This article or section uses citations that link to broken or outdated sources, and are deemed unreliable. Please improve the article or discuss this issue on the talk page. Help on using footnotes is available. This article has been tagged since August 2007. 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 travel Light speed travel Main article: Teleportation Interstellar travel via transmission Main articles: faster-than-light and Faster than Light Travel Faster than light travel According to
General Relativity,
spacetime is curved, according to the
Einstein equation:

General relativity may permit the travel of an object faster than light in curved spacetime . One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the
Warp Drive concept.
In physics, the
Alcubierre drive is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of
exotic matter, or "negative mass". As a practical means of interstellar transportation, this idea has been criticized; see
Alcubierre Drive.
NASA research Eugene Mallove and Gregory Matloff (1989). The Starflight Handbook. John Wiley & Sons, Inc. ISBN 0-471-61912-4. Zubrin, Robert (1999). Entering Space: Creating a Spacefaring Civilization. Tarcher / Putnam. ISBN 1-58542-036-0. Eugene F. Mallove, Robert L. Forward, Zbigniew Paprotny, Jurgen Lehmann: "Interstellar Travel and Communication: A Bibliography,"
Journal of the British Interplanetary Society, Vol. 33, pp. 201-248, 1980.
Geofffrey A. Landis, "The Ultimate Exploration: A Review of Propulsion Concepts for Interstellar Flight," in
Interstellar Travel and Multi-Generation Space Ships, Kondo, Bruhweiller, Moore and Sheffield., eds., pp. 52-61, Apogee Books (2003),
ISBN 1-896522-99-8.
Zbigniew Paprotny, Jurgen Lehmann: "Interstellar Travel and Communication Bibliography: 1982 Update,"
Journal of the British Interplanetary Society, Vol. 36, pp. 311-329, 1983.
Zbigniew Paprotny, Jurgen Lehmann, John Prytz: "Interstellar Travel and Communication Bibliography: 1984 Update"
Journal of the British Interplanetary Society, Vol. 37, pp. 502-512, 1984.
Zbigniew Paprotny, Jurgen Lehmann, John Prytz: "Interstellar Travel and Communication Bibliography: 1985 Update"
Journal of the British Interplanetary Society, Vol. 39, pp. 127-136, 1986.