Interstellar space travel is unmanned or manned travel between stars, though the term usually denotes the latter. The concept of interstellar travel in starships is a staple in science fiction. There is a tremendous difference between interstellar travel and interplanetary travel, mainly in the distances involved.
As a practical goal interstellar travel has been debated fiercely by various scientists, science fiction authors, hobbyists and enthusiasts.
Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, unmanned 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.
The difficulty of interstellar travel
Interstellar travel poses a number of difficulties. There are all the difficulties of interplanetary travel, including hard vacuum, radiation, micrometeoroids, and free fall. These difficulties seem tractable; robot missions have been sent to almost every planet in the Solar system, humans have been sent to the Moon, and manned missions to Mars have been planned for years. Interstellar travel is made immesurably more difficult by the tremendous distance to even the nearest stars.
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 approximately 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 a 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 short, current spacecraft propulsion technology cannot send objects fast enough to reach the stars in a reasonable time.
Even theoretical interstellar travel is expected to be slow. Current theories of physics predict that it will be impossible or very difficult to travel faster than light, and that if it were possible, it would also be possible to build a time machine. This possibility seems unlikely, although Robert L. Forward has written a novel, Timemaster, which describes how this could be done, assuming that one could find negative matter.
However, special relativity and general relativity offer the possibility of shortening the apparent travel time: with sufficiently advanced engines, a starship could make interstellar voyages at nearly the speed of light, and relativistic time dilation would make the voyage seem much shorter. However, upon arrival, the passengers would discover that the full time had passed in the world outside the starship.
Even this possibility still leads to very long travel times without the use of exotic physics. For a lengthy voyage, the spacecraft cannot accelerate at much more than one Earth gravity, since its acceleration will provide artificial gravity for the passengers, and the passengers cannot long tolerate high gravity. This means that if the ship accelerates throughout the voyage, accelerating on the way out and decelerating on the way back, in a year of ship time, the ship can travel half a light year. Because the ship can accelerate for longer, in two years the ship can go four times as far, or two light years. In three years, the ship could reach Alpha Centauri. More than five years will have passed on Earth. This is a long voyage, but still not much worse than ancient sailing voyages.
Speculative interstellar travel
Interstellar travel designs fall into two categories. The first, which we will call slow interstellar travel, takes a great deal of time, longer than a human lifespan. The second, which we will call fast interstellar travel assumes that the difficulties above can be conquered.
Slow interstellar travel
Slow interstellar travel designs generally use near future spacecraft propulsion techniques. As a result, voyages are extremely long, hundreds or thousands of years. Voyages would be one-way trips to set up colonies. Exactly one near-term interstellar propulsion technology is known: nuclear pulse propulsion, and it is controversial. All other proposals require substantial technology development, and many require massive investments in space-based infrastructure.
Many scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic freezing. While neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long years of the voyage.
A second possibility is generation ships, ships that would be very large, large enough to hold a colony of people. These people would live out their lives on board the ship, and their distant descendants would arrive at a new solar system. After living happily for hundreds or thousands of years inside the ship, they might have lost interest in establishing a colony, stopping only to resupply and perhaps to build other ships. Some science fiction even suggests that the descendants may eventually forget that they are on a spaceship.
Generation ships are not currently feasible, both because building such an enormous ship would have to be done in space, and because such a sealed habitat would be extremely difficult to construct. Biosphere 2, an artificial closed ecosystem, was built in an attempt to work out the engineering difficulties in such a system.
Fast interstellar travel
Also: Faster than Light Travel
More interesting to scientists and to writers is the possibility of starships that reach the stars quickly (or at least, within a human lifespan). This would require some sort of exotic propulsion methods or exotic physics, but the idea captures the imagination.
A vehicle that would allow fast interstellar travel is called a starship. They are very common in fiction. A number of propulsion methods have been proposed that would allow this.
In 1957 it was deemed possible to build 8 million ton starships with nuclear pulse propulsion engines, 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 be 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.
A second possibility is that wormholes would provide some way of connecting distant locations. Wormholes are warps in space-time which cause it to bend back on itself. It is not known whether or not wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.
There are two types of wormholes that may enable interstellar travel. The first kind originates with the same process as a black hole: the death of a star. Wormholes of this kind safe enough for a human being to navigate would probably have to be supermassive and rotating, on a similar scale to Sagittarius A* at the centre of the Milky Way Galaxy; smaller black holes produce intense tidal forces that would completely destroy anybody falling into them. Theorists postulate that wormholes connect arbitrary points in -- or between -- universes, across an Einstein-Rosen Bridge.
Another kind of wormhole is based on quantum gravity. Some have speculated that Euclidean wormholes that spontaneously come into being and disappear again, and exist at scales of Planck length. It may be that this wormhole could be "propped open" using negative energy (also known as vacuum energy), though the quantity of the energy would be immense. However, it is not clear that any of this is possible, largely because there is no widely accepted theory of quantum gravity.
A third possibility are starships that utilize Alcubierre's warp drive. In this theory, a starship warps space, expanding space behind it and contracting space ahead of it. The concept of manipulating space-time in this manner is based on Einstein's General Theory of Relativity, which states that gravitational fields cause space-time to be warped. Being able to warp space would probably require a control over gravity that we currently do not have.