View source for Engineering:Interstellar travel

{{Short description|Hypothetical travel between stars or planetary systems}}
[[File:Bussard Interstellar Ramjet Engine.jpg|thumb|A Bussard ramjet, one of many possible methods that could serve to propel spacecraft.]]

'''Interstellar travel''' is the hypothetical travel of [[Engineering:Spacecraft|spacecraft]] from one [[Astronomy:Star system|star system]], solitary [[Astronomy:Star|star]], or [[Astronomy:Planetary system|planetary system]] to another. Interstellar travel is expected to prove much more difficult than [[Astronomy:Interplanetary spaceflight|interplanetary spaceflight]] due to the vast difference in the scale of the involved distances. Whereas the distance between any two [[Astronomy:Planet|planet]]s in the [[Astronomy:Solar System|Solar System]] is less than 55 [[Astronomy:Astronomical unit|astronomical unit]]s (AU), stars are typically separated by hundreds of thousands of AU, causing these distances to typically be expressed instead in [[Astronomy:Light-year|light-year]]s. Because of the vastness of these distances, non-generational interstellar travel based on known physics would need to occur at a high percentage of the [[Physics:Speed of light|speed of light]]; even so, travel times would be long, at least decades and perhaps millennia or longer.<ref name="Mauldin">{{Cite book|last=Mauldin|first=John H.|url=https://archive.org/details/prospectsforinte0000maul|url-access=registration|quote=Interstellar travel.|title=Prospects for interstellar travel|date=May 1992|publisher=Published for the American Astronautical Society by Univelt|language=en}}</ref>

As of 2022, five uncrewed spacecraft, all launched and operated by the United States, have achieved the escape velocity required to leave the [[Astronomy:Solar System|Solar System]] as part of missions to explore parts of the outer system. They will therefore continue to travel through interstellar space indefinitely. However, they will not approach another star for hundreds of thousands of years, long after they have ceased to operate (though in theory the [[Astronomy:Voyager Golden Record|Voyager Golden Record]] would be playable in the event that the spacecraft is retrieved by an extraterrestrial civilization).

The speeds required for interstellar travel in a human lifetime far exceed what current methods of space travel can provide. Even with a hypothetically perfectly efficient propulsion system, the [[Physics:Kinetic energy|kinetic energy]] corresponding to those speeds is enormous by today's standards of [[Physics:Energy development|energy development]]. Moreover, collisions by spacecraft with [[Astronomy:Cosmic dust|cosmic dust]] and gas at such speeds would be very dangerous for both passengers and the spacecraft itself.<ref name="Mauldin"/>

A number of strategies have been proposed to deal with these problems, ranging from [[Engineering:Generation ship|giant arks that would carry entire societies and ecosystems]], to microscopic [[Engineering:Space probe|space probe]]s. Many different [[Astronomy:Spacecraft propulsion|spacecraft propulsion]] systems have been proposed to give spacecraft the required speeds, including nuclear propulsion, [[Astronomy:Beam-powered propulsion|beam-powered propulsion]], and methods based on speculative physics.<ref>{{cite web|url=http://www.bis-space.com/what-we-do/advocacy/interstellar-travel|title=Interstellar Travel|website=www.bis-space.com|language=en-US|access-date=2017-06-16|archive-date=2017-06-16|archive-url=https://web.archive.org/web/20170616131942/http://www.bis-space.com/what-we-do/advocacy/interstellar-travel|url-status=dead}}</ref>

Humanity would need to overcome considerable technological and economic challenges to achieve either crewed or uncrewed interstellar travel. Even the most optimistic views forecast that it will be decades before this milestone is reached. However, in spite of the challenges, a wide range of scientific benefits are expected should interstellar travel become a reality.<ref>{{cite journal|last1=Crawford|first1=I. A.|title=The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight|journal=Journal of the British Interplanetary Society|date=2009|volume=62|pages=415–421|arxiv=1008.4893|bibcode = 2009JBIS...62..415C }}</ref>

Most interstellar travel concepts require a developed space logistics system capable of moving millions of tonnes to a construction / operating location, and most would require gigawatt-scale power for construction or power (such as [[Physics:Starwisp|Star Wisp]]{{ndash}} or [[Physics:Solar sail|Light Sail]]{{ndash}}type concepts). Such a system could grow organically if [[Physics:Space-based solar power|space-based solar power]] became a significant component of Earth's [[Physics:Energy mix|energy mix]]. Consumer demand for a multi-terawatt system would create the necessary multi-million ton/year logistical system.<ref>Conclusion of the 2016 Tennessee Valley Interstellar Workshop Space Solar Power Working Track run by Peter Garretson & Robert Kennedy.</ref>

== Challenges ==

=== Interstellar distances ===
Distances between the planets in the Solar System are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some {{convert|1.5E8|km|e6mi|sp=us|abbr=off}}. [[Astronomy:Venus|Venus]], the closest planet to Earth is (at closest approach) 0.28 AU away. [[Astronomy:Neptune|Neptune]], the farthest planet from the Sun, is 29.8 AU away. As of January 20, 2023, [[Astronomy:Voyager 1|Voyager 1]], the farthest human-made object from Earth, is 159 AU away.<ref name="voyager">{{cite web |author=Howell |first=Elizabeth |date=19 January 2022 |title=Voyager 1: Earth's farthest spacecraft |url=https://www.space.com/17688-voyager-1.html |accessdate=8 April 2022 |website=Space.com |archive-date=22 February 2022 |archive-url=https://web.archive.org/web/20220222120811/https://www.space.com/17688-voyager-1.html |url-status=live }}</ref>

The closest known star, [[Astronomy:Proxima Centauri|Proxima Centauri]], is approximately {{convert|4.243|ly|AU|0|disp=output only}} away, or over 9,000 times farther away than Neptune.
<!--
1 A.U. = 149,597,870,700 meters,
1 light-year = 9,460,730,472,580,800 meters or 63241.077 AU,
Venus perihelion = 0.7184 AU
Neptune perihelion = 29.809946 AU
Voyager 1 distance (2021/Jan) = 156 AU
Proxima = 4.243 ± 0.002 light years away.
-->

{| class="wikitable" style="margin: 1em auto 1em auto;"
! Object
! Distance<br>(AU)
! Light time
|-
| [[Astronomy:Moon|Moon]]
| 0.0026
| 1.3 seconds
|-
| [[Astronomy:Sun|Sun]]
| 1
| 8 minutes
|-
| [[Astronomy:Venus|Venus]] (nearest planet)
| 0.28
| 2.41 minutes
|-
| [[Astronomy:Neptune|Neptune]] (farthest planet)
| 29.8
| 4.1 hours
|-
| [[Astronomy:Voyager 2|Voyager 2]]
|133.1
|18.45 hours
|-
| [[Astronomy:Voyager 1|Voyager 1]]
| 159.4
| 22.1 hours
|-
| [[Astronomy:Proxima Centauri|Proxima Centauri]] (nearest star and exoplanet)&nbsp; &nbsp;
| 268,332&nbsp;
| 4.24 years
|}

Because of this, distances between stars are usually expressed in [[Astronomy:Light-year|light-year]]s (defined as the distance that light travels in vacuum in one [[Astronomy:Julian year|Julian year]]) or in [[Astronomy:Parsec|parsec]]s (one parsec is 3.26 ly, the distance at which [[Astronomy:Stellar parallax|stellar parallax]] is exactly one [[Arcsecond|arcsecond]], hence the name). Light in a vacuum travels around {{convert|300000|km|mi|-3}} per second, so 1 light-year is about {{convert|{{convert|1|ly|Pm|3|disp=number}}e12|km|e12mi|abbr=off|sp=us}} or {{convert|1|ly|AU|0|disp=number}} AU. Hence, Proxima Centauri is approximately 4.243 light-years from Earth.

Another way of understanding the vastness of interstellar distances is by scaling: One of the closest stars to the Sun, Alpha Centauri A (a Sun-like star that is one of two companions of Proxima Centauri), can be pictured by scaling down the [[Astronomy:Astronomical unit|Earth–Sun distance]] to {{convert|1|m|ft|2|spell=in|sp=us}}. On this scale, the distance to Alpha Centauri A would be {{convert|276|km|mi|abbr=off|sp=us}}<!--Using 4.37 ly from Alpha Centauri page-->.

The fastest outward-bound spacecraft yet sent, [[Astronomy:Voyager 1#Interstellar mission|Voyager 1]], has covered 1/600 of a light-year in 30 years and is currently moving at 1/18,000 the speed of light. At this rate, a journey to Proxima Centauri would take 80,000 years.<ref>{{cite web | url= http://www.nasa.gov/centers/glenn/technology/warp/scales.html | title= A Look at the Scaling | website= nasa.gov | publisher= NASA Glenn Research Center | date= 2015-03-11 | access-date= 28 June 2013 | archive-date= 8 July 2013 | archive-url= https://web.archive.org/web/20130708041902/http://www.nasa.gov/centers/glenn/technology/warp/scales.html | url-status= live }}</ref>

=== Required energy ===
A significant factor contributing to the difficulty is the energy that must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the [[Physics:Kinetic energy|kinetic energy]] <math>K = \tfrac{1}{2}mv^2</math> where <math>m</math> is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the lower bound for the required energy is doubled to <math>mv^2</math>.<ref>{{cite arXiv |last=Millis |first=Marc G. |year=2011 |title=Energy, incessant obsolescence, and the first interstellar missions |class=physics.gen-ph |eprint=1101.1066 }}</ref>

The velocity for a crewed round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles. This means that due to the <math>v^2</math> term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least {{convert|450|PJ|J TWh|sigfig=3|abbr=off|disp=or}}<ref>{{cite journal|last1=Zirnstein|first1=E.J|title=Simulating the Compton-Getting Effect for Hydrogen Flux Measurements: Implications for IBEX-Hi and -Lo Observations |journal=Astrophysical Journal|date=2013|volume=778|issue=2|pages=112–127|doi=10.1088/0004-637x/778/2/112|bibcode=2013ApJ...778..112Z|doi-access=free}}</ref> (world energy consumption 2008 was 143,851&nbsp;terawatt-hours),<ref>{{Cite book|title=Outer Solar System : prospective energy and material resources|last1=Badescu|first1=Viorel|last2=Zacny|first2=Kris|isbn=9783319738451|location=Cham, Switzerland|oclc=1033673323|date = 2018-04-28}}</ref> without factoring in efficiency of the propulsion mechanism. This energy has to be generated onboard from stored fuel, harvested from the interstellar medium, or projected over immense distances.

