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Interstellar space travel is unmanned or manned travel between stars. The concept of interstellar travel in starships is a staple in science fiction. Interstellar travel is tremendously more difficult than interplanetary travel due to the vastly larger distances involved, and intergalactic travel more difficult yet.
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, both unmanned and generational interstellar travel seem possible, though representing a very considerable technological and economic challenge unlikely to be met for some time, particularly for crewed probes. 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
Astronomical distances are sometimes measured in the amount of time it would take a beam of light to travel between two points (see lightyear). Light in a vacuum travels 299,792,458 meters per second or about 186,000 miles per second.
The distance from Earth to the Moon is 1.3 light-seconds. With current spacecraft propulsion technologies, a trip to the moon will typically take about three days. The distance from Earth to other planets in the solar system ranges from three light-minutes to about four 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 known star to the Sun is Proxima Centauri, which is 4.23 light-years away. The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/600th of a light-year in 30 years and is currently moving at 1/18000 c. At that rate, a journey to Proxima Centauri would take 72,000 years. Of course, this mission was not specifically intended to travel fast to the stars, and current technology could do much better. The travel time could be reduced to a few millennia using lightsails, or to a century or less using nuclear pulse propulsion (Orion).
No current technology can propel a craft fast enough to reach other stars in a reasonable time (Star-Trek-like time). Current theories of physics indicate that it is impossible to travel faster than light, and suggest that if it were possible, it might also be possible to build a time machine using similar methods.
However, special relativity offers the possibility of shortening the apparent travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light, relativistic time dilation would make the voyage seem much shorter for the traveller. However, it would still take many years of elapsed time as viewed by the people remaining on Earth, and upon returning to Earth, the travellers would find that far more time had elapsed (on Earth) than the subjective travel time. (This effect is referred to as the twin paradox.)
General relativity offers the theoretical possibility that faster than light travel may be possible without violating fundamental laws of physics, for example, via wormholes, although it is still debated whether this is possible in the real world. Proposed mechanisms for faster than light travel within the theory of General Relativity require the existence of exotic matter.
Interstellar distances
The mass of any craft capable of carrying humans would inevitably be several orders of magnitude greater than that necessary for an unmanned interstellar probe. For instance, the first space probe, Luna 1, had a payload of 361 kg; while the first spacecraft to carry a living passenger (Laika the dog), Sputnik 2, had a payload over 20 times that at 7,314 kg. This in fact severely underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle life support system.
Probes versus human travel
Interstellar travel designs fall into two categories. The first, which we will call slow interstellar travel, takes a great deal of time, sometimes longer than a human lifespan. The second, which we will call fast interstellar travel assumes that the difficulties above can be conquered.
Speculative interstellar travel
Slow interstellar travel designs such as Project Longshot generally use near-future spacecraft propulsion technologies. As a result, voyages are extremely long, starting from about one hundred years and reaching to thousands of years. Crewed voyages might be one-way trips to set up colonies. The propulsion systems required for such slow travel are less speculative than those for fast interstellar travel, but the duration of such a journey would present a huge obstacle in itself. The following are the major proposed solutions to that obstacle.
Slow interstellar travel
A generation ship is a type of interstellar ark in which the travellers live normally (not in suspended animation) and the crew who arrive at the destination are descendants of those who started the journey.
Generation ships are not currently feasible, both because of the enormous scale of such a ship 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.
Generation ships would also have to solve major biological and social problems (Sex and Society Aboard the First Starships). Estimates of the minimum viable population vary - 150 is about the lowest, but such a small population would be vulnerable to genetic drift, which might reduce the gene pool below a safe level. A generation ship in fiction typically takes thousands of years to reach its destination, i.e. longer than most human civilizations have lasted. Hence there is a risk that the culture which arrives may be incapable of doing what is needed- in the worst case it may have fallen into barbarism. Also, they may forget that they are on a generation ship. Stephen Baxter's story Mayflower II (in the collection Resplendent) explores both of these risks.
Generation ships
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.
Suspended animation
A variant on this possibility is based on the development of substantial human life extension, such as the "Engineered Negligible 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.
Extended human lifespan
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.)
Frozen embryos
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.
Fast interstellar travel
If a spaceship could average 10 percent of light speed, this would be enough to reach Proxima Centauri in forty years. Several propulsion systems are able to achieve this, but none of them is reasonably cheap.
In the sixties it was already technically possible to build 8 million ton spaceships with nuclear pulse propulsion engines, perhaps capable of reaching speeds of about 7 percent of light speed. One problem with such a propulsion method is that it uses nuclear explosions as a driving force, and, paradoxically enough, under current nuclear test ban treaties, nuclear explosions are only legal on Earth. See Project Orion for details.
Fusion rocket starships, using foreseeable fusion reactors which hopefully should be feasible roughly about 2040, should be able to reach speeds of approximately 10 percent of that of light. These would "burn" deuterium.
Light sails powered by massive ground-based lasers could potentially reach even greater speeds, because there is no need to accelerate the fuel. But decelerating a solar sail looks very problematic. An hybrid design (accelerate by laser sail, decelerate by fusion rocket) would probably be more effective.
In 1960 Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton-proton fusion reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected en route, the craft could theoretically accelerate to near the speed of light.
Linear Accelerator Propulsion (LINAC) using invariant mass electron / microwave beam as propellant. An inexhaustable supply of electrons in space makes the technology capable of continuous 24 x 7 non-stop propulsion operation constantly accelerating at 1 g where NLS would then be possible. "Einstein For Dummies", By Dr. Carlos I. Calle, PhD, NASA senior research scientist Pub. Date: June 2005, ISBN 978-0-7645-8348-3, Pages: 384 Pages. If the total distance is X, then the total travel time T is given by the expression
If X = 4.3 light-years, then T = 3.6 years. Dozens of stars could be reached in five to six years. In fact, a traveler could even go the Andromeda galaxy (2,000,000 light years) in under 29 years (Ship Time in Years) if a constant acceleration could be maintained. Dr. Steve Schaefer Ph.D. Princeton University (Physics).
Finally, there is the possibility of the Antimatter rocket. 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.
Sub-light-speed travelLight speed travel
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