Editing Interstellar travel (section)

== Proposed methods ==

=== Slow, uncrewed probes ===
{{Main|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 [[Voyager program]].<ref>{{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 [[Project Daedalus]], [[Project Icarus (interstellar)|Project Icarus]], [[Project Dragonfly (space study)|Project Dragonfly]], [[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 [[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|Interstellar probe}}

==== Nanoprobes ====
[[File:Project Dragonfly.PNG|thumb|Rendering of the Dragonfly-Probe: This concept won the Project Dragonfly Design Competition. Its sail is not depicted to scale,<ref name="v036">{{cite journal | last=Perakis | first=Nikolaos | last2=Schrenk | first2=Lukas E. | last3=Gutsmiedl | first3=Johannes | last4=Koop | first4=Artur | last5=Losekamm | first5=Martin J. | title=Project Dragonfly: A feasibility study of interstellar travel using laser-powered light sail propulsion | journal=Acta Astronautica | volume=129 | date=2016 | doi=10.1016/j.actaastro.2016.09.030 | pages=316–324}}</ref> it would be kilometres across.<ref name="g891">{{cite journal | last=Häfner | first=Tobias | last2=Kushwaha | first2=Manisha | last3=Celik | first3=Onur | last4=Bellizzi | first4=Filippo | title=Project Dragonfly: Sail to the stars | journal=Acta Astronautica | volume=154 | date=2019 | doi=10.1016/j.actaastro.2018.05.018 | pages=311–319}}</ref>]]
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 [[University of Michigan]] are developing thrusters that use [[nanoparticle]]s as propellant. Their technology is called "nanoparticle field extraction thruster", or [[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=dead }}</ref>

[[Michio Kaku]], a theoretical physicist, has suggested that clouds of "smart dust" be sent to the stars, which may become possible with advances in [[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 |author-link=Michio Kaku}}</ref>

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

Starseed is a similar proposed method of launching interstellar nanoprobes at one-third light speed. The proposed launcher uses a 1,000&nbsp;km-long small-diameter hollow wire, with [[electrode]]s lining the hollow wire, an electrostatic accelerator tube, similar to [[K. Eric Drexler]]'s ideas.<ref>{{Cite web |title=Starseed/Launcher |url=https://www.iase.cc/starseed.htm |access-date=2023-07-24 |archive-date=2023-07-24 |archive-url=https://web.archive.org/web/20230724122950/https://www.iase.cc/starseed.htm |url-status=dead }}</ref>

=== 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|Generation ship}}
A [[generation ship]] (or '''world ship''') is a type of [[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>{{cite journal|last=Hein|first=A. M.|display-authors=etal|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|Sleeper ship}}

Scientists and writers have postulated various techniques for [[suspended animation]]. These include human [[hibernation]] and [[cryonics|cryonic preservation]]. Although neither is currently practical, they offer the possibility of [[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|Embryo space colonization}}
A [[robotic]] interstellar mission carrying some number of frozen early stage human [[embryo]]s is another theoretical possibility. This method of [[space colonization]] requires, among other things, the development of an [[artificial uterus]], the prior detection of a habitable [[terrestrial planet]], and advances in the field of fully autonomous [[robotics|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 ([[Oort cloud]]) to possible [[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 [[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|§ Propulsion]] below), but none of them are ready for near-term (few decades) developments at acceptable cost.

==== Time dilation ====
{{Main|Time dilation}}

Physicists generally believe faster-than-light travel is impossible. 
Relativistic [[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 |author3-link=Penina Axelrad}}</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 [[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-[[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.]]
{{See also|Space travel under constant acceleration}}

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 [[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 [[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 [[time dilation]] and [[length contraction]].

The result is an impressively fast journey for the crew.