=== Interstellar medium ===
A knowledge of the properties of the [[Astronomy:Interstellar medium|interstellar gas and dust]] through which the vehicle must pass is essential for the design of any interstellar space mission.<ref name="ism" /> A major issue with traveling at extremely high speeds is that due to the requisite high relative speeds and large kinetic energies, collisions with interstellar dust could cause considerable damage to the craft. Various shielding methods to mitigate this problem have been proposed.<ref name="NASA">{{cite conference | url= http://www.nasa.gov/pdf/637131main_radiation%20shielding_symposium_r1.pdf | archive-url= https://web.archive.org/web/20140211144338/http://www.nasa.gov/pdf/637131main_radiation%20shielding_symposium_r1.pdf | url-status= dead | archive-date= 11 February 2014 | title= Active Radiation Shielding Utilizing High Temperature Superconductors | author= Westover, Shayne | conference= NIAC Symposium | date= 27 March 2012 }}</ref> Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects and mitigation methods have been discussed in literature, but many unknowns remain.<ref name="HGarrett">{{cite report | url= http://www.kiss.caltech.edu/workshops/systems2012/presentations/garrett.pdf | title= There and Back Again: A Layman's Guide to Ultra-Reliability for Interstellar Missions | author= Garrett, Henry | date= 30 July 2012 | url-status= dead | archive-url= https://web.archive.org/web/20140508062130/http://www.kiss.caltech.edu/workshops/systems2012/presentations/garrett.pdf | archive-date= 8 May 2014 }}</ref> An additional consideration is that due the non-homogeneous distribution of interstellar matter around the Sun, these risks would vary between different trajectories.<ref name=ism>{{cite journal|last1=Crawford|first1=I. A.|title=Project Icarus: A review of local interstellar medium properties of relevance for space missions to the nearest stars|journal=Acta Astronautica|date=2011|volume=68|issue=7–8|pages=691–699|doi=10.1016/j.actaastro.2010.10.016|arxiv=1010.4823|bibcode = 2011AcAau..68..691C |s2cid=101553}}</ref> Although a high density interstellar medium may cause difficulties for many interstellar travel concepts, [[Physics:Bussard ramjet|interstellar ramjets]], and some proposed concepts for decelerating interstellar spacecraft, would actually benefit from a denser interstellar medium.<ref name="ism" />

=== Hazards ===
The crew of an interstellar ship would face several significant hazards, including the psychological effects of long-term isolation, the physiological effects of extreme acceleration, the effects of exposure to ionising radiation, and the physiological effects of [[Physics:Weightlessness|weightlessness]] to the muscles, joints, bones, immune system, and eyes. There also exists the risk of impact by [[Astronomy:Micrometeoroid|micrometeoroid]]s and other [[Astronomy:Space debris|space debris]]. These risks represent challenges that have yet to be overcome.<ref>{{cite book|last=Gibson|first=Dirk C.|title=Terrestrial and Extraterrestrial Space Dangers: Outer Space Perils, Rocket Risks and the Health Consequences of the Space Environment|url=https://books.google.com/books?id=dvYADgAAQBAJ&pg=PP1|year=2015|publisher=Bentham Science Publishers|isbn=978-1-60805-991-1|page=1}}</ref>

=== Wait calculation ===
The speculative fiction writer and physicist [[Biography:Robert L. Forward|Robert L. Forward]] has argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity and not yet having reached the limit, the resources 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 (the incessant obsolescence postulate).<ref name="Bob Forward 1996" /> In 2006, Andrew Kennedy calculated ideal departure dates for a trip to Barnard's Star using a more precise concept of the wait calculation where for a given destination and growth rate in propulsion capacity there is a departure point that overtakes earlier launches and will not be overtaken by later ones and concluded "an interstellar journey of 6 light years can best be made in about 635 years from now if growth continues at about 1.4% per annum", or approximately 2641 AD.<ref>{{Cite journal |last=Kennedy |first=Andrew |date=July 2006 |title=Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress |url=https://gwern.net/doc/statistics/decision/2006-kennedy.pdf |journal=Journal of the British Interplanetary Society |volume=59 |pages=239–246 |bibcode=2006JBIS...59..239K |access-date=9 June 2023 |number=7}}</ref> It may be the most significant calculation for competing cultures occupying the galaxy.<ref>Kennedy, A., "The Wait Calculation: The Broader Consequences
of the minimum time from now to interstellar destinations and its significance to the space economy". JBIS, 66:96-109, 2013</ref>

== Prime targets for interstellar travel ==
There are 59 known stellar systems within 40 light years of the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:<ref name="Bob Forward 1996">{{Cite journal |first=Robert L. |last=Forward |title=Ad Astra! |journal=Journal of the British Interplanetary Society |volume=49 |issue=1 |pages=23–32 |date=1996|bibcode = 1996JBIS...49...23F }}</ref>

{| class="wikitable" style="text-align:left"
|-
! System !! Distance (ly) !! Remarks
|-
| Alpha&nbsp;Centauri || 4.3 || Closest system. Three stars (G2, K1, M5). Component&nbsp;A is similar to the Sun (a G2 star). On August 24, 2016, the discovery of an Earth-size [[Astronomy:Exoplanet|exoplanet]] ([[Astronomy:Proxima Centauri b|Proxima Centauri b]]) orbiting in the habitable zone of [[Astronomy:Proxima Centauri|Proxima Centauri]] was announced.
|-
| [[Astronomy:Barnard's Star|Barnard's Star]] || 6 || Small, low-luminosity M5&nbsp;[[Astronomy:Red dwarf|red dwarf]]. Second closest to Solar System.
|-
| [[Astronomy:Sirius|Sirius]] || 8.6 || Large, very bright A1&nbsp;star with a [[Astronomy:White dwarf|white dwarf]] companion.
|-
| [[Astronomy:Epsilon Eridani|Epsilon Eridani]] || 10.5 || Single K2&nbsp;star slightly smaller and colder than the Sun. It has two asteroid belts. It is also believed to host a gas giant (AEgir),<ref>{{cite encyclopedia | url= https://exoplanet.eu/catalog/eps_eridani_b--73/ | title= Planet eps Eridani b | date= 16 December 1995 | encyclopedia= [[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] | access-date= 2023-08-09}}</ref> possibly another smaller planet,<ref>{{cite encyclopedia | url= https://exoplanet.eu/catalog/eps_eridani_c--287/ | title= Planet eps Eridani c | date= 16 December 1995 | encyclopedia= [[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] | access-date= 2023-08-09}}</ref> and may possess a Solar-System-type planetary system.
|-
| [[Astronomy:Tau Ceti|Tau Ceti]] || 11.8 || Single [[Astronomy:G-type main-sequence star|G8 star]] similar to the Sun. High probability of possessing a Solar-System-type planetary system: current evidence shows four planets with potentially two in the habitable zone.
|-
|[[Astronomy:Luyten's Star|Luyten's Star]]
|12.36
|M3 red dwarf with the super-Earth [[Astronomy:Luyten b|Luyten b]] orbiting in the habitable zone.
|-
| [[Astronomy:Wolf 1061|Wolf 1061]] || 14.1 || Wolf 1061 c is 1.6 times the size of Earth; it may have rocky terrain. It also sits within the 'Goldilocks' zone where it might be possible for liquid water to exist.<ref>{{Cite web |date=18 December 2015 |title=Astronomers Have Discovered The Closest Potentially Habitable Planet |url=https://ca.news.yahoo.com/astronomers-discovered-closest-potentially-habitable-105244680.html |access-date=2023-05-06 |website=[[Software:Yahoo! News|Yahoo! News]] |language=en-CA |archive-date=14 February 2021 |archive-url=https://web.archive.org/web/20210214141503/https://ca.news.yahoo.com/astronomers-discovered-closest-potentially-habitable-105244680.html |url-status=live }}</ref>
|-
|-
| [[Astronomy:Vega|Vega]] || 25 || A very young system possibly in the process of planetary formation.<ref>{{cite web |author=Croswell, Ken |date=3 December 2012 |title=ScienceShot: Older Vega Mature Enough to Nurture Life |url=https://www.science.org/content/article/scienceshot-older-vega-mature-enough-nurture-life |url-status=live |archive-url=https://web.archive.org/web/20121204103521/http://news.sciencemag.org/sciencenow/2012/12/scienceshot-older-vega-mature-en.html |archive-date=4 December 2012 |website=Science |doi=10.1126/article.26684|doi-broken-date=1 August 2023 }}</ref>
|-
| [[Astronomy:TRAPPIST-1|TRAPPIST-1]] ||40.7|| A system which boasts seven Earth-like planets, some of which may have liquid water. The discovery is a major advancement in finding a habitable planet and in finding a planet that could support life.
|}

Existing astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.

== Proposed methods ==

=== Slow, uncrewed probes ===
{{Main|Astronomy:Interstellar probe}}
"Slow" interstellar missions (still fast by other standards) based on current and near-future propulsion technologies are associated with trip times starting from about several decades to thousands of years. These missions consist of sending a robotic probe to a nearby star for exploration, similar to interplanetary probes like those used in the [[Astronomy:Voyager program|Voyager program]].<ref name="National Aeronautics and Space Administration">{{cite book|title=Voyager|date=1977|publisher=ERIC Clearing House|location=Louisiana State University|page=12|url=http://eds.a.ebscohost.com.libezp.lib.lsu.edu/eds/detail/detail?vid=5&sid=76390b8e-3776-42f2-87d0-14f9ce6435e2%40sessionmgr4001&hid=4210&bdata=JnNpdGU9ZWRzLWxpdmUmc2NvcGU9c2l0ZQ%3d%3d#AN=lalu.2943532&db=cat00252a|access-date=2015-10-26}}</ref> By taking along no crew, the cost and complexity of the mission is significantly reduced, as is the mass that needs to be accelerated, although technology lifetime is still a significant issue next to obtaining a reasonable speed of travel. Proposed concepts include [[Engineering:Project Daedalus|Project Daedalus]], [[Engineering:Project Icarus (interstellar)|Project Icarus]], [[Engineering:Project Dragonfly (space study)|Project Dragonfly]], [[Astronomy:Project Longshot|Project Longshot]],<ref name="centauri-dreams.org">{{cite web |url=http://www.centauri-dreams.org/?p=31478 |title=Project Dragonfly: The case for small, laser-propelled, distributed probes |work=Centauri Dreams |first=Paul |last=Gilster |date=September 5, 2014 |access-date=12 June 2015 |archive-date=2 July 2018 |archive-url=https://web.archive.org/web/20180702150806/https://www.centauri-dreams.org/2014/09/05/project-dragonfly-the-case-for-small-laser-propelled-distributed-probes/ |url-status=live }}</ref> and more recently [[Astronomy:Breakthrough Starshot|Breakthrough Starshot]].<ref>{{cite web|url=http://www.bbc.com/future/story/20161003-the-myths-and-reality-about-interstellar-travel|title=The myths and reality about interstellar travel|last=Nogrady|first=Bianca|work=BBC Future|date=4 October 2016|access-date=2017-06-16|archive-date=12 July 2017|archive-url=https://web.archive.org/web/20170712143201/http://www.bbc.com/future/story/20161003-the-myths-and-reality-about-interstellar-travel|url-status=live}}</ref>

=== Fast, uncrewed probes ===
{{Main|Astronomy:Interstellar probe}}

==== Nanoprobes ====
Near-lightspeed nano spacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the [[Organization:University of Michigan|University of Michigan]] are developing thrusters that use [[Physics:Nanoparticle|nanoparticle]]s as propellant. Their technology is called "nanoparticle field extraction thruster", or [[Engineering:Nano-particle field extraction thruster|nanoFET]]. These devices act like small particle accelerators shooting conductive nanoparticles out into space.<ref>{{Cite news |first=Daniel H. |last=Wilson |url=http://www.nbcnews.com/id/31665236 |title=Near-lightspeed nano spacecraft might be close |date=July 8, 2009 |work=NBC News |access-date=13 November 2019 |archive-date=15 April 2016 |archive-url=https://web.archive.org/web/20160415170459/http://www.nbcnews.com/id/31665236/ |url-status=live }}</ref>

[[Biography:Michio Kaku|Michio Kaku]], a theoretical physicist, has suggested that clouds of "smart dust" be sent to the stars, which may become possible with advances in [[Engineering:Nanotechnology|nanotechnology]]. Kaku also notes that a large number of nanoprobes would need to be sent due to the vulnerability of very small probes to be easily deflected by magnetic fields, micrometeorites and other dangers to ensure the chances that at least one nanoprobe will survive the journey and reach the destination.<ref>{{Cite book |last=Kaku |first=Michio |title=Physics of the Impossible |publisher=Anchor Books |year=2008 |}}</ref>

As a near-term solution, small, laser-propelled interstellar probes, based on current CubeSat technology were proposed in the context of [[Engineering:Project Dragonfly (space study)|Project Dragonfly]].<ref name="centauri-dreams.org" />

=== Slow, crewed missions ===
In crewed missions, the duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.<ref>{{cite web |last=Hein |first=Andreas |date=April 17, 2012 |title=How Will Humans Fly to the Stars? |url=http://www.centauri-dreams.org/?p=22561 |access-date=12 April 2013 |website=Centauri Dreams |archive-date=20 January 2013 |archive-url=https://web.archive.org/web/20130120060728/http://www.centauri-dreams.org/?p=22561 |url-status=live }}</ref> They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.

