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What is the actual feasibility of interstellar travel?

For instance, a manned journey to Proxima Centauri, given the distance, what technology would need to be developed/perfected for such a journey?

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    $\begingroup$ I have voted to reopen the question, I have not been super active on the site recently but, given the two new and good answers below, it does not seem overly broad. $\endgroup$ – James Jenkins Dec 28 '17 at 19:04
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Given no time constraint, there are only two pieces missing for a manned journey to Proxima Centauri. Resources (money) and commitment. Getting there and back in a single human life time would be a totally different question/answer.

There are several, drives that can get us there, the solar sail probably being the simplest and easiest to maintain over the long time/distance. All of current propulsion options would take time measured in generations.

We have had humans constantly in space for decades now, so technologically speaking all the major hurdles of maintaining life in space are overcome.

There are some medical and psychological considerations, most of the medical having to do with adapting to space in a way that make re-adapting to earth difficult. The psychological considerations can likely not be fully addressed without going on the journey.

There are several logistical issues, to balance available resources to allow for a successful mission. You could capture an asteroid (preferably mostly metal) and hollow it out, put 500 people in it, and farms/gardens to support 5,000, and send it on its way. but the resources to accomplish this would be significantly greater then a simple trip to the moon, which we have not repeated for several decades.

Practically you could send a small envoy in a smaller man made craft, a crew of 3 to 10, with frozen sperm/egg/embryo DNA to maintain genetic diversity.

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    $\begingroup$ I think this answer is too optimistic. Maintaining several humans in low-earth-orbit, close to rescue, solar powered, and with regular food supplies, is a totally different beast than sending many humans out into interstellar orbit, entirely self-sufficient, and in what will probably have to be nuclear powered. $\endgroup$ – gerrit♦ Jul 24 '15 at 10:51
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    $\begingroup$ @gerrit Pessimists stay home, All the Flat Earth people still live in Europe. :) Seriously though you are correct, but history teaches us, people with the desire to explore, tend to be optimistic. $\endgroup$ – James Jenkins Jul 24 '15 at 11:04
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    $\begingroup$ Oh, I'm not saying technology can't be developed, I'm just saying we're not there quite yet. $\endgroup$ – gerrit♦ Jul 24 '15 at 11:21
  • $\begingroup$ @gerrit But is there actually any new technology that needs to be developed or is it just figuring out how to use current technology to do the extremely difficult task of manned interstellar travel. (I'd think more the later.) $\endgroup$ – Brooks Nelson Jul 27 '17 at 2:31
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    $\begingroup$ If you send a manned mission to say Sedna with say a 35 year mission length, knowing the older members will die, the younger will have children and take over some duties you can 'test' at least part of the psychological effects and generation ship aspects. $\endgroup$ – Brooks Nelson Oct 30 '17 at 14:15
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I'd say that we need the construction capability to build an O'Neill cylinder that is strong enough to survive thrust along it's axis, that contains a balanced biosphere, and an energy source that will last the journey.

This will give you a generation ship or, another way to look at it, an independent space station that can move from star to star.

We could do some of that today.

The cylinder itself is just an engineering and resource problem. Once we have the infrastructure in space to make it, we can make it.

The closed cycle biosphere is a bit tricky but there is no real new science involved, just experience balancing things and finding stable balances so things self correct instead of spinning out of control like we have seen in several experiments.

The kicker will be the energy source. Solar power only really helps out to the asteroid belt. After that, we need a different power source.

  1. You could use fissionables with a long half live but you will need a lot of it (the longer the half life the less energy it puts out at any given time).
  2. Another possibility would be fusion once we figure out how to make that self sustaining and can store enough hydrogen/deuterium/tritium for the trip.
  3. A new branch of science for a new power source. If there are other options that I missed, please post them in the comments.

Point 3, while possible, is not something we should try to plan for. To use it in a story you have to invent handwavium (though less handwavium, possibly than FTL).

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  • $\begingroup$ Does anyone know how to use a pound sign (hashtag to the young'uns) in a post? Everything I tried changed the font of the last sentence into a type of header. $\endgroup$ – ShadoCat Dec 27 '17 at 20:56
  • $\begingroup$ HashTags are not a StackExchange thing. If you are trying to use it at the start of a sentence it is markup (as you discovered) either put a character (not a space) before the # or use the word "Bullet", "Item" or as you did "Point" $\endgroup$ – James Jenkins Dec 27 '17 at 23:37
  • $\begingroup$ @JamesJenkins, thank you. Disappointing but expected. $\endgroup$ – ShadoCat Dec 28 '17 at 18:37
  • $\begingroup$ A forth option for power is having it beamed from your home system. This would probably take the form of lasers but could also be fissionables fired from a mass driver fast enough to catch up to the colony ship and then caught by another mass driver on the colony ship. $\endgroup$ – Lex Dec 29 '17 at 17:33
  • $\begingroup$ @Lex, the problem with beamed power is the same as solar power. the farther from the source you get, the less power you get. Even a laser beam spreads over distance. The more it spreads, the less of that power gets to the target. $\endgroup$ – ShadoCat Dec 29 '17 at 19:37
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Edit: An alternative for building a huge ship to sustain a whole colony for thousands of years of travel would be to send cryogenized people, and unfreeze them once arrived; a middle term solution in case we do not manage to handle adult unfreezing shortly is to send frozen embryos and have robots hatch them once arrived on / near habitable land.

