Definitions

Interstellar travel

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. Intergalactic travel, the travel between different galaxies, is even more difficult.

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

The main difficulty of interstellar travel is the vast distances that have to be covered and therefore the time it takes with most realistic propulsion methods - from decades to millennia. Hence an interstellar ship would be much more severely exposed to the hazards found in interplanetary travel, including hard vacuum, radiation, weightlessness, and micrometeoroids. The long travel times make it difficult to design manned missions, and make a primarily economic justification of any interstellar mission nearly impossible, since benefits that do not become available for decades or longer have a present value close to zero.

It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, the money should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion.

Intergalactic travel involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.

Interstellar distances

Astronomical distances are often 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 approximately 300,000 kilometers per second or 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 the speed of light. At this 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 under 50 years' time. Current theories of physics indicate that it is impossible to travel faster than light within a flat region of space-time, 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 travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light, relativistic time dilation would make the voyage much shorter for the traveler. However, it would still take many years of elapsed time as viewed by the people remaining on Earth, and upon returning to Earth, the travelers would find that far more time had elapsed on Earth than had for them. (For more on this effect, see 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.

Prime targets for interstellar travel

There are 59 stellar systems within 20 light years from the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:

Stellar System Distance (ly) Remarks
Alpha Centauri 4.3 Closest system. Two stars, (G2, M5). Component A almost identical to our sun (a G2 star).
Barnard's Star 6.0 Small, low luminousity M5 red dwarf. Next closest to Solar System.
Sirius 8.7 Large, very bright A1 star with a white dwarf companion.
Epsilon Eridani 10.8 Single K2 star slightly smaller and colder than the Sun. May have solar system type planetary system.
Tau Ceti 11.8 Single G8 star similar to the Sun. High probability of possessing a solar system type planetary system.

Manned missions

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.

Proposed methods of 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 affordable.

Nuclear pulse propulsion

Since the 1960s it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. ships driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high speed, and therefore of reaching the nearest star in decades rather than centuries; construction and operational costs per unit of payload were expected to be similar to those of ships using chemical rockets.

Proposed interstellar spacecraft using nuclear pulse propulsion include Project Orion and Project Longshot. Using miniature nuclear bombs as fuel, Orion would be able to reach a velocity of 10% of the speed of light. It is one of very few known interstellar spacecraft proposals that could be constructed entirely with today's technology.

Fusion rockets

Fusion rocket starships, using foreseeable fusion reactors, should be able to reach speeds of approximately 10 percent of that of light. These would "burn" deuterium. One proposal using a fusion rocket is Project Daedalus.

The problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it quite heavy. The following three methods attempt to solve this problem.

Interstellar ramjets

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.

Antimatter rockets

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. Assuming that 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.

Beamed propulsion

A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need reaction mass and therefore would not need to accelerate that as well as the payload. In theory a lightsail driven by a laser or other beam from Earth can be used to decelerate a spacecraft approaching a distant star or planet, by detaching part of the sail and using it to focus the beam on the forward-facing surface of the rest of the sail. A magnetic sail could also decelerate to its destination without fuel, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium.

Beamed propulsion seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes, and would be considerably cheaper than nuclear pulse propulsion.

The following table lists some example concepts using beamed lased propulsion as proposed by the physicist Robert L. Forward

Mission Laser Power Vehicle Mass Acceleration Sail Diameter Maximum Velocity (% of the speed of light)
1. Flyby 65 GW 1 t 0.036 g 3.6 km 0.11 @ 0.17 ly
2. Rendezvous
outbound stage 7,200 GW 785 t 0.3 g 100 km 0.21 @ 2.1 ly
deleceration stage 26,000 GW 71 t 0.2 g 30 km 0.21 @ 4.3 ly
3. Manned
outbound stage 75,000,000 GW 78,500 t 0.3 g 1000 km 0.50 @ 0.4 ly
deleceration stage 17,000,000 GW 7,850 t 0.3 g 320 km 0.50 @ 10.4 ly
return stage 17,000,000 GW 785 t 0.3 g 100 km 0.50 @ 10.4 ly
deceleration stage 430,000 GW 785 t 0.3 g 100 km 0.50 @ 0.4 ly

Further speculative methods

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. Tachyons could not be used for communication.

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 speculative.

Warped spacetime

According to Einstein's equation of General Relativity, spacetime is curved:

$G_\left\{munu\right\}=8pi,GT_\left\{munu\right\}$

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.

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

Methods for slow manned missions

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 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. 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 at the destination 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. However, even assuming all relevant space-faring and suspended animation technologies could be developed, an automated means of guiding the ship to its destination with little or no human influence would be required.

Extended human lifespan

A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by 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 and educational robots which would replace human parents (see embryo space colonization).

NASA research

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

1. A method of propulsion able to reach the maximum speed which it is possible to attain
2. 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 passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years.

References

• (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.
• Geoffrey 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.