==== Generation ships ====
{{Main|Engineering:Generation ship}}
A [[Engineering:Generation ship|generation ship]] (or '''world ship''') is a type of [[Engineering:Interstellar ark|interstellar ark]] in which the crew that arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.<!---So? nothing else in this article is, either---><ref name="Hein 2012 119–133">{{cite journal|last=Hein|first=A. M.|display-authors=0|title=World Ships: Architectures & Feasibility Revisited|journal=Journal of the British Interplanetary Society|date=2012|volume=65|pages=119–133|url=https://www.academia.edu/2111006|bibcode=2012JBIS...65..119H|access-date=1 November 2017|archive-date=16 December 2021|archive-url=https://web.archive.org/web/20211216042335/https://www.academia.edu/2111006|url-status=live}}</ref><ref name="Hein 2020 75–104">{{cite journal|last=Hein|first=A.M.|author2=Smith, C.|author3=Marin, F.|author4=Staats, K.|title=World Ships: Feasibility and Rationale|journal=Acta Futura|date=2020|volume=12|pages=75–104|doi=10.5281/zenodo.3747333|arxiv=2005.04100|s2cid=218571111|url=https://zenodo.org/record/3747333|access-date=1 June 2020|archive-date=16 May 2021|archive-url=https://web.archive.org/web/20210516224552/https://zenodo.org/record/3747333|url-status=live}}</ref><ref>{{cite journal|last=Bond|first=A.|author2=Martin, A.R.|title=World Ships – An Assessment of the Engineering Feasibility|journal=Journal of the British Interplanetary Society|date=1984|volume=37|pages=254–266|bibcode = 1984JBIS...37..254B }}</ref><ref>{{cite book|last=Frisbee|first=R.H.|title=Limits of Interstellar Flight Technology in Frontiers of Propulsion Science|date=2009|publisher=Progress in Astronautics and Aeronautics}}</ref><ref>{{cite web |last=Hein |first=Andreas M. |title=Project Hyperion: The Hollow Asteroid Starship – Dissemination of an Idea |url=http://www.icarusinterstellar.org/project-hyperion-the-hollow-asteroid-starship-dissemination-of-an-idea/ |url-status=dead |archive-url=https://web.archive.org/web/20130410004948/http://www.icarusinterstellar.org/project-hyperion-the-hollow-asteroid-starship-dissemination-of-an-idea/ |archive-date=10 April 2013 |access-date=12 April 2013 |website=Icarus Interstellar}}</ref>

==== Suspended animation ====
{{Main|Engineering:Sleeper ship}}

Scientists and writers have postulated various techniques for [[Biology:Suspended animation|suspended animation]]. These include human [[Biology:Hibernation|hibernation]] and [[Physics:Cryonics|cryonic preservation]]. Although neither is currently practical, they offer the possibility of [[Engineering:Sleeper ship|sleeper ship]]s in which the passengers lie inert for the long duration of the voyage.<ref>{{cite journal|title=Various articles on hibernation|journal=Journal of the British Interplanetary Society|date=2006|volume=59|pages=81–144}}</ref>

==== Frozen embryos ====
{{Main|Unsolved:Embryo space colonization}}
A robotic interstellar mission carrying some number of frozen early stage human [[Biology:Embryo|embryo]]s is another theoretical possibility. This method of [[Engineering:Space colonization|space colonization]] requires, among other things, the development of an [[Engineering:Artificial uterus|artificial uterus]], the prior detection of a habitable [[Astronomy:Terrestrial planet|terrestrial planet]], and advances in the field of fully autonomous mobile robots and educational robots that would replace human parents.<ref>{{cite journal|last=Crowl|first=A.|author2=Hunt, J.|author3=Hein, A.M.|title=Embryo Space Colonisation to Overcome the Interstellar Time Distance Bottleneck|journal=Journal of the British Interplanetary Society|date=2012|volume=65|pages=283–285|url=http://www.jbis.org.uk/paper.php?p=2012.65.283|bibcode=2012JBIS...65..283C|access-date=12 April 2013|archive-date=31 July 2020|archive-url=https://web.archive.org/web/20200731004527/http://www.jbis.org.uk/paper.php?p=2012.65.283|url-status=live}}</ref>

==== Island hopping through interstellar space ====
Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids ([[Astronomy:Oort cloud|Oort cloud]]) to possible [[Astronomy:Rogue planet|rogue planet]]s. There may be ways to take advantage of these resources for a good part of an interstellar trip, slowly hopping from body to body or setting up waystations along the way.<ref>{{cite web |last=Gilster |first=Paul |date=12 February 2012 |title='Island-Hopping' to the Stars |url=http://www.centauri-dreams.org/?p=21719 |access-date=12 June 2015 |work=Centauri Dreams |archive-date=18 November 2021 |archive-url=https://web.archive.org/web/20211118074304/https://www.centauri-dreams.org/2012/02/13/island-hopping-to-the-stars/ |url-status=live }}</ref>

=== Fast, crewed missions ===
If a spaceship could average 10&nbsp;percent of light speed (and decelerate at the destination, for human crewed missions), this would be enough to reach [[Astronomy:Proxima Centauri|Proxima Centauri]] in forty years. Several propulsion concepts have been proposed<ref name=crawist>{{cite journal|last1=Crawford|first1=I. A.| date= 1990|title=Interstellar Travel: A Review for Astronomers|journal=Quarterly Journal of the Royal Astronomical Society|volume=31|pages=377–400|bibcode=1990QJRAS..31..377C}}</ref> that might be eventually developed to accomplish this (see § Propulsion below), but none of them are ready for near-term (few decades) developments at acceptable cost.

==== Time dilation ====
{{Main|Physics:Time dilation}}

Physicists generally believe faster-than-light travel is impossible. 
Relativistic [[Physics:Time dilation|time dilation]] allows a traveler to experience time more slowly, the closer their speed is to the speed of light.<ref>{{cite book |last1=Parkinson |first1=Bradford W. |url=http://app.knovel.com.libezp.lib.lsu.edu/hotlink/pdf/rcid:kpGPSVTA03/id:kt00C9LNA2/global-positioning-system/time-dilation?kpromoter=federation. |title=18.2.2.1Time Dilation |last2=Spilker |first2=James J. Jr. |last3=Axelrad |first3=Penina |last4=Enge |first4=Per |date=2014 |publisher=American Institute of Aeronautics and Astronautics |isbn=978-1-56347-106-3 |}}</ref> This apparent slowing becomes noticeable when velocities above 80% of the speed of light are attained. Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were capable of continuously generating around 1&nbsp;g of acceleration (which is comfortable for humans), the ship could reach almost anywhere in the galaxy and return to Earth within 40 years ship-time (see diagram). Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth.

For example, a spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g (i.e. 10.1&nbsp;m/s<sup>2</sup>) for 1.32 years (ship time), then stopping its engines and coasting for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination. After a short visit, the astronaut could return to Earth the same way. After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 light years per ship-year. The universe would appear contracted along the direction of travel to half the size it had when the ship was at rest; the distance between that star and the Sun would seem to be 16 light years as measured by the astronaut.

At higher speeds, the time on board will run even slower, so the astronaut could travel to the center of the [[Astronomy:Milky Way|Milky Way]] (30,000 light years from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 light year per Earth year, so, when back home, the astronaut will find that more than 60 thousand years will have passed on Earth.

==== Constant acceleration ====
[[File:Roundtriptimes.png|thumb|upright=1.75|This plot shows a ship capable of 1-[[Physics:Gravitational acceleration|g]] (10&nbsp;m/s<sup>2</sup> or about 1.0 ly/y<sup>2</sup>) "felt" or proper-acceleration<ref>{{cite web | title= Clock paradox III | url= http://www.eftaylor.com/pub/spacetime/STP1stEdExercP81to100.pdf | access-date= 2014-08-31 | archive-url= https://web.archive.org/web/20170721192831/http://www.eftaylor.com/pub/spacetime/STP1stEdExercP81to100.pdf | archive-date= 2017-07-21 | url-status= dead }} {{cite book | author1= Taylor, Edwin F. | author2= Wheeler, John Archibald | date= 1966 | title= Spacetime Physics | chapter-url= https://archive.org/details/spacetimephysics0000tayl | chapter-url-access= registration | publisher= W.H. Freeman, San Francisco | isbn= 978-0-7167-0336-5 | chapter= Chapter 1 Exercise 51 | pages= [https://archive.org/details/spacetimephysics0000tayl/page/97 97–98]}}</ref> can go far, except for the problem of accelerating on-board propellant.]]

Regardless of how it is achieved, a propulsion system that could produce acceleration continuously from departure to arrival would be the fastest method of travel. A constant acceleration journey is one where the propulsion system accelerates the ship at a constant rate for the first half of the journey, and then decelerates for the second half, so that it arrives at the destination stationary relative to where it began. If this were performed with an acceleration similar to that experienced at the Earth's surface, it would have the added advantage of producing artificial "gravity" for the crew. Supplying the energy required, however, would be prohibitively expensive with current technology.<ref>{{Cite book |last=Crowell |first=Benjamin |url=https://open.umn.edu/opentextbooks/textbooks/59 |title=Light and Matter |publisher=Benjamin Crowell |year=2010 |chapter=4 (Force and motion) |access-date=6 May 2023 |archive-date=26 September 2022 |archive-url=https://web.archive.org/web/20220926091944/https://open.umn.edu/opentextbooks/textbooks/59 |url-status=live }}</ref>

From the perspective of a planetary observer, the ship will appear to accelerate steadily at first, but then more gradually as it approaches the speed of light (which it cannot exceed). It will undergo [[Physics:Hyperbolic motion (relativity)|hyperbolic motion]].<ref>{{cite journal |last1 = Yagasaki|first1 = Kazuyuki|title = Invariant Manifolds And Control Of Hyperbolic Trajectories On Infinite- Or Finite-Time Intervals|journal = Dynamical Systems|date = 2008|volume = 23|issue = 3|pages = 309–331|doi = 10.1080/14689360802263571|s2cid = 123409581}}</ref> The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.

From the perspective of an onboard observer, the crew will feel a [[Physics:Gravitational field|gravitational field]] opposite the engine's acceleration, and the universe ahead will appear to fall in that field, undergoing hyperbolic motion. As part of this, distances between objects in the direction of the ship's motion will gradually contract until the ship begins to decelerate, at which time an onboard observer's experience of the gravitational field will be reversed.

When the ship reaches its destination, if it were to exchange a message with its origin planet, it would find that less time had elapsed on board than had elapsed for the planetary observer, due to [[Physics:Time dilation|time dilation]] and [[Length contraction|length contraction]].

The result is an impressively fast journey for the crew.

== Propulsion ==

=== Rocket concepts ===
All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (''M''<sub>0</sub>, including fuel) to final (''M''<sub>1</sub>, fuel depleted) mass.

