Editing Interstellar travel (section)

=== 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>