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.

The plausibility of 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 difficulties of interstellar travel

Interstellar travel poses a number of difficulties. There are all the difficulties of interplanetary travel, including hard vacuum, radiation, weightlessness, and micrometeoroids. These difficulties seem tractable; robot missions have been sent to 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.

Interstellar distances

Astronomical distances are sometimes measured in the length of time it would take a beam of light to travel between two points (see lightyear). Light in a vacuum travels 299,792,458 metres per second or 186,282.37 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 the speed of light. 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. 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 had for them. (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.

Probes versus human travel

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.

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, sometimes 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 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:

Generation ships

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 - 180 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 as does Robert A. Heinlein's two-part 1941 novel Orphans of the Sky.

Suspended animation

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 lifespan

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.

Frozen embryos

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 uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots. (See embryo space colonization.)

Fast interstellar travel

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.

Sub-light-speed 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 are 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^{[citation needed]}. 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 could 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. A 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 inexhaustible supply of electrons in space makes the technology capable of continuous propulsion providing constant 1 g acceleration where NLS would then be possible.^[1] If the total distance is x, then the total travel time T is given by the expression

{\frac {x}{2}}={\frac {c2}{g}}\cosh \left({\frac {0.5gT}{c}}-1\right)

T={\frac {2c}{g}}\cosh ^{-1}\left(1+{\frac {0.5gX}{c2}}\right)

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).^{[verification needed]}

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.

Light speed travel

Interstellar travel via transmission

If physical entities could be transmitted as information and reconstructed at a destination, travel precisely at the speed of light would be possible. Note that, under General Relativity, information cannot travel faster than light. The speed increase when compared to near-light-speed travel would therefore be minimal for outside observers, but for the travelers the journey would become instantaneous.

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.

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. Even the most serious-minded of these are extremely speculative.

Warped Spacetime

According to General Relativity, spacetime is curved, according to the Einstein equation:

$G_{\mu \,v}=8\pi \,GT_{\mu \,v}$

General relativity may permit the travel of an object faster than light in curved spacetime.^[2] 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.

Wormholes

Wormholes are conjectural 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.^[3] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.^[4] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes^[5]

NASA research

The NASA Breakthrough Propulsion Physics Project identified two breakthroughs which are needed for interstellar travel to be possible [2]:

A method of propulsion able to reach the maximum speed which is possible to attain
A new method of on-board energy production which would power those devices.

In other words, any engine short of the best conceivable engine won't work, and that engine cannot be powered by currently known energy sources. Analogies for 'breakthroughs' in technology are steam engines supplanting sailing ships, and jet aircraft replacing propeller aircraft.

Geoffery A. Landis, of NASA's Glenn Research Center, says that a laser-powered interstellar sail ship could possibly be launched within 50 years, utilizing new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with gigantic sails, propelled by laser light to about one tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it did not have to stop. Stopping at a destination could take 100 years.^[6]

References

^ "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.
^ http://rst.gsfc.nasa.gov/Sect20/A10.html
^ (http://www.nasa.gov/centers/glenn/research/warp/ideachev.html#worm)
^ John G. Cramer, Robert L. Forward, Michael S. Morris, Matt Visser, Gregory Benford, and Geoffrey A. Landis, "Natural Wormholes as Gravitational Lenses," Phys. Rev. D51 (1995) 3117-3120
^ M. Visser (1995) Lorentzian Wormholes: from Einstein to Hawking, AIP Press, Woodbury NY, ISBN 1-56396-394-9
^ [1] Malik, Tariq, "Sex and Society Aboard the First Starships." Science Tuesday, Space.com March 19, 2002.

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.

External links

[1] "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.

[2] ttp://rst.gsfc.nasa.gov/Sect20/A10.html

[3] (http://www.nasa.gov/centers/glenn/research/warp/ideachev.html#worm)

[4] John G. Cramer, Robert L. Forward, Michael S. Morris, Matt Visser, Gregory Benford, and Geoffrey A. Landis, "Natural Wormholes as Gravitational Lenses," Phys. Rev. D51 (1995) 3117-3120

[5] M. Visser (1995) Lorentzian Wormholes: from Einstein to Hawking, AIP Press, Woodbury NY, ISBN 1-56396-394-9

[6] [1] Malik, Tariq, "Sex and Society Aboard the First Starships." Science Tuesday, Space.com March 19, 2002.

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