Very high [[Engineering:Power-to-weight ratio|specific power]], the ratio of thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.<ref>{{cite report | title= VISTA – A Vehicle for Interplanetary Space Transport Application Powered by Inertial Confinement Fusion | author= Orth, C. D. | date= 16 May 2003 | publisher= Lawrence Livermore National Laboratory | url= https://e-reports-ext.llnl.gov/pdf/318478.pdf | access-date= 9 April 2013 | archive-date= 21 December 2016 | archive-url= https://web.archive.org/web/20161221174826/http://e-reports-ext.llnl.gov/pdf/318478.pdf | url-status= live }}</ref> Some heat transfer is inevitable, resulting in an extreme thermal load.

Thus, for interstellar rocket concepts of all technologies, a key engineering problem (seldom explicitly discussed) is limiting the heat transfer from the exhaust stream back into the vehicle.<ref>{{cite book | title= The Exploration of Space | url= https://archive.org/details/explorationofspa00clar | url-access= registration | author= Clarke, Arthur C. | publisher= New York: Harper | date= 1951}}</ref>

==== Ion engine ====
A type of electric propulsion, spacecraft such as ''[[Engineering:Dawn (spacecraft)|Dawn]]'' use an ion engine. In an ion engine, electric power is used to create charged particles of the propellant, usually the gas xenon, and accelerate them to extremely high velocities. The exhaust velocity of conventional rockets is limited to about 5&nbsp;km/s by the chemical energy stored in the fuel's molecular bonds. They produce a high thrust (about 10<sup>6</sup> N), but they have a low specific impulse, and that limits their top speed. By contrast, ion engines have low force, but the top speed in principle is limited only by the electrical power available on the spacecraft and on the gas ions being accelerated. The exhaust speed of the charged particles range from 15&nbsp;km/s to 35&nbsp;km/s.<ref name="http://www.iflscience.com">{{citation| url= http://www.iflscience.com/space/dawn-new-era-revolutionary-ion-engine-took-spacecraft-ceres| title= Dawn Of A New Era: The Revolutionary Ion Engine That Took Spacecraft To Ceres| date= 10 March 2015| access-date= 13 March 2015| archive-date= 13 March 2015| archive-url= https://archive.today/20150313060144/http://www.iflscience.com/space/dawn-new-era-revolutionary-ion-engine-took-spacecraft-ceres| url-status= live}}</ref>

==== Nuclear fission powered ====

===== Fission-electric =====
Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles or nuclear-thermal rockets. Such vehicles probably have the potential to power solar system exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation. [[Engineering:Electrically powered spacecraft propulsion|Electrically powered spacecraft propulsion]] powered by a portable power-source, say a [[Physics:Nuclear reactor|nuclear reactor]], producing only small accelerations, would take centuries to reach for example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.<ref name="scholar.googleusercontent.com">{{citation|url=http://daedalus-zvezdolet.narod.ru/doceng/07eng.doc |title=Project Daedalus: The Propulsion System Part 1; Theoretical considerations and calculations. 2. REVIEW OF ADVANCED PROPULSION SYSTEMS |url-status=dead |archive-url=https://web.archive.org/web/20130628001133/http://daedalus-zvezdolet.narod.ru/doceng/07eng.doc |archive-date=2013-06-28 }}</ref>

===== Fission-fragment =====
[[Engineering:Fission-fragment rocket|Fission-fragment rocket]]s use [[Physics:Nuclear fission|nuclear fission]] to create high-speed jets of fission fragments, which are ejected at speeds of up to {{convert|12,000|km/s|mi/s|abbr=on}}. With fission, the energy output is approximately 0.1% of the total mass-energy of the reactor fuel and limits the effective exhaust velocity to about 5% of the velocity of light. For maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, so no extra reaction mass need be bookkept in the mass ratio.

===== Nuclear pulse =====
{{Main|Physics:Nuclear pulse propulsion}}
[[File:Modern Pulsed Fission Propulsion Concept.jpg|thumb|Modern Pulsed Fission Propulsion Concept.]]Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse and high [[Engineering:Power-to-weight ratio|specific power]].<ref name="GeneralDynamics1964NuclearPulseVehicleCondensed">{{cite web
 | url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf
 | title=Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp.)
 | date=January 1964
 | author=[[Company:General Dynamics|General Dynamics]] Corp.
 | publisher=U.S. Department of Commerce National Technical Information Service
 | access-date=7 July 2017
 | archive-date=11 May 2010
 | archive-url=https://web.archive.org/web/20100511090218/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf
 | url-status=live
 }}</ref>

[[Astronomy:Project Orion (nuclear propulsion)|Project Orion]] team member [[Biography:Freeman Dyson|Freeman Dyson]] proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that used pure [[Astronomy:Deuterium fusion|deuterium fusion]] detonations with a very high fuel-[[Engineering:Burnup|burnup]] fraction. He computed an exhaust velocity of 15,000&nbsp;km/s and a 100,000-tonne space vehicle able to achieve a 20,000&nbsp;km/s [[Astronomy:Delta-v|delta-v]] allowing a flight-time to [[Astronomy:Alpha Centauri|Alpha Centauri]] of 130 years.<ref name="Interstellar Transport">{{cite journal
 |author=Freeman J. Dyson
 |date=October 1968
 |title=Interstellar Transport
 |journal=Physics Today
 |volume=21 |issue=10 |page=41
 |doi=10.1063/1.3034534
|bibcode=1968PhT....21j..41D}}</ref> Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).<ref>Cosmos by Carl Sagan</ref> An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% and 80% of the speed of light. In each case saving fuel for slowing down halves the maximum speed. The concept of using a [[Physics:Magnetic sail|magnetic sail]] to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.<ref>{{cite journal | title= Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation | author1= Lenard, Roger X. | author2= Andrews, Dana G. | journal= Acta Astronautica | date= June 2007 | volume= 61 | issue= 1–6 | url= http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf | doi= 10.1016/j.actaastro.2007.01.052 | pages= 450–458 | bibcode= 2007AcAau..61..450L | access-date= 2013-11-24 | archive-date= 2014-06-17 | archive-url= https://web.archive.org/web/20140617053903/http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf | url-status= dead }}</ref> Alternative designs utilizing similar principles include [[Astronomy:Project Longshot|Project Longshot]], [[Engineering:Project Daedalus|Project Daedalus]], and [[Engineering:Mini-Mag Orion|Mini-Mag Orion]]. The principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.

In the 1970s the Nuclear Pulse Propulsion concept further was refined by [[Engineering:Project Daedalus|Project Daedalus]] by use of externally triggered [[Physics:Inertial confinement fusion|inertial confinement fusion]], in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then, [[Physics:Laser|laser]]s, [[Physics:Ion beam|ion beam]]s, neutral particle beams and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.<ref name="The Release of Thermonuclear Energy by Inertial Confinement">{{cite book |author=Winterberg |first=Friedwardt |title=The Release of Thermonuclear Energy by Inertial Confinement |date=2010 |publisher=World Scientific |isbn=978-981-4295-91-8}}</ref>

A current impediment to the development of ''any'' nuclear-explosion-powered spacecraft is the 1963 Partial Test Ban Treaty, which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. This treaty would, therefore, need to be renegotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international cooperation on at least the scale of the [[Astronomy:International Space Station|International Space Station]].

Another issue to be considered, would be the [[Physics:G-force|g-force]]s imparted to a rapidly accelerated spacecraft, cargo, and passengers inside (see [[Physics:Inertia negation|Inertia negation]]).

==== Nuclear fusion rockets ====
[[Physics:Fusion rocket|Fusion rocket]] starships, powered by [[Physics:Nuclear fusion|nuclear fusion]] reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light.<ref name="L.D. Jaffe 1963, pp. 49-58" /> These would "burn" such light element fuels as deuterium, tritium, <sup>3</sup>He, <sup>11</sup>B, and <sup>7</sup>Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4–10% of the speed of light. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, although these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.

[[File:Daedalus ship.png|thumb|Daedalus interstellar probe.]]Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II",<ref>[https://web.archive.org/web/20110610051632/http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213559.pdf PDF] C. R. Williams et al., 'Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion', 2001, 52 pages, NASA Glenn Research Center</ref> designed and optimized for crewed Solar System exploration, based on the D<sup>3</sup>He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300&nbsp;km/s with an acceleration of ~1.7•10<sup>−3</sup> ''g'', with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. Although these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% [[Engineering:Burnup|burnup]] fraction it could achieve a pure fusion product exhaust velocity of ~3,000&nbsp;km/s.

==== Antimatter rockets ====
{{Main|Physics:Antimatter rocket}}
An [[Physics:Antimatter rocket|antimatter rocket]] would have a far higher energy density and specific impulse than any other proposed class of rocket.<ref name="crawist" /> If energy resources and efficient production methods are found to make [[Physics:Antimatter|antimatter]] in the quantities required and store<ref>{{cite web|url=http://home.web.cern.ch/about/engineering/storing-antimatter|title=Storing antimatter - CERN|website=home.web.cern.ch|access-date=5 August 2015|archive-date=28 August 2015|archive-url=https://web.archive.org/web/20150828182849/http://home.web.cern.ch/about/engineering/storing-antimatter|url-status=live}}</ref><ref>{{cite web|url=https://newscenter.lbl.gov/2011/06/05/alpha-quarter-hour/|title=ALPHA Stores Antimatter Atoms Over a Quarter of an Hour – and Still Counting - Berkeley Lab|date=5 June 2011|access-date=5 August 2015|archive-date=6 September 2015|archive-url=https://web.archive.org/web/20150906020619/https://newscenter.lbl.gov/2011/06/05/alpha-quarter-hour/|url-status=live}}</ref> it safely, it would be theoretically possible to reach speeds of several tens of percent that of light.<ref name="crawist" /> Whether antimatter propulsion could lead to the higher speeds (>90% that of light) at which relativistic [[Physics:Time dilation|time dilation]] would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, is doubtful owing to the large quantity of antimatter that would be required.<ref name="crawist" /><ref>{{Cite book|last=Rouaud|first=Mathieu|date=2020|title=Interstellar travel and antimatter|publisher=Mathieu Rouaud |url=http://www.voyagepourproxima.fr/SR.pdf|isbn=9782954930930|access-date=10 September 2021|archive-date=10 September 2021|archive-url=https://web.archive.org/web/20210910152837/http://www.voyagepourproxima.fr/SR.pdf|url-status=live}}</ref>

Speculating that production and storage of antimatter should become feasible, two further issues need to be considered. First, in the annihilation of antimatter, much of the energy is lost as high-energy gamma radiation, and especially also as [[Physics:Neutrino|neutrino]]s, so that only about 40% of ''mc''<sup>2</sup> would actually be available if the antimatter were simply allowed to annihilate into radiations thermally.<ref name="crawist" /> Even so, the energy available for propulsion would be substantially higher than the ~1% of ''mc''<sup>2</sup> yield of nuclear fusion, the next-best rival candidate.

Second, heat transfer from the exhaust to the vehicle seems likely to transfer enormous wasted energy into the ship (e.g. for 0.1''g'' ship acceleration, approaching 0.3 trillion watts per ton of ship mass), considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming shielding was provided to protect the payload (and passengers on a crewed vehicle), some of the energy would inevitably heat the vehicle, and may thereby prove a limiting factor if useful accelerations are to be achieved.

