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If Alpha Centauri (approx 4.5 light years) is our nearest star, at the highest possible speed for human spacecraft, how long would it take to travel there? People talk about interstellar travel. People do not realize the vastness and distances involved. Inhabiting alien planets or moons would have to therefore reside inside our solar system. Interstellar distances are simply too vast for this possibility.

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    $\begingroup$ Not true thanks to relativity: math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html $\endgroup$ – barrycarter Oct 22 '18 at 2:07
  • $\begingroup$ I think a question about multi-generation relativistic crewed spaceflight is okay, and the question is certainly not unclear. This may have one or more answers about this here already. $\endgroup$ – uhoh Oct 22 '18 at 2:18
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    $\begingroup$ Do you mean the highest speed he attained for crewed spacecraft, or hypothetically possible speeds? There’s a huge range of possibilities, requiring varying degrees of handwavium and unobtainium to achieve. $\endgroup$ – Russell Borogove Oct 22 '18 at 2:33
  • $\begingroup$ Yes - there are several concepts that could work, reducing the travel time to something attainable in less than a generation - and they are all blocked by us either not knowing how to make some crucial components of the propulsion or just not being desperate/insane enough to go with the entire concept (we pretty much know how to make the Project Orion... still finding someone insane enough to implement it is a different matter.) $\endgroup$ – SF. Oct 22 '18 at 14:59
  • $\begingroup$ @SF, project Orion has a theoretical maximum $I_{sp}$ of 100000 seconds, according to wikipedia en.wikipedia.org/wiki/Nuclear_pulse_propulsion, corresponding to an exhaust velocity of about 1000 $km/s$. To get to alpha centauri in 100 years needs a velocity of $0.04 c$ about $13000 km/s$ so a delta-V of twice that. The mass ratio you would need is still ridiculous. Project Orion doesn't cut it. $\endgroup$ – Steve Linton Oct 22 '18 at 15:52
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You are absolutely right about many people not understanding the vastness and distances involved in space travel, especially once leaving our solar system. The New Horizons craft (which visited Pluto) would take about 78,000 years to reach Alpha Centauri. https://earthsky.org/space/alpha-centauri-travel-time. And that was on a craft designed especially small so it could make it to Pluto very quickly, probably faster than speeds a manned ship could reach.

Then there is the problem of once a spacecraft reached Alpha Centauri travelling 36,000 mph, how would it slow down and not just pass it at speeds difficult for humans to comprehend in real world experiences. Here is one illustration that might help. https://www.sciencealert.com/this-gif-shows-how-mind-blowingly-fast-new-horizons-is-travelling-through-space

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