Editing Interstellar travel (section)

== 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>{{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 [[Alpha Centauri]], as planned for by the [[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 [[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 [[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>