More recently, [[Biography:Friedwardt Winterberg|Friedwardt Winterberg]] proposed that a matter-antimatter GeV gamma ray laser photon rocket is possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by the [[Physics:Mössbauer effect|Mössbauer effect]] to the spacecraft.<ref name="AA-20120821">{{cite journal |last=Winterberg |first=F. |title=Matter–antimatter gigaelectron volt gamma ray laser rocket propulsion |date=21 August 2012 |journal=Acta Astronautica |volume=81 |issue=1 |pages=34–39 |bibcode = 2012AcAau..81...34W |doi = 10.1016/j.actaastro.2012.07.001 }}</ref>

==== Rockets with an external energy source ====
Rockets deriving their power from external sources, such as a [[Physics:Laser|laser]], could replace their internal energy source with an energy collector, potentially reducing the mass of the ship greatly and allowing much higher travel speeds. [[Biography:Geoffrey A. Landis|Geoffrey A. Landis]] proposed an [[Astronomy:Interstellar probe|interstellar probe]] propelled by an [[Engineering:Ion thruster|ion thruster]] powered by the energy beamed to it from a base station laser.<ref name="geoffreylandis.com">{{cite conference | url= http://www.geoffreylandis.com/laser_ion_pres.htp | title= Laser-powered Interstellar Probe | author= Landis, Geoffrey A. | conference= Conference on Practical Robotic Interstellar Flight | location= NY University, New York, NY | date= 29 August 1994 | url-status= dead | archive-url= https://web.archive.org/web/20131002200923/http://www.geoffreylandis.com/laser_ion_pres.htp | archive-date= 2 October 2013 }}</ref> Lenard and Andrews proposed using a base station laser to accelerate nuclear fuel pellets towards a [[Engineering:Mini-Mag Orion|Mini-Mag Orion]] spacecraft that ignites them for propulsion.<ref>{{cite journal | title= Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation | author1= Lenard, Roger X. | author2= Andrews, Dana G. | journal= Acta Astronautica | date= June 2007 | volume= 61 | issue= 1–6 | url= http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf | doi= 10.1016/j.actaastro.2007.01.052 | pages= 450–458 | bibcode= 2007AcAau..61..450L | access-date= 2013-11-24 | archive-date= 2014-06-17 | archive-url= https://web.archive.org/web/20140617053903/http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf | url-status= dead }}</ref>

=== Non-rocket concepts ===
A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive, in accordance with the rocket equation. Several concepts attempt to escape from this problem:<ref name="crawist" /><ref>A. Bolonkin (2005). ''Non Rocket Space Launch and Flight''. Elsevier. {{ISBN|978-0-08-044731-5}}</ref>

==== RF resonant cavity thruster ====
A radio frequency (RF) resonant cavity thruster is a device that is claimed to be a [[Astronomy:Spacecraft propulsion|spacecraft thruster]]. In 2016, the Advanced Propulsion Physics Laboratory at [[Organization:NASA|NASA]] reported observing a small apparent thrust from one such test, a result not since replicated.<ref>{{Cite web|url=https://www.nationalgeographic.com/news/2016/11/nasa-impossible-emdrive-physics-peer-review-space-science/|title=NASA Team Claims 'Impossible' Space Engine Works—Get the Facts|date=2016-11-21|website=National Geographic News|language=en|access-date=2019-11-12|archive-date=12 November 2019|archive-url=https://web.archive.org/web/20191112135532/https://www.nationalgeographic.com/news/2016/11/nasa-impossible-emdrive-physics-peer-review-space-science/|url-status=dead}}</ref> One of the designs is called EMDrive. In December 2002, Satellite Propulsion Research Ltd described a working prototype with an alleged total thrust of about 0.02 newtons powered by an 850 W [[Engineering:Cavity magnetron|cavity magnetron]]. The device could operate for only a few dozen seconds before the magnetron failed, due to overheating.<ref>{{Cite web|url=http://rexresearch.com/shawyer/shawyer.htm|title=Roger SHAWYER -- EM Space Drive -- Articles & Patent|website=rexresearch.com|access-date=2019-11-12|archive-date=14 September 2019|archive-url=https://web.archive.org/web/20190914002837/http://www.rexresearch.com/shawyer/shawyer.htm|url-status=live}}</ref> The latest test on the EMDrive concluded that it does not work.<ref>{{Cite web|url=https://www.sciencealert.com/impossible-em-drive-test-concludes-external-thrust|title=The Latest Test on The 'Impossible' EM Drive Concludes It Doesn't Work|last=McRae|first=Mike|website=ScienceAlert|date=24 May 2018|language=en-gb|access-date=2019-11-12|archive-date=12 November 2019|archive-url=https://web.archive.org/web/20191112135539/https://www.sciencealert.com/impossible-em-drive-test-concludes-external-thrust|url-status=live}}</ref>

==== Helical engine ====
Proposed in 2019 by NASA scientist Dr. David Burns, the helical engine concept would use a particle accelerator to accelerate particles to near the speed of light. Since particles traveling at such speeds acquire more mass, it is believed that this mass change could create acceleration. According to Burns, the spacecraft could theoretically reach 99% the speed of light.<ref>{{Cite web|url=https://www.sciencealert.com/no-this-new-space-engine-isn-t-going-to-break-physics|title=NASA Engineer Claims 'Helical Engine' Concept Could Reach 99% The Speed of Light Without Propellant|last=Starr|first=Michelle|website=ScienceAlert|date=15 October 2019|language=en-gb|access-date=2019-11-12|archive-date=30 November 2019|archive-url=https://web.archive.org/web/20191130113556/https://www.sciencealert.com/no-this-new-space-engine-isn-t-going-to-break-physics|url-status=live}}</ref>

==== Interstellar ramjets ====
In 1960, Robert W. Bussard proposed the [[Physics:Bussard ramjet|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 [[Astronomy:Proton–proton chain reaction|proton–proton chain reaction]], and expel it out of the back. Later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design.{{citation needed|date=May 2016}} Yet the idea is attractive because the fuel would be collected ''en route'' (commensurate with the concept of ''energy harvesting''), so the craft could theoretically accelerate to near the speed of light. The limitation is due to the fact that the reaction can only accelerate the propellant to 0.12c. Thus the drag of catching interstellar dust and the thrust of accelerating that same dust to 0.12c would be the same when the speed is 0.12c, preventing further acceleration.

==== Beamed propulsion ====
[[File:Forward-sailcraft-scheme.png|thumb|This diagram illustrates Robert L. Forward's scheme for slowing down an interstellar light-sail at the star system destination.]]

A [[Physics:Solar sail|light sail]] or [[Physics:Magnetic sail|magnetic sail]] powered by a massive [[Physics:Laser|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 to carry its own reaction mass and therefore would only need to accelerate the craft's payload. [[Biography:Robert L. Forward|Robert L. Forward]] proposed a means for decelerating an interstellar craft with a light sail of 100 kilometers in the destination star system without requiring a laser array to be present in that system. In this scheme, a secondary sail of 30 kilometers is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.<ref name="Forward1984RoundtripInterstellar">{{cite journal | author=Forward, R.L. | title=Roundtrip Interstellar Travel Using Laser-Pushed Lightsails | journal=J Spacecraft | volume=21 | issue=2 | pages=187–195 | date=1984 | doi=10.2514/3.8632 |bibcode = 1984JSpRo..21..187F }}</ref> In 2002, [[Biography:Geoffrey A. Landis|Geoffrey A. Landis]] of [[Organization:NASA|NASA]]'s Glen Research center also proposed a laser-powered, propulsion, sail ship that would host a diamond sail (of a few nanometers thick) powered with the use of [[Physics:Solar energy|solar energy]].<ref>{{cite web|url=http://go.galegroup.com/ps/i.do?p=ITOF&id=GALE{{pipe}}A444067493&v=2.1&it=r&sid=summon|title=Alpha Centauri: Our First Target for Interstellar Probes|via=go.galegroup.com}}</ref> With this proposal, this interstellar ship would, theoretically, be able to reach 10 percent the speed of light. It has also been proposed to use beamed-powered propulsion to accelerate a spacecraft, and electromagnetic propulsion to decelerate it; thus, eliminating the problem that the Bussard ramjet has with the drag produced during acceleration.<ref>{{Cite web|last=Delbert|first=Caroline|date=2020-12-09|title=The Radical Spacecraft That Could Send Humans to a Habitable Exoplanet|url=https://www.popularmechanics.com/space/deep-space/a34907687/solar-one-radical-spacecraft-crewed-interstellar-travel-light-sail-fusion-reactor/|access-date=2020-12-12|website=Popular Mechanics|language=en-US|archive-date=11 December 2020|archive-url=https://web.archive.org/web/20201211070301/https://www.popularmechanics.com/space/deep-space/a34907687/solar-one-radical-spacecraft-crewed-interstellar-travel-light-sail-fusion-reactor/|url-status=live}}</ref>

A [[Physics:Magnetic sail|magnetic sail]] could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium.<ref>{{cite journal |title=Magnetic Sails and Interstellar Travel |journal=Journal of the British Interplanetary Society |date=1990 |last1=Andrews |first1=Dana G. |last2=Zubrin |first2=Robert M. |volume=43 |pages=265–272 |url=http://www.lunarsail.com/LightSail/msit.pdf |archive-url=https://web.archive.org/web/20141012182359/http://www.lunarsail.com/LightSail/msit.pdf |url-status=dead |archive-date=2014-10-12 |access-date=2014-10-08 }}</ref><ref>{{cite web |url=http://www.niac.usra.edu/files/library/meetings/fellows/nov99/320Zubrin.pdf |title=NIAC Study of the Magnetic Sail |last1=Zubrin |first1=Robert |last2=Martin |first2=Andrew |date=1999-08-11 |access-date=2014-10-08 |archive-date=24 May 2015 |archive-url=https://web.archive.org/web/20150524181108/http://www.niac.usra.edu/files/library/meetings/fellows/nov99/320Zubrin.pdf |url-status=live }}</ref>

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist [[Biography:Robert L. Forward|Robert L. Forward]]:<ref>{{cite book | author= Landis, Geoffrey A. | chapter= The Ultimate Exploration: A Review of Propulsion Concepts for Interstellar Flight | title= Interstellar Travel and Multi-Generation Space Ships | editor= Yoji Kondo | editor2= Frederick Bruhweiler | editor3= John H. Moore, Charles Sheffield |page=52 | publisher= Apogee Books | date= 2003 | isbn= 978-1-896522-99-9}}</ref>

{| class="wikitable"
|-
! Mission !! Laser Power !! Vehicle Mass !! Acceleration !! Sail Diameter !! Maximum Velocity <br /> (% of the speed of light)
|-
| colspan=6| 1. Flyby – Alpha Centauri, 40 years
|-
| ''outbound stage'' || 65 GW || 1 t || 0.036 g || 3.6&nbsp;km || 11% @ 0.17 ly
|-
| colspan=6| 2. Rendezvous – Alpha Centauri, 41 years
|-
| ''outbound stage'' || 7,200 GW|| 785 t || 0.005 g || 100&nbsp;km || 21% @ 4.29 ly{{dubious|date=May 2016}}<!--confused with flyby numbers?-->
|-
| ''deceleration stage'' || 26,000 GW || 71 t || 0.2 g || 30&nbsp;km || 21% @ 4.29 ly
|-
| colspan=6| 3. Crewed – Epsilon Eridani, 51 years (including 5 years exploring star system)
|-
| ''outbound stage'' || 75,000,000 GW || 78,500 t || 0.3 g || 1000&nbsp;km || 50% @ 0.4 ly
|-
| ''deceleration stage'' || 21,500,000 GW || 7,850 t || 0.3 g || 320&nbsp;km || 50% @ 10.4 ly
|-
| ''return stage'' || 710,000 GW || 785 t || 0.3 g || 100&nbsp;km || 50% @ 10.4 ly
|-
| ''deceleration stage'' || 60,000 GW || 785 t || 0.3 g || 100&nbsp;km || 50% @ 0.4 ly
|}

=====Interstellar travel catalog to use photogravitational assists for a full stop=====
The following table is based on work by Heller, Hippke and Kervella.<ref>{{Cite journal|arxiv=1704.03871|last1=Heller|first1=René|title=Optimized trajectories to the nearest stars using lightweight high-velocity photon sails|journal=The Astronomical Journal|volume=154|issue=3|pages=115|last2=Hippke|first2=Michael|last3=Kervella|first3=Pierre|year=2017|doi=10.3847/1538-3881/aa813f|bibcode=2017AJ....154..115H|s2cid=119070263 |doi-access=free }}</ref>

