Editing Interstellar travel (section)

== 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}}. [[Venus]], the closest planet to Earth is (at closest approach) 0.28 AU away. [[Neptune]], the farthest planet from the Sun, is 29.8 AU away. As of January 20, 2023, [[Voyager 1]], the farthest human-made object from Earth, is 163 AU away, exiting the Solar System at a speed of 17 km/s (0.006% of the speed of light).<ref name="voyager">{{cite web |title=Voyager - Mission Status |url=https://voyager.jpl.nasa.gov/mission/status/ |accessdate=22 March 2024 |website=[[nasa.gov]] }}</ref>

The closest known star, [[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) = 163 AU
Proxima = 4.243 ± 0.002 light years away.
-->

{| class="wikitable" style="margin: 1em auto 1em auto;"
! Object
! Distance<br>(AU)
! Light time
|-
| [[Moon]]
| 0.0026
| 1.3 seconds
|-
| [[Sun]]
| 1
| 8 minutes
|-
| [[Venus]] (nearest planet)
| 0.28
| 2.4 minutes
|-
| [[Neptune]] (farthest planet)
| 29.8
| 4.1 hours
|-
| [[Voyager 2]]
|136.1
|18.9 hours
|-
| [[Voyager 1]]
| 163.0
| 22.6 hours
|-
| [[Proxima Centauri]] (nearest star and exoplanet)
| 268,332
| 4.24 years
|}

Because of this, distances between stars are usually expressed in [[light-year]]s (defined as the distance that light travels in vacuum in one [[Julian year (astronomy)|Julian year]]) or in [[parsec]]s (one parsec is 3.26 ly, the distance at which [[stellar parallax]] is exactly one [[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 [[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, [[Voyager 1#Interstellar mission|Voyager 1]], has covered 1/390 of a light-year in 46 years and is currently moving at 1/17,600 the speed of light. At this rate, a journey to Proxima Centauri would take 75,000 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><ref name="voyager">{{cite web |title=Voyager - Mission Status |url=https://voyager.jpl.nasa.gov/mission/status/ |accessdate=22 March 2024 |website=[[nasa.gov]] }}</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 [[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>.

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 [[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>{{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>{{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 to 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, [[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 [[solitude|isolation]], the physiological effects of extreme acceleration, the effects of exposure to [[ionising radiation]], and the physiological effects of [[weightlessness]] to the muscles, joints, bones, immune system, and eyes. There also exists the risk of impact by [[micrometeoroid]]s and other [[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 [[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>