Interstellar space travel is travel between the stars. Dreamed of for centuries, interstellar travel as a theme has dominated much of the total body of science fiction literature and film. There is a major difference between interstellar travel and interplanetary travel, mainly the distances involved.
As a serious concept, interstellar travel has been debated fiercely by various scientists, science fiction authors, hobbyists and eccentrics, but has seldom been taken seriously in the academic community or the mainstream public. Many ideas for managing this feat have been thrown around, ranging from Star Trek's Warp Drive and hyperspace engines of various sorts, to trips which involve very long waits and either multi-generation crews or cryogenic freezing. As current technology stands, we have no good solution; unless a dodge involving theoretical higher dimensions is found, any travel to other stars is doomed to be slow and dull.
However, a number of science fiction novels deal with interstellar travel quasi-realistically by using Generation star-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, leaving the tedium of interstellar travel to automated systems and awakening unaged at their destination.
While interstellar travel may prove difficult or impossible to accomplish, interplanetary travel (travel between the planets of the solar system) may prove quite feasible. Time will tell as to whether our space program lives up to this task, and looks to the stars.
The notion is taken seriously in the scientific community, and many papers have been published on concepts about it. It is certainly not impossible, given sufficient travel time and solid engineering work, but at present calling it "feasible" would be a stretch of the term. NASA has been engaging in dedicated research into these topics for several years now, and has accumulated a number of theoretical approaches. Among the in-our-lifetime achievable technologies are nuclear engines (nuclear thermal or nuclear electric, primarily). Interstellar travel times would still be very, very long compared with a single human lifespan, but would be within the realm of the possible, given generation ships or some sort of organic stasis approach, as alluded to in this article.
Interplanetary versus interstellar travel
At the end of the 20th century, we have sent 12 people to land on the moon and return safely. We have sent robot spacecraft to fly past most of the major planets of our solar system, and land on two of them. Four of these spacecraft (Pioneer 10 & 11, Voyager 1 & 2) are on course to leave the Solar System, but will cease to function long before reaching the Oort Cloud.
At this stage there remains much to do in our own solar system (for that matter, on our own planet) in the way of research and exploration, and perhaps, if we choose, in the way of settlement and economic exploitation.
But already the question is being asked, can we explore further afield, to other stars? And if so, how? At this stage of our technology, the answers are not encouraging, but it seems that in long run if our technology continues to advance, it might eventually be possible.
The problem of huge interstellar distances
Distances for things that are very far away are measured in the amount of time it would take a beam of light to travel that far. Light in a vacuum travels 3x108 meters per second which is usually denoted with the letter c, so a light second is 3x108 meters.
From earth to the moon is about one and a quarter light seconds. Our spacecraft that have made that trip have typically taken about three days.
The distance from the earth to other planets in the solar system ranges from three light minutes to about five and a half light hours, depending on which planet and their alignment at the time. Our spacecraft that have made these trips have taken from a few months to a little over a decade.
These interplanetary missions have all used small uncrewed, robotic spacecraft, since at the beginning of the 21st century we do not have the technology to build and run a crewed craft that will travel for that long away from the earth. There is little doubt that such technology could be developed soon with a budget of a few trillion dollars, but it doesn't exist yet.
The nearest star to the Sun is the triple system Alpha Centauri. Light shining from that star takes just over four years to reach us. We have not yet attemped to send a spacecraft there, as even the fastest spacecraft yet built can only achieve a speed of about 30 km per second. At that rate, the journey would take about 40,000 years. At this stage in space exploration, the longest space missions we have initiated are expected to have an operational lifetime of about forty years before failure of electical power and other key components.
Clearly, if we are going to travel between the stars, we need major advances in technology. A much faster space propulsion system would be one solution. Technology that remains reliable after millenia, and patience to match, would be another. Most likely, we will need both of these. Ships that would travel this way could have crews that would live their entire lifespans aboard, for multiple generations; this type of ship is called a multi-generation ship, or generation ship. As an alternative, some suggest that the crew could instead be frozen in suspended animation during the journey. This sort of ship is sometimes referred to as a sleeper ship.
Special relativity and the speed of light as a limit
In science fiction, and particularly in television science fiction, it is normal for starships to travel from star to star in time for next week's episode. Given the distances involved, that would require travelling faster than the speed of light. Currently we have no way to achieve this, but it is interesting to ask if it might be possible one day.
According to our current understanding of physics, probably not.
There are two objections to faster than light travel, both of them from the theory of relativity, which is stated:
E = mc2 / √( 1 - V2/C2 )
Neither completely rules out the possibility, but they certainly raise major doubts.
First, the amount of energy required to accelerate an object increases as its speed increases (the one minus velocity2 divided by C2 term). At normal everday speeds, the increase is too small to measure, but as the speed becomes a significant fraction of the speed of light, the increase is substantial. The theory suggests the energy approaches infinity as the speed approaches c. This has been verified up to a point close to the speed of light in laboratory experiments, and does not seem to be in doubt.
So, the idea of exceeding the speed of light just by accelerating normally from lower speeds can probably be ruled out. Is it possible to go faster than light by more devious (and so far unspecified) methods?
This brings up the second objection. 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 sufficently unsettling that many physicists believe it to be so.
(In this section, it should be noted that "experimentally verified", means just 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.)
"Practical" interstellar travel
However, even without faster-than-light travel, multi-generation starships, or dramatic extensions to human lifespan, it may be feasible in the medium to long-term future to travel to the nearer stars.
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 we can ever develop energy resources and efficient production methods to make antimatter in the quantities required, we could theoretically 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.
What this does suggest, however, is that without the development of faster-than-light travel or orders-of-magnitude extension of current human lifespans, interstellar travel is likely to involve one-way voyages of colonization rather than scouting trips.
Whilst Star Trek and Star Wars feature interstellar travel, neither does so in a way that pays much attention to the engineering used to achieve it (resorting to unexplained warp drives). The novels of Isaac Asimov, Arthur C. Clarke, Robert A. Heinlein and Larry Niven cover such territory with much greater attention given to plausible ideas as to how this could be achieved.