{| class="wikitable"
|-
! Name !! Travel time<br> (yr)!!  Distance<br> (ly)!! Luminosity<br> ([[Astronomy:Sun|L<sub>☉</sub>]])
|-
| ''Sirius A'' || 68.90 || 8.58 || 24.20
|-
| ''α Centauri A'' || 101.25 || 4.36 || 1.52
|-
| ''α Centauri B'' || 147.58|| 4.36 ||  0.50
|-
| ''Procyon A'' || 154.06 || 11.44 || 6.94
|-
| ''Vega'' || 167.39 ||25.02 || 50.05
|-
| '' Altair'' || 176.67 ||  16.69 ||  10.70
|-
| ''Fomalhaut A'' || 221.33 || 25.13 || 16.67
|-
| ''Denebola'' || 325.56 ||  35.78||  14.66
|-
| ''Castor A'' || 341.35|| 50.98 || 49.85
|-
| ''Epsilon Eridani'' ||  363.35 || 10.50 || 0.50
|}
* Successive assists at α Cen A and B could allow travel times to 75 yr to both stars.
* Lightsail has a nominal mass-to-surface ratio (σ<sub>nom</sub>) of 8.6×10<sup>−4</sup> gram m<sup>−2</sup> for a nominal graphene-class sail.
* Area of the Lightsail, about 10<sup>5</sup> m<sup>2</sup> = (316 m)<sup>2</sup> 
* Velocity up to 37,300&nbsp;km s<sup>−1</sup> (12.5% c)

==== Pre-accelerated fuel ====
Achieving start-stop interstellar trip times of less than a human lifetime require mass-ratios of between 1,000 and 1,000,000, even for the nearer stars. This could be achieved by multi-staged vehicles on a vast scale.<ref name="L.D. Jaffe 1963, pp. 49-58">{{cite journal |author1=D.F. Spencer |author2=L.D. Jaffe |title=Feasibility of Interstellar Travel |journal=Astronautica Acta |volume=9 |year=1963 |pages=49–58|url=https://apps.dtic.mil/sti/citations/AD0274312 |archive-url=https://web.archive.org/web/20171204204839/http://www.dtic.mil/docs/citations/AD0274312 |url-status=live |archive-date=December 4, 2017 }}</ref> Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.<ref name="Interstellar rendezvous missions employing fission propulsion systems">{{cite journal |author1=Roger X. Lenard  |author2=Ronald J. Lipinski  |title=Interstellar rendezvous missions employing fission propulsion systems |year=2000 |journal=AIP Conference Proceedings |volume=504 |pages=1544–1555|doi=10.1063/1.1290979  |bibcode=2000AIPC..504.1544L }}</ref>

==== Dynamic soaring ====
Dynamic soaring as a way to travel across interstellar space has been proposed.<ref name="SA-20221206">{{cite news |last=Mcrae |first=Mike |title='Dynamic Soaring' Trick Could Speed Spacecraft Across Interstellar Space |url=https://www.sciencealert.com/dynamic-soaring-trick-could-speed-spacecraft-across-interstellar-space |date=6 December 2022 |work=ScienceAlert |accessdate=6 December 2022 |archive-date=6 December 2022 |archive-url=https://web.archive.org/web/20221206063650/https://www.sciencealert.com/dynamic-soaring-trick-could-speed-spacecraft-across-interstellar-space |url-status=live }}</ref><ref name="FST-20221128">{{cite journal |last1=Larrouturou |first1=Mathias N. |last2=Higgns |first2=Andrew J. |last3=Greason |first3=Jeffrey K. |title=Dynamic soaring as a means to exceed the solar wind speed |date=28 November 2022 |journal=  Frontiers in Space Technologies|volume=3 |doi=10.3389/frspt.2022.1017442 |arxiv=2211.14643 |bibcode=2022FrST....317442L |doi-access=free }}</ref>

=== Theoretical concepts ===

==== Transmission of minds with light ====
[[Philosophy:Mind uploading|Uploaded human minds]] or AI could be transmitted with laser or radio signals at the [[Physics:Speed of light|speed of light]].<ref>{{cite web |title=Michio Kaku foretells humanity's extraordinary future |website=NBC News |date=2 March 2018 |url=https://www.nbcnews.com/mach/science/michio-kaku-sees-amazing-things-our-future-except-those-scary-ncna851226 |quote=We're going to have the Human Connectome Project map the human brain before the end of this century, I think. We're going to put the connectome on a laser beam and shoot it to the moon. In one second, our consciousness is on the moon. In 20 minutes we're on Mars, eight hours we're on Pluto, in four years our consciousness has reached the nearest star. |access-date=20 December 2021 |archive-date=20 December 2021 |archive-url=https://web.archive.org/web/20211220134353/https://www.nbcnews.com/mach/science/michio-kaku-sees-amazing-things-our-future-except-those-scary-ncna851226 |url-status=live }}</ref> This requires a receiver at the destination which would first have to be set up e.g. by humans, probes, self replicating machines (potentially along with AI or uploaded humans), or an alien civilization (which might also be in a different galaxy, perhaps a [[Astronomy:Kardashev scale#Type III|Kardashev type III civilization]]).

==== Faster-than-light travel ====
[[File:Wormhole travel as envisioned by Les Bossinas for NASA.jpg|thumb|Artist's depiction of a hypothetical ''Wormhole Induction Propelled Spacecraft'', based loosely on the 1994 [[Alcubierre drive|"warp drive"]] paper of Miguel Alcubierre.]]
{{Main|Physics:Faster-than-light}}

Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light, but even the most serious-minded of these are highly speculative.<ref name="crawftl">{{cite journal|last1=Crawford|first1=Ian A.|title=Some thoughts on the implications of faster-than-light interstellar space travel|journal=Quarterly Journal of the Royal Astronomical Society|date=1995|volume=36|pages=205–218|bibcode=1995QJRAS..36..205C}}</ref>

It is also debatable whether faster-than-light travel is physically possible, in part because of [[Physics:Tachyonic antitelephone|causality]] concerns: travel faster than light may, under certain conditions, permit travel backwards in time within the context of [[Physics:Special relativity|special relativity]].<ref>{{cite journal|last1=Feinberg|first1=G.|title=Possibility of faster-than-light particles|journal=Physical Review|date=1967|volume=159|issue=5|pages=1089–1105|doi=10.1103/physrev.159.1089|bibcode = 1967PhRv..159.1089F }}</ref> Proposed mechanisms for [[Physics:Faster-than-light|faster-than-light]] travel within the theory of general relativity require the existence of [[Physics:Exotic matter|exotic matter]]<ref name="crawftl" /> and, it is not known if it could be produced in sufficient quantities, if at all.

===== Alcubierre drive =====

{{Main|Alcubierre drive}}
In physics, the [[Alcubierre drive]] is based on an argument, within the framework of [[Physics:General relativity|general relativity]] and without the introduction of [[Physics:Wormhole|wormhole]]s, that it is possible to modify spacetime in a way that allows a spaceship to travel with an arbitrarily large speed by a local expansion of spacetime behind the spaceship and an opposite contraction in front of it.<ref name="Warp Alcubierre">{{cite journal |title=The warp drive: hyper-fast travel within general relativity |journal=Classical and Quantum Gravity |year=1994 |last=Alcubierre |first=Miguel |volume=11 |issue=5 |doi=10.1088/0264-9381/11/5/001 |pages=L73–L77|arxiv = gr-qc/0009013 |bibcode = 1994CQGra..11L..73A |citeseerx=10.1.1.338.8690 |s2cid=4797900 }}</ref> Nevertheless, this concept would require the spaceship to incorporate a region of [[Physics:Exotic matter|exotic matter]], or the hypothetical concept of [[Physics:Negative mass|negative mass]].<ref name="Warp Alcubierre" />

===== Artificial black hole =====

{{Main|Engineering:Black hole starship}}
A theoretical idea for enabling interstellar travel is to propel a starship by creating an artificial black hole and using a parabolic reflector to reflect its [[Physics:Hawking radiation|Hawking radiation]]. Although beyond current technological capabilities, a black hole starship offers some advantages compared to other possible methods. Getting the black hole to act as a power source and engine also requires a way to convert the Hawking radiation into energy and thrust. One potential method involves placing the hole at the focal point of a parabolic reflector attached to the ship, creating forward thrust. A slightly easier, but less efficient method would involve simply absorbing all the gamma radiation heading towards the fore of the ship to push it onwards, and let the rest shoot out the back.<ref name="Crane Westmoreland p. ">{{cite arXiv | last1=Crane | first1=Louis | last2=Westmoreland | first2=Shawn | title=Are Black Hole Starships Possible | year=2009 | page=| class=gr-qc | eprint=0908.1803 }}</ref><ref>{{Cite journal |url=https://www.newscientist.com/article/mg20427361.000-dark-power-grand-designs-for-interstellar-travel.html |title=Dark power: Grand designs for interstellar travel |journal=New Scientist |date=25 November 2009 |issue=2736 |last=Chown |first=Marcus |access-date=1 September 2017 |archive-date=26 April 2015 |archive-url=https://web.archive.org/web/20150426175526/http://www.newscientist.com/article/mg20427361.000-dark-power-grand-designs-for-interstellar-travel.html |url-status=live }}{{subscription required}}</ref><ref>{{Cite news |url=http://io9.com/5391989/a-black-hole-engine-that-could-power-spaceships |title=A Black Hole Engine That Could Power Spaceships |first=Tim |last=Barribeau |date=November 4, 2009 |work=io9 |access-date=11 August 2016 |archive-date=22 November 2015 |archive-url=https://web.archive.org/web/20151122040407/http://io9.com/5391989/a-black-hole-engine-that-could-power-spaceships |url-status=live }}</ref>

===== Wormholes =====
[[Physics:Wormhole|Wormhole]]s are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an Einstein–Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity that allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of [[Physics:Negative mass|negative mass]], which may be unphysical.<ref>{{cite web |url=http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html#worm |title=Ideas Based On What We'd Like To Achieve: Worm Hole transportation |date=11 March 2015 |publisher=NASA Glenn Research Center |access-date=4 September 2012 |archive-date=24 September 2013 |archive-url=https://web.archive.org/web/20130924060726/http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html#worm |url-status=live }}</ref> However, Cramer ''et al.'' argue that such wormholes might have been created in the early universe, stabilized by [[Astronomy:Cosmic string|cosmic string]]s.<ref>{{cite journal | author= John G. Cramer| author2= Robert L. Forward| author3= Michael S. Morris| author4= Matt Visser| author5= Gregory Benford| author6= Geoffrey A. Landis | title= Natural Wormholes as Gravitational Lenses | journal= Physical Review D | volume= 51 | issue= 3117 | date= 15 March 1995 | pages= 3117–3120 | doi= 10.1103/PhysRevD.51.3117 | pmid= 10018782| arxiv= ph/9409051|bibcode = 1995PhRvD..51.3117C | s2cid= 42837620}}</ref> The general theory of wormholes is discussed by Visser in the book ''Lorentzian Wormholes''.<ref>{{cite book | author= Visser, M. | date= 1995 | title= Lorentzian Wormholes: from Einstein to Hawking | publisher= AIP Press, Woodbury NY | isbn= 978-1-56396-394-0}}</ref>

