Editing Interstellar travel (section)

== 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 [[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 ''[[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>{{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. [[Electrically powered spacecraft propulsion]] powered by a portable power-source, say a [[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>{{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 =====
[[Fission-fragment rocket]]s use [[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|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 [[Power-to-weight ratio|specific power]].<ref>{{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=[[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>

[[Project Orion (nuclear propulsion)|Project Orion]] team member [[Freeman Dyson]] proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that used pure [[deuterium fusion]] detonations with a very high fuel-[[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 [[delta-v]] allowing a flight-time to [[Alpha Centauri]] of 130 years.<ref>{{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 [[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 [[Project Longshot]], [[Project Daedalus]], and [[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 [[Project Daedalus]] by use of externally triggered [[inertial confinement fusion]], in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then, [[laser]]s, [[ion beam]]s, [[neutral particle beam]]s and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.<ref>{{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 [[Partial Test Ban Treaty|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 [[International Space Station]].

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

==== Nuclear fusion rockets ====
[[Fusion rocket]] starships, powered by [[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% [[burnup]] fraction it could achieve a pure fusion product exhaust velocity of ~3,000&nbsp;km/s.

==== Antimatter rockets ====
{{Main|Antimatter rocket}}
{{more citations needed section|date=August 2015}}
An [[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 [[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 [[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 [[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, [[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 [[Mössbauer effect]] to the spacecraft.<ref>{{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 [[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. [[Geoffrey A. Landis]] proposed an [[interstellar probe]] propelled by an [[ion thruster]] powered by the energy beamed to it from a base station laser.<ref>{{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 [[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 [[Spacecraft propulsion|spacecraft thruster]]. In 2016, the [[Advanced Propulsion Physics Laboratory]] at [[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 [[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 [[Bussard ramjet]], a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a [[proton–proton 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 [[solar sail|light-sail]] at the star system destination.]]

A [[solar sail|light sail]] or [[magnetic sail]] powered by a massive [[laser]] or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own [[reaction mass]] and therefore would only need to accelerate the craft's [[Payload (air and space craft)|payload]]. [[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>{{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, [[Geoffrey A. Landis]] of [[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 [[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 [[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 [[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"
|-
! Journey !! Mission !! Laser Power !! Vehicle Mass !! Acceleration !! Sail Diameter !! Maximum Velocity <br /> (% of the speed of light) !! Total duration
|-
! Flyby – Alpha Centauri
| ''outbound stage'' || 65 GW || 1 t || 0.036 g || 3.6&nbsp;km || 11% @ 0.17 ly
| 40 years
|-
! rowspan=2 | Rendezvous – Alpha Centauri
| ''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?-->
| rowspan=2 | 41 years
|-
| ''deceleration stage'' || 26,000 GW || 71 t || 0.2 g || 30&nbsp;km || 21% @ 4.29 ly
|-
! rowspan=4 | Crewed – Epsilon Eridani
| ''outbound stage'' || 75,000,000 GW || 78,500 t || 0.3 g || 1000&nbsp;km || 50% @ 0.4 ly
| rowspan=4 | 51 years (including 5 years exploring star system)
|-
| ''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> ([[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>{{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>{{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>{{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 ====
[[Mind uploading|Uploaded human minds]] or [[AI]] could be transmitted with laser or radio signals at the [[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 [[Kardashev scale#Type_III|Kardashev type III civilization]]).

==== Artificial black hole ====

{{Main|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 [[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>{{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>

==== 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|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 [[Tachyonic antitelephone|causality]] concerns: travel faster than light may, under certain conditions, permit travel backwards in time within the context of [[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 [[faster-than-light]] travel within the theory of general relativity require the existence of [[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 [[general relativity]] and without the introduction of [[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 [[exotic matter]], or the hypothetical concept of [[negative mass]].<ref name="Warp Alcubierre" />

===== Wormholes =====
[[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 [[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 [[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>