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 due to the much larger 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 seems 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 enormously more difficult by the million-fold greater distances to nearby stars. Intergalactic travel would involve distances a million-fold greater than interstellar distances.
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 the current stage of space technology, the longest space missions that have been initiated are expected to have an operational lifetime of about 40 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 indicate that it is impossible to travel faster than light, and that if it were possible, it would also be possible to build a time machine. Most proposed mechanisms for faster than light travel require the existence of negative mass.
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 for the traveller. However, it would be slow for the people on Earth interested in the results of the mission, and upon return to Earth, the travellers would find that far more time had elapsed (on Earth) than their subjective travel time would indicate.
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. Artificial closed ecosystems, including Biosphere 2, have been built in an attempt to work out the engineering difficulties in such a system, with mixed results.
A third possibility is a robotic space mission carrying a large number of frozen early stage human embryos. This method of space colonization requires, among other things, the development of a method to replicate conditions in a woman's womb, the prior detection of another terrestrial planet with a nitrogen-oxygen, non-toxic atmosphere as well as advances in the field of fully autonomous mobile robots. (See embryo space colonization.)
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 the interstellar medium 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 idea involves 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, but would require the existence of negative-mass materials, which are neither observed nor predicted by current physical theory.
Light-speed interstellar travel
The fastest possible travel is at light-speed, which can be achieved by sending photons (or other massless particles that might be shown to exist). This is equivalent to communication using any part of the electromagnetic spectrum.
It may not be practical or even feasible to transport a massive organism (i.e. one that contains mass) across interstellar distances, but it is within the realms of physical possibility to transmit enough information that the organism could be reconstructed at the receiving end. It may even be sufficient to send software that in all practical purposes duplicates the neural function of the organism, rather than sending an atom by atom description of the entire body.
It is conceivable that at our present technology, humankind would be able to implement (after suitable translation) such software on our present day computational hardware. This is possible and maybe even probable when one considers that all classical computing devices are all in some sense equivalent Turing Machines.
As part of the NASA Breakthrough Propulsion Physics Project, it identified three things which must happen, or breakthroughs which are needed, in order for interstellar travel to be possible :
- A new propulsion method which has less need for propellant
- A method of propulsion which is able to reach the maximum speed which is possible to attain
- A new method of on board energy production method which would power those devices.
Analogies for 'breakthroughs' in technology are steam engines supplanting sailing ships, and jet aircraft replacing propeller aircraft. The breakthrough event means that they are not looking for a better way of designing a rocket engine, but instead a substantially new technology. It comes where the benefits of a past technology advancing gradually diminish, where there becomes a need for a new technology.