== Designs and studies ==

=== Project Hyperion ===
{{Main|Engineering:Project Hyperion (interstellar)}}
[[Engineering:Project Hyperion (interstellar)|Project Hyperion]] has looked into various feasibility issues of crewed interstellar travel.<ref>{{cite web|title=Icarus Interstellar – Project Hyperion|url=http://www.icarusinterstellar.org/projects/project-hyperion/|access-date=13 April 2013|archive-date=20 April 2013|archive-url=https://web.archive.org/web/20130420063618/http://www.icarusinterstellar.org/projects/project-hyperion/|url-status=live}}</ref><ref name="HEIN">{{cite report|url=https://www.academia.edu/2111006|title=World Ships – Architectures & Feasibility Revisited|author=Hein, Andreas|date=January 2012|access-date=7 February 2013|display-authors=0|archive-date=16 December 2021|archive-url=https://web.archive.org/web/20211216042335/https://www.academia.edu/2111006|url-status=live}}</ref><ref>{{cite journal | last1 = Smith | first1 = Cameron M | year = 2014 | title = Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion | journal = Acta Astronautica | volume = 97 | pages = 16–29 | doi = 10.1016/j.actaastro.2013.12.013 | bibcode = 2014AcAau..97...16S }}</ref> Its members continue to publish on crewed interstellar travel in collaboration with the [[Organization:Initiative for Interstellar Studies|Initiative for Interstellar Studies]].<ref name="Hein 2020 75–104"/>

=== Enzmann starship ===
{{Main|Unsolved:Enzmann starship}}
The Enzmann starship, as detailed by [[Biography:G. Harry Stine|G. Harry Stine]] in the October 1973 issue of ''Analog'', was a design for a future starship, based on the ideas of Robert Duncan-Enzmann. The spacecraft itself as proposed used a 12,000,000 ton ball of frozen [[Physics:Deuterium|deuterium]] to power 12–24 thermonuclear pulse propulsion units. Twice as long as the Empire State Building is tall and assembled in-orbit, the spacecraft was part of a larger project preceded by [[Astronomy:Interstellar probe|interstellar probe]]s and telescopic observation of target star systems.<ref>{{cite web |url=http://www.centauri-dreams.org/?p=1142 |work=Centauri Dreams |title=A Note on the Enzmann Starship |first=Paul |last=Gilster |date=April 1, 2007 |access-date=18 November 2010 |archive-date=30 June 2011 |archive-url=https://web.archive.org/web/20110630042240/http://www.centauri-dreams.org/?p=1142 |url-status=live }}</ref>

=== NASA research ===
[[Organization:National Aeronautics and Space Administration|NASA]] has been researching interstellar travel since its formation, translating important foreign language papers and conducting early studies on applying fusion propulsion, in the 1960s, and laser propulsion, in the 1970s, to interstellar travel.

In 1994, NASA and JPL cosponsored a "Workshop on Advanced Quantum/Relativity Theory Propulsion" to "establish and use new frames of reference for thinking about the faster-than-light (FTL) question".<ref>{{cite journal |last1=Bennett |first1=Gary |last2=Forward |first2=Robert |last3=Frisbee |first3=Robert |title=Report on the NASA/JPL Workshop on advanced quantum/relativity theory propulsion |url=https://arc.aiaa.org/doi/pdf/10.2514/6.1995-2599 |journal=31st Joint Propulsion Conference and Exhibit |publisher=American Institute of Aeronautics and Astronautics |access-date=8 September 2020 |doi=10.2514/6.1995-2599 |date=10 July 1995}}</ref>

The [[Organization:National Aeronautics and Space Administration|NASA]] [[Engineering:Breakthrough Propulsion Physics Program|Breakthrough Propulsion Physics Program]] (terminated in FY 2003 after a 6-year, $1.2-million study, because "No breakthroughs appear imminent.")<ref>[https://web.archive.org/web/19991002003540/http://www.grc.nasa.gov/WWW/bpp "Breakthrough Propulsion Physics"] project at NASA Glenn Research Center, Nov 19, 2008</ref> identified some breakthroughs that are needed for interstellar travel to be possible.<ref name="NASAWarpDriveBreakthruTechs">{{Cite web |url=http://www.nasa.gov/centers/glenn/technology/warp/warp.html |title=Warp Drive, When? |work=NASA Breakthrough Technologies |date=January 26, 2009 |access-date=2 April 2010 |archive-date=7 July 2008 |archive-url=https://web.archive.org/web/20080707043144/http://www.nasa.gov/centers/glenn/technology/warp/warp.html |url-status=live }}</ref>

[[Biography:Geoffrey A. Landis|Geoffrey A. Landis]] of NASA's [[Organization:Glenn Research Center|Glenn Research Center]] states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I&nbsp;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 extensive 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 without stopping. Slowing down to stop at Alpha Centauri could increase the trip to 100 years,<ref name="Malik20020319">{{cite web|url=http://www.space.com/scienceastronomy/generalscience/star_voyage_020319-1.html |title=Sailing to the Stars: Sex and Society Aboard the First Starships |website=Space.com |access-date=2009-04-03 |url-status=dead |archive-url=https://web.archive.org/web/20090327233721/http://www.space.com/scienceastronomy/generalscience/star_voyage_020319-1.html |archive-date=2009-03-27 }} Malik, Tariq, "Sex and Society Aboard the First Starships." Science Tuesday, Space.com March 19, 2002.</ref> whereas a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.

=== 100 Year Starship study ===
The [[Astronomy:100 Year Starship|100 Year Starship]] (100YSS) study was the name of a one-year project to assess the attributes of and lay the groundwork for an organization that can carry forward the 100 Year Starship vision. 100YSS-related symposia were organized between 2011 and 2015.

Harold ("Sonny") White<ref>{{cite web|url=http://www.icarusinterstellar.org/team/harold-white/|title=Dr. Harold "Sonny" White – Icarus Interstellar|work=icarusinterstellar.org|access-date=12 June 2015|url-status=dead|archive-url=https://web.archive.org/web/20150601090330/http://www.icarusinterstellar.org/team/harold-white/|archive-date=1 June 2015}}</ref> from NASA's Johnson Space Center is a member of Icarus Interstellar,<ref name="icarusinterstellar.org">{{cite web|url=http://www.icarusinterstellar.org|title=Icarus Interstellar – A nonprofit foundation dedicated to achieving interstellar flight by 2100.|work=icarusinterstellar.org|access-date=12 June 2015|archive-date=2 December 2013|archive-url=https://web.archive.org/web/20131202234741/http://www.icarusinterstellar.org/|url-status=dead}}</ref> the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a [[Physics:White–Juday warp-field interferometer|laser to try to warp spacetime]] by 1 part in 10 million with the aim of helping to make interstellar travel possible.<ref>{{cite web | url= http://www.space.com/17628-warp-drive-possible-interstellar-spaceflight.html | title= Warp Drive May Be More Feasible Than Thought, Scientists Say | author= Moskowitz, Clara | date= 17 September 2012 | website= space.com | access-date= 29 December 2012 | archive-date= 17 August 2013 | archive-url= https://web.archive.org/web/20130817082143/http://www.space.com/17628-warp-drive-possible-interstellar-spaceflight.html | url-status= live }}</ref>

=== Other designs ===
* [[Astronomy:Project Orion (nuclear propulsion)|Project Orion]], human crewed interstellar ship (1958–1968).
* [[Engineering:Project Daedalus|Project Daedalus]], uncrewed interstellar probe (1973–1978).
* Starwisp, uncrewed interstellar probe (1985).<ref>{{cite journal |title= Starwisp – An ultra-light interstellar probe |last= Forward |first= R. L. |journal= Journal of Spacecraft and Rockets |volume= 22 |number= 3 |date= May–June 1985 |pages= 345–350 |doi= 10.2514/3.25754 |bibcode= 1985JSpRo..22..345F}}</ref>
* [[Astronomy:Project Longshot|Project Longshot]], uncrewed interstellar probe (1987–1988).
* [[Engineering:Starseed launcher|Starseed/launcher]], fleet of uncrewed interstellar probes (1996).
* [[Physics:Project Valkyrie|Project Valkyrie]], human crewed interstellar ship (2009).
* [[Engineering:Project Icarus (interstellar)|Project Icarus]], uncrewed interstellar probe (2009–2014).
* Sun-diver, uncrewed interstellar probe.<ref>{{cite journal | url= http://www.ugcs.caltech.edu/~diedrich/solarsails/docs/Near-term_beam_sail.pdf | author= Benford, James |author2=Benford, Gregory | title= Near-Term Beamed Sail Propulsion Missions: Cosmos-1 and Sun-Diver | journal= Beamed Energy Propulsion | year= 2003 | volume= 664 | page= 358 | publisher= Department of Physics, University of California, Irvine | doi= 10.1063/1.1582124 | bibcode= 2003AIPC..664..358B | archive-url= https://web.archive.org/web/20141024185623/http://www.ugcs.caltech.edu/~diedrich/solarsails/docs/Near-term_beam_sail.pdf | archive-date= 2014-10-24 | url-status= dead}}</ref>
* [[Engineering:Project Dragonfly (space study)|Project Dragonfly]], small laser-propelled interstellar probe (2013–2015).
* [[Astronomy:Breakthrough Starshot|Breakthrough Starshot]], fleet of uncrewed interstellar probes, announced on 12 April 2016.<ref name="Starshot home">{{cite web |url=http://breakthroughinitiatives.org/Initiative/3 |title=Breakthrough Starshot |work=Breakthrough Initiatives |date=12 April 2016 |access-date=2016-04-12 |archive-date=12 April 2016 |archive-url=https://web.archive.org/web/20160412180305/http://www.breakthroughinitiatives.org/Initiative/3 |url-status=live }}</ref><ref name="concept'">[http://breakthroughinitiatives.org/Concept/3 Starshot – Concept] {{Webarchive|url=https://web.archive.org/web/20160903181459/http://breakthroughinitiatives.org/Concept/3 |date=3 September 2016 }}.</ref><ref>{{cite web|url=http://breakthroughinitiatives.org/Target/3|title=Breakthrough Initiatives|website=breakthroughinitiatives.org|access-date=14 April 2016|archive-date=28 April 2017|archive-url=https://web.archive.org/web/20170428235912/http://breakthroughinitiatives.org/Target/3|url-status=live}}</ref>
* [[Unsolved:Solar One (interstellar)|Solar One]], crewed spacecraft that would combine beamed-powered propulsion, electromagnetic propulsion, and nuclear propulsion (2020).<ref>{{Cite web|date=2020-05-22|title=Solar One – a concept for interstellar travel|url=https://www.innovationnewsnetwork.com/solar-one-a-concept-for-interstellar-travel/5239/|access-date=2020-12-07|website=Innovation News Network|language=en-GB|archive-date=7 January 2023|archive-url=https://web.archive.org/web/20230107214530/https://www.innovationnewsnetwork.com/solar-one-a-concept-for-interstellar-travel/5239/|url-status=live}}</ref>

== Non-profit organizations ==
A few organisations dedicated to interstellar propulsion research and advocacy for the case exist worldwide. These are still in their infancy, but are already backed up by a membership of a wide variety of scientists, students and professionals.
* [[Organization:Initiative for Interstellar Studies|Initiative for Interstellar Studies]] (UK)<ref>{{cite web|url=http://www.i4is.org/|title=Initiative For Interstellar Studies|author=Webpole Bt.|work=i4is.org|access-date=12 June 2015|archive-date=1 June 2015|archive-url=https://web.archive.org/web/20150601135124/http://i4is.org/|url-status=live}}</ref>
* Tau Zero Foundation (USA)<ref name="https://tauzero.aero">{{cite web| url = https://tauzero.aero/| title = Pioneering Interstellar Flight - Tau Zero Foundation| access-date = 18 April 2018| archive-date = 19 April 2018| archive-url = https://web.archive.org/web/20180419053543/https://tauzero.aero/| url-status = live}}</ref>
* Limitless Space Institute (USA)<ref name="https://www.limitlessspace.org/">{{cite web| url = https://www.limitlessspace.org/| title = Limitless Space Institute| access-date = 7 September 2022| archive-date = 7 September 2022| archive-url = https://web.archive.org/web/20220907062107/https://www.limitlessspace.org/| url-status = live}}</ref>
* Tennessee Valley Interstellar Workshop (TVIW), business name Interstellar Research Group (IRG) (USA)<ref name="irg.space">{{cite web| url =https://irg.space/| title =Interstellar Research Group| access-date =April 22, 2023| archive-date =23 April 2023| archive-url =https://web.archive.org/web/20230423003916/https://irg.space/| url-status =live}}</ref>

