Editing Interstellar travel (section)

=== 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>