based spacecraft. You need a staggering 300 MW of power for every newton (100 grams) of thrust! Thus, interstellar travel by laser sail would probably only be feasible for a large solar system-wide civilization that for example doesn't mind building thousands of square kms of solar panels on the moon, to power lasers of terawatt proportions.. Also, slowing down again could be problematic, though theoretical schemes for doing so have been proposed (tip: you need a second sail). On the plus side, the laser system would remain in the solar system, and be reusable for many missions. Also, the spacecraft itself is very simple, elegant and low tech. And unlike all the designs mentioned before, no radioactivity of any kind is produced.