As a practical goal interstellar travel has been debated fiercely by various scientists, science fiction authors, hobbyists and enthusiasts.
Many ideas for managing this goal have been suggested, especially in science fiction themes, ranging from Star Trek's warp drive and hyperspace engines of various sorts to trips that involve very long waits and either multi-generation crews (generation ships) or cryogenic freezing (sleeper ships). Other science fiction authors such as Isaac Asimov, Arthur C. Clarke and others, cover such concepts with greater attention given to plausible ideas as to how this could be achieved. As current knowledge of physics stands there is no effective solution to achieving faster than light travel, and the current technological limits to slower-than-light travel make interstellar journeys shorter than centuries unrealistic.
A number of science fiction novels deal with interstellar travel quasi-realistically by using generation ships where generations of people are born, live and die onboard a massive star ship as it travels to its destination. Alternately, the crew of a starship could spend the bulk of the journey frozen in suspended animation on board a sleeper ship, leaving the tedium of interstellar travel to automated systems and awakening unaged at their destination.
While manned interstellar travel may prove difficult or impossible to accomplish, manned interplanetary travel (travel between the planets of the solar system) is technically feasible, though economic factors, health and other risks regarding a person's continueous stay in space, and other motivational factors may suspend the achivement.
Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, interstellar travel is certainly possible. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches. Among the technologies suggested are nuclear engines (nuclear thermal or nuclear electric, primarily). With any of these technologies interstellar travel times would still be very long compared to a single human lifespan, but it would be within the realm of the possible given generation ships or some sort of organic stasis approach as mentioned previously.
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Astronomical distances are sometimes measured in the amount of time it would take a beam of light to travel. Light in a vacuum travels in approximately 3×108 metres per second, which is denoted with the letter c, so a light second is app. 3×108 metres.
The distance between Earth and its Moon is about one and a quarter light seconds. With current propulsion technologies, such a trip will typically take about three days for a spacecraft.
The distance from Earth to other planets in the solar system ranges from three light minutes to about five and a half light hours. Depending on the planet and its alignment to earth, for an typical unmanned spacecraft these trips will take from a few months to a little over a decade.
The nearest star to the Sun is the triple system Alpha Centauri. Light radiating from that star takes a bit more than four years to reach Earth.
Currently, the fastest spacecraft built can achieve a velocity of about 30 km per second (relative to Earth). At that rate, the journey would take about 40,000 years. Additionally, at current stage of space technology, the longest space missions that have been initiated are expected to have an operational lifetime of about forty years before failure of key components is likely to happen. Significant engineering advances such as automated self-repair may be required to ensure survival.
In many science fiction stories starships to travel from star to star faster than light. Currently there is no known way to achieve this, and according to the current understanding of physics there may never be one.
There are two objections to faster than light travel, both originate from the theory of relativity which states that the speed of light is constant in all reference frames.
The first is that because the speed of light is constant to all observers, an attempt to overtake
a beam of light will fail. No matter how much energy you put into an spaceship, a beam of light will appear from that spaceship to be moving away at the same speed. While this appears to rule out accelerating past a beam of light, it does not immediately appear to prevent one from exploiting particles that already are moving faster than light or perhaps by somehow sending information faster than light without having any
physical object move faster than light.
However even this causes problems. According to a section of the theory of relativity called special relativity, travelling faster than light is equivalent to travelling backwards in time, or time travel, according to some observers. In particular, if faster than light travel is possible without too many arbitrary restrictions, it is possible to have events in the future cause events in the past. This is called a causality loop.
This concept has not been verified experimentally, because no-one has yet exceeded the speed of light in the laboratory. However, it seems that this reasoning must apply to any theory in which the speed of light in a vacuum is equal to all observers, something that has been carefully verified in many experiments.
It is not immediately obvious that a causality loop is impossible, but the idea is sufficiently unsettling that many physicists believe it to be so.
(In this section it should be noted that "experimentally verified" only means that several different repeatable experiments have been performed that support the theory. Of course these cannot prove the theory correct, they can only give confidence that the theory appears to work for the cases that were tested. That is about as good as it gets in science.)
Even without faster-than-light travel, multi-generation starships, or dramatic extensions to human lifespan, it may still be feasible in the medium to long-term future to travel to the nearer stars.
A practical short-term approach that has been proposed is nuclear pulse propulsion. In 1957 it was deemed possible to build 8 million ton starships with this type of engine, even though they would be limited to about 1/10 the speed of light. One problem with it is that such a propulsion method uses nuclear explosives as fuel, and may be highly controversial due to the risk of radiation or other hazards in using such a method.
Another early proposal for an interstellar propulsion system was the Bussard ramjet, in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it using a proton-proton fusion reaction, and expel it out the back. As the fuel would be collected en route, the craft could have theoretically accelerated to near the speed of light. Proposed in 1960, later calculations with more accurate estimates suggest that the thrust generated would have been less than the drag caused by any conceivable scoop design.
Fusion-powered starships should be able to reach speeds of approximately 10 percent of that of light. Light sails powered by massive lasers could potentially reach similar or greater speeds. Finally, if energy resources and efficient production methods are found to make antimatter in the quantities required, theoretically it would be possible to reach speeds near that of light, where time dilation would shorten perceived trip times for the travellers considerably (though shielding the spacecraft from stray atoms in interstellar space would become a very serious issue as faster speeds were achieved). Even given the assumption of 10 percent of light speed, this would be enough to reach Alpha Centauri in forty years, only half a present human lifetime.
See also: interstellar communication, relativistic rocketThe problem of interstellar distances
Special relativity and the speed of light as a limit
"Practical" interstellar travel