== Feasibility ==
The energy requirements make interstellar travel very difficult. It has been reported that at the 2008 Joint Propulsion Conference, multiple experts opined that it was improbable that humans would ever explore beyond the Solar System.<ref name="O'Neill">{{cite web |last=O'Neill |first=Ian |date=Aug 19, 2008 |title=Interstellar travel may remain in science fiction |url=http://www.universetoday.com/2008/08/19/bad-news-insterstellar-travel-may-remain-in-science-fiction/ |work=Universe Today |access-date=25 August 2009 |archive-date=26 January 2009 |archive-url=https://web.archive.org/web/20090126105034/http://www.universetoday.com/2008/08/19/bad-news-insterstellar-travel-may-remain-in-science-fiction/ |url-status=live }}</ref> Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated that at least 100 times the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.<ref name="O'Neill" />

Astrophysicist Sten Odenwald stated that the basic problem is that through intensive studies of thousands of detected exoplanets, most of the closest destinations within 50 light years do not yield Earth-like planets in the star's habitable zones.<ref name="Odenwald">{{cite web |first=Sten |last=Odenwald |date=April 2, 2015 |work=Huffington Post Blog |title=Interstellar travel: Where should we go? |url=https://www.huffingtonpost.com/dr-sten-odenwald/interstellar-travel-so-where-should-we-go_b_6971584.html |access-date=20 February 2020 |archive-date=22 February 2017 |archive-url=https://web.archive.org/web/20170222144923/http://www.huffingtonpost.com/dr-sten-odenwald/interstellar-travel-so-where-should-we-go_b_6971584.html |url-status=live }}</ref> Given the multitrillion-dollar expense of some of the proposed technologies, travelers will have to spend up to 200 years traveling at 20% the speed of light to reach the best known destinations. Moreover, once the travelers arrive at their destination (by any means), they will not be able to travel down to the surface of the target world and set up a colony unless the atmosphere is non-lethal. The prospect of making such a journey, only to spend the rest of the colony's life inside a sealed habitat and venturing outside in a spacesuit, may eliminate many prospective targets from the list.

Moving at a speed close to the speed of light and encountering even a tiny stationary object like a grain of sand will have fatal consequences. For example, a gram of matter moving at 90% of the speed of light contains a kinetic energy corresponding to a small nuclear bomb (around 30kt TNT).

One of the major stumbling blocks is having enough Onboard Spares & Repairs facilities for such a lengthy time journey assuming all other considerations are solved, without access to all the resources available on Earth.<ref>{{Cite web |url=https://www.scientificamerican.com/article/interstellar-travel-as-delusional-fantasy-excerpt/ |title=Interstellar Travel as Delusional Fantasy [Excerpt] |first=Ed |last=Regis |date=October 3, 2015 |website=Scientific American |access-date=24 January 2021 |archive-date=18 January 2021 |archive-url=https://web.archive.org/web/20210118180017/https://www.scientificamerican.com/article/interstellar-travel-as-delusional-fantasy-excerpt/ |url-status=live }}</ref>

=== Interstellar missions not for human benefit ===

Explorative high-speed missions to [[Astronomy:Alpha Centauri|Alpha Centauri]], as planned for by the [[Astronomy:Breakthrough Starshot|Breakthrough Starshot initiative]], are projected to be realizable within the 21st century.<ref>{{Cite journal|arxiv=1710.10732|last1=Kulkarni|first1=Neeraj|title=Relativistic Spacecraft Propelled by Directed Energy|journal=The Astronomical Journal|volume=155|issue=4|pages=155|last2=Lubin|first2=Philip|last3=Zhang|first3=Qicheng|year=2017|doi=10.3847/1538-3881/aaafd2|bibcode=2018AJ....155..155K|s2cid=62839612 |doi-access=free }}</ref> It is alternatively possible to plan for uncrewed slow-cruising missions taking millennia to arrive. These probes would not be for human benefit in the sense that one can not foresee whether there would be anybody around on Earth interested in then back-transmitted science data. An example would be the Genesis mission,<ref>{{cite journal |last1=Gros |first1=Claudius |title=Developing ecospheres on transiently habitable planets: the genesis project |journal=Astrophysics and Space Science |date=5 September 2016 |volume=361 |issue=10 |pages=324 |doi=10.1007/s10509-016-2911-0|arxiv=1608.06087 |bibcode=2016Ap&SS.361..324G |s2cid=6106567 }}</ref> which aims to bring unicellular life, in the spirit of [[Unsolved:Directed panspermia|directed panspermia]], to habitable but otherwise barren planets.<ref>{{Cite web |last=Andersen |first=Ross |date=August 25, 2016 |title=How to Jumpstart Life Elsewhere in Our Galaxy |url=https://www.theatlantic.com/science/archive/2016/08/genesis-missions/497258/ |work=The Atlantic |access-date=29 January 2018 |archive-date=18 June 2022 |archive-url=https://web.archive.org/web/20220618182442/https://www.theatlantic.com/science/archive/2016/08/genesis-missions/497258/ |url-status=live }}</ref> Comparatively slow cruising Genesis probes, with a typical speed of <math>c/300</math>, corresponding to about <math>1000\,\mbox{km/s}</math>, can be decelerated using a [[Physics:Magnetic sail|magnetic sail]]. Uncrewed missions not for human benefit would hence be feasible.<ref>{{Cite web |last=Romero |first=James |date=13 November 2017 |title=Should we seed life through the cosmos using laser-driven ships? |url=https://www.newscientist.com/article/2153165-should-we-seed-life-through-the-cosmos-using-laser-driven-ships/ |work=New Scientist |access-date=16 November 2017 |archive-date=14 November 2017 |archive-url=https://web.archive.org/web/20171114211810/https://www.newscientist.com/article/2153165-should-we-seed-life-through-the-cosmos-using-laser-driven-ships/ |url-status=live }}</ref>

== Discovery of Earth-like planets ==

In February 2017, NASA announced that its [[Astronomy:Spitzer Space Telescope|Spitzer Space Telescope]] had revealed seven Earth-size planets in the TRAPPIST-1 system orbiting an ultra-cool dwarf star 40 light-years away from the Solar System.<ref name="NASA Press Release Feb 22nd 2017">{{cite web |date=22 February 2017 |title=Release 17-015: NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star |url=https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around |website=[[Organization:NASA|NASA]] |access-date=25 February 2017 |archive-date=5 March 2017 |archive-url=https://web.archive.org/web/20170305055703/https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around/ |url-status=live }}</ref> Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water. The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside the Solar System. All of these seven planets could have liquid water – the key to life as we know it – under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.

== See also ==
<!--Please do not add links that already exist in the body of the article (like [[Physics:Bussard ramjet|Bussard ramjet]]), or links that are space-related but not specifically with interstellar travel (like [[Biography:Eugen Sänger|Eugen Sänger]] or [[Biography:Freeman Dyson|Freeman Dyson]]). There are *thousands* of Wikipedia articles that deal with deep space travel in some fashion. If you can't come up with a good reason for properly integrating a link into the article itself, ask yourself why it would belong here.-->
* {{annotated link|Astronomy:Effect of spaceflight on the human body}}
* {{annotated link|Astronomy:Health threat from cosmic rays}}
* {{annotated link|Astronomy:Human spaceflight}}
* {{annotated link|Astronomy:Intergalactic travel}}
* [[Physics:Wormhole#Interuniversal travel|Interuniversal travel]] – Hypothetical travel between universes
* {{annotated link|Astronomy:Interstellar communication}}
*[[Astronomy:Interstellar object|Interstellar object]]
* List of artificial objects leaving the Solar System
* {{annotated link|Astronomy:List of nearest terrestrial exoplanet candidates}}
* [[Astronomy:List of potentially habitable exoplanets|List of potentially habitable exoplanets]]
* {{annotated link|Astronomy:Spacecraft propulsion}}
* [[Engineering:Space travel in science fiction|Space travel in science fiction]]

== References ==
{{Reflist}}

== Further reading ==
* {{cite journal|last1=Crawford|first1=Ian A.|title=Interstellar Travel: A Review for Astronomers |url=https://articles.adsabs.harvard.edu/pdf/1990QJRAS..31..377C|journal=Quarterly Journal of the Royal Astronomical Society|date=1990|volume=31|pages=377–400|bibcode=1990QJRAS..31..377C}}
* {{cite journal | author= Hein, A.M. | url= https://www.academia.edu/2654836 | title= Evaluation of Technological-Social and Political Projections for the Next 100-300 Years and the Implications for an Interstellar Mission | journal= Journal of the British Interplanetary Society | volume= 33 | number= 9/10 | pages= 330–340 | date= September 2012| bibcode= 2012JBIS...65..330H }}
* {{cite book |last=Long |first=Kelvin |title=Deep Space Propulsion: A Roadmap to Interstellar Flight |publisher=Springer |date=2012 |isbn=978-1-4614-0606-8 |doi=10.1007/978-1-4614-0607-5}}
* {{cite book | last = Mallove | first = Eugene | title=The Starflight Handbook | url = https://archive.org/details/starflighthandbo0000mall | url-access = registration | publisher = John Wiley & Sons, Inc. | date = 1989 | isbn = 978-0-471-61912-3 }}
* {{cite book | last = Odenwald | first = Sten | title = Interstellar Travel: An Astronomer's Guide | date = 2015 | publisher = CreateSpace Independent Publishing Platform | isbn = 978-1-5120-5627-3 }}
* {{cite book | last = Woodward | first = James | title = Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes | publisher = Springer | date = 2013 | isbn = 978-1-4614-5622-3 }}
* {{cite book | last = Zubrin | first = Robert | title = Entering Space: Creating a Spacefaring Civilization | url = https://archive.org/details/enteringspacecre00zubr | url-access = registration | publisher = Tarcher / Putnam | date = 1999 | isbn = 978-1-58542-036-0 }}

== External links ==
* [https://www.nbcnews.com/id/3741674 Leonard David – Reaching for interstellar flight (2003) – MSNBC] (MSNBC Webpage)
* [https://web.archive.org/web/20040402092917/http://www.grc.nasa.gov/WWW/bpp/ NASA Breakthrough Propulsion Physics Program] (NASA Webpage)
* [http://www.ibiblio.org/lunar/school/library/bib.html Bibliography of Interstellar Flight] (source list)
* [http://news.cnet.com/8301-13772_3-20060476-52.html DARPA seeks help for interstellar starship] {{Webarchive|url=https://web.archive.org/web/20140304163503/http://news.cnet.com/8301-13772_3-20060476-52.html |date=2014-03-04 }}
* [http://theconversation.com/how-to-build-a-starship-and-why-we-should-start-thinking-about-it-now-53417/ How to build a starship – and why we should start thinking about it now] (Article from [http://theconversation.com/uk The Conversation], 2016)

{{Spaceflight| state=collapsed}}
{{emerging technologies|topics=yes|space=yes}}

[[Category:Interstellar travel| ]]
[[Category:Spaceflight concepts|*]]

{{Sourceattribution|Interstellar travel}}