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 travelEdit
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 between two points. 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. As a point of comparison, Voyager 1, launched in 1977, is the most far-traveled of space probes. As of 2005, it has reached a distance from Earth of approximately 12 light hours.
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 travelEdit
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 travelEdit
Slow interstellar travel designs generally use near future spacecraft propulsion technologies. As a result, voyages are extremely long, lasting hundreds or thousands of years. Voyages might be one-way trips to set up colonies. The propulsion system required for such slow travel are less speculative than those for fast interstellar travel, but the duration of such journeys would present a huge obstacle in itself. The following are the major proposed solutions to that obstacle:
A generation ship would be large enough to hold a colony of people. These people would live out their lives on board the ship, and their descendants would arrive at a new solar system. These descendants might establish a colony, or perhaps stop only to explore and perhaps to build other ships. Generation ships have long been a popular plot device in science fiction; such stories often have negative outcomes involving a deterioration of the ship-borne culture.
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, self-sustaining habitat would be 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.
Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. 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.
Extended human lifespanEdit
A variant on this possibility is based on the development of substantial human life extension, such as the "Engineered Neglible Senescence" strategy of Dr. Aubrey de Grey. If a ship crew had lifespans of some thousands of years, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.
A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of a method to replicate conditions in a womb, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots. (See embryo space colonization.)
Fast interstellar travelEdit
The possibility of starships that can reach the stars quickly (or at least, within a human lifespan) is naturally more attractive. This would require some sort of exotic propulsion methods or exotic physics.
In 1957 it was deemed possible to build 8 million ton spaceships with nuclear pulse propulsion engines, perhaps capable of reaching speeds of about 10 percent of light speed. One problem with such a propulsion method is that it uses nuclear explosions as a driving force, 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 travelers considerably. 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.
With any ship traveling at a significant fraction of light speed, shielding the spacecraft from the sparse dust and gas of the interstellar medium would become a serious issue.
Faster than light travelEdit
Main article: Faster than Light Travel
Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light. Unfortunately, even the most serious-minded of these are extremely speculative at this point.
Wormholes are probably the least conjectural of faster-than-light options under current science. Wormholes are distortions in space-time that theorists postulate could connect two arbitrary points in the universe, across an Einstein-Rosen Bridge. 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 scale similar to Sagittarius A* at the centre of the Milky Way Galaxy; smaller black holes produce intense tidal forces that would completely destroy any macroscopic object falling into them.
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. It is not clear that any of this is even theoretically possible, largely because there is no widely accepted theory of quantum gravity.
String theory (and all other theories involving hidden dimensions) predict that gravity and electromagnetism unify in hidden dimensions and that the hidden dimensions are indetectible because of their small size. It does also predict that sufficiently short-waved photons, with wavelengths shorter than the size of the hidden dimensions, can enter them. Producing ultra-short photons can thus manipulate gravity, with revolutionizing space travel applications such as cheap anti-gravity launches. The problem that it would require high energy can be practically solved by concentrating several laser beams on a nanoparticle, heating it to locally extreme temperatures. An Alcubierre metric can be created by ejecting multiple nanoparticles from the craft and then beam perfectly timed laser beams on them (fire at the most distant first so that they are hit simultaneously), so each nanoparticle contributes a slower than light effect but together add up to faster than light, creating no discrete event horizon and thus no Hawking radiation.
Interstellar travel via transmissionEdit
If physical entities could be transmitted as information and reconstructed at a destination, travel at the speed of light would be possible.
Encoding, sending and then reconstructing an atom by atom description of (say) a human body is a daunting prospect, but it may be sufficient to send software that in all practical purposes duplicates the neural function of a person. Presumably, the receiver/reconstructor for such transmissions would have to be sent to the destination by more conventional means.
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.