A baby girl recently was born from a 25 years frozen embryo. Plus babies as young as 24 gestational weeks are now able to live a normal life after months of careful mothering... nothing seems impossible when you've seen this.

I know this is horrible for the children who will come into life on this other stellar system.

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    $\begingroup$ This is not a proper stackexchange answer to the question, it is a personal opinion or comment. $\endgroup$ – uhoh Dec 27 '17 at 22:55
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    $\begingroup$ @uhoh, I tried to remove the "I think" part to make it fit the rules. $\endgroup$ – J. Chomel Dec 28 '17 at 6:22
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    $\begingroup$ @uhoh, at the point people unfreeze, the mission becomes "manned" again. In my view, the point of sending a manned mission that far is to send seeds of humanity. $\endgroup$ – J. Chomel Dec 28 '17 at 7:05
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    $\begingroup$ The body of the question asks "what technology would need to be developed/perfected for such a journey?" An answer of the form "you can freeze people and robots will thaw them, but it will be horrible for the children" is not answering the question as asked. From your profile I can see that you can write very direct, focused answers on other technology-related SE sites and in SO. $\endgroup$ – uhoh Dec 28 '17 at 8:02
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    $\begingroup$ @uhoh, I don't know what's wrong today, I've seen you have an open mind. Here I'm just trying to focus on that freeze /unfreeze technology that could be used, and talk about some ethical issues it has... But if you do insist, I can say that you are right. $\endgroup$ – J. Chomel Dec 28 '17 at 8:22
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The question assumes interstellar travel, so we're already indulging in far fetched science fiction. So I'll put aside my pessimism and indulge my inner space cadet.

We have a greater incentive to open a new frontier when our population growth becomes limited by resources available. Then the apparently exponential growth levels out and an S curve asymptotically approaches our ceiling. This is called logistic growth.

If we manage to break free of Cradle Earth, there is enough real estate and resources in the Main Belt to postpone bumping into the logistic growth ceiling for centuries or even millennia. This would be a great setting for gaining experience in maintaining biomes. Sunlight is still fairly abundant. The asteroid city states need not be entirely self sufficient as they could trade with other asteroid city states as well Earth, Moon and Mars.

Beyond the Main Belt are two healthy asteroid populations: the Trojans residing at the sun earth L4 and L5 points. The Hilda asteroids are natural cyclers between the Main Belt and the Trojans.

Building asteroid biomes would give some experience in burrowing. The moons of the Jupiter, Saturn, Uranus and Neptune could host subsurface biomes.

But as we get further from the sun, solar energy is harder to come by. At Jupiter and Trojan distance, insolation is about 1/25 what we enjoy on earth. At Saturn 1/100. At Neptune nearly a 1/1000.

I believe developing fusion energy sources would be prerequisite for developing real estate in the Kuiper Belt and the Oort Cloud. Perhaps non stellar fusion power is possible, who knows?

Oort city states would be more isolated than Main Belt or Trojan city states. There is more incentive for self sufficiency.

By the time human kinds expands to the outer Oort we would already have plenty of experience in maintaining biomes that are self sufficient for centuries or even millennia. These city states could easily drift off into interstellar space.

Given fusion power and lots of water ice (as many Oort bodies are thought to have), the city sates could send off hydrogen ions at a good fraction of light speed. The thrust would be pathetic but given centuries to do a burn, the interstellar city states could manage substantial delta V.

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  • $\begingroup$ Good answer, essentially I think you are saying "We don't have the motivation to go interstellar, there are lots of places closer that we would go first, that would teach us whatever else we might need to know" $\endgroup$ – James Jenkins Dec 28 '17 at 19:01
  • $\begingroup$ The only sensible text in this answer is "But as we get further from the sun, solar energy is harder to come by. At Jupiter and Trojan distance, insolation is about 1/25 what we enjoy on earth. At Saturn 1/100. At Neptune nearly a 1/1000." Everything else is science fiction. $\endgroup$ – RonJohn Jun 10 '19 at 2:54
  • $\begingroup$ @RonJohn I acknowledged it's science fiction in the first paragraph. Reading comprehension -- get some. $\endgroup$ – HopDavid Jun 10 '19 at 18:48