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If you take real physics, you could travel to the next star in moving cities.

Imagine independent, self-sufficient mining outposts attached to an asteroid. While the asteroid is being used up, the youth will start to search the next target. People will follow when one is found. Latest when the last scrap of material is used, also the most stagnant family will move on.

You can live that way, in a few generations, from asteroid to asteroid to Jupiter's trojans, to Saturn's rings, into the plutonoids, from there into the Oort cloud.... well and at the end of our Oort cloud, the next star's gravity well begins. From snowball to snowball, in a few 100 years you arrive at Proxima Centauri.

It is doable with today's technology or very little advancement from here. But you have to say goodbye to the dream of going there, taking some photos and coming back to show them your wife.

Instead you have to develop an entirely self sufficient life style which is religiously or philosophically focused on moving on to the next snowball, you have to take your entire family with you and you have to say goodbye to earth.

Question

What tech would be needed that is absolutely not available today? For the sake of this question, I would understand rotating hammers or rings for artificial gravity as existing technology: even if it has not yet been built, the concept is understood and technically feasible.

Which prerequisites are missing today?

What problems could those space faring families encounter?

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    $\begingroup$ You need arc reactors. $\endgroup$ – user6760 Jun 15 '20 at 11:40
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    $\begingroup$ Your question is based on a false premise - the technology isn't available today (or in the near future) or we'd be doing it. I'm an electrical engineer and I can't see the ability to do this for at least a century. Also: (a) there are very few asteroids, moons, or chunks of rock in most of the locations you list large enough to mine in the way you suggest and (b) I don't know that we know of such rocks in interstellar space, but the "oort cloud" is so ambiguous at this time that I guess one could suspend disbelief. $\endgroup$ – JBH Jun 15 '20 at 16:04
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    $\begingroup$ If you want some fun realizing what I just said about your premise, check out this poster from Rockwell. I have one of the originals on my wall. What's amazing is how badly (and understandably) they misjudged how fast things can happen. According to the poster, we should have had a self-supporting lunar base two years ago. I'd be pleasantly shocked to see one in another 20 years. $\endgroup$ – JBH Jun 15 '20 at 16:06
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    $\begingroup$ We are laughably far from putting a chip fab in space. $\endgroup$ – Daniel B Jun 15 '20 at 20:06
  • $\begingroup$ What kind of mining economy you envision? You have to sell something, transport it to somewhere more populous? $\endgroup$ – fraxinus Jun 15 '20 at 20:15
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It is doable with today's technology or very little advancement from here

Not at all. We are still struggling to find a way to protect the astronauts on their way to Mars, which as compared to what you describe is just the grocery store around the corner!

Few months of permanence in microgravity, as seen with the astronauts living on the ISS, just to cite the most recent examples, severely weaken the human body, by weakening bones and immune systems among others.

And the ISS orbits under the protection of Van Allen belts, meaning that the astronauts are not showered in highly energetic particles, which would further damage a living organism.

And then the people living on ISS can rely on constant cargo supplies from Earth, they don't have to grow their own food, do their own laundry and so on and so forth.

We don't even know if a woman can successfully start and complete a pregnancy in microgravity!

If you really want to enable ice hopping, as you call it, you need to:

  • mitigate microgravity damages to human organism (pregnancy included)
  • mitigate high energy particles damages to human organism (pregnancy included)
  • find a way to locally produce food and other needed supplies
  • find a reliable energy source to supply the stations
  • find a suitable propulsion mechanism: moving all that mass around will require a huge lot of rocket propellant!

I highly recommend reading this NASA informative site

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  • $\begingroup$ Thanks for that NASA link. They mention of course only American astronauts :-) radiation shielding might be the biggest problem indeed, as also the energy source aboard those flying cities will have to be nuclear, adding to the cosmic radiation danger. I am willing to handwave water and food production... maybe those flying cities will be very green indeed yes. $\endgroup$ – Anderas Jun 15 '20 at 12:09
  • $\begingroup$ Why do you think propulsion will be a problem? Hydrogen rockets, or better, Ion drives, are known. If those snowballs contain something it's water as a source for propellant. $\endgroup$ – Anderas Jun 15 '20 at 12:15
  • $\begingroup$ @Anderas you can't use water. You really can't use any consumable unless you can periodically restock it - and once you leave the solar system that's a big "if." Also, bear in mind that Voyager discovered interstellar space is hot (89,000F, hotter than the sun's photosphere) so shielding and heat control will be a Big Deal. $\endgroup$ – JBH Jun 15 '20 at 16:10
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    $\begingroup$ @JBH, there's a big difference between temperature and heat: interstellar space may be hot, but it's also incredibly sparse, so there isn't much heat there. Heat control won't be a problem. (As a point of comparison, heat your oven to 100C/212F and see how long you can hold your hand in it. Then heat a pot of water to boiling and decide if you dare stick your hand in it.) $\endgroup$ – Mark Jun 15 '20 at 23:32
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    $\begingroup$ A little satellite can carry sensitive instruments that can measure plasma properties that would have no noticeable effect on a radiator. Equilibrium temperatures for objects in interstellar space get down to around 3 K. The interstellar medium is not a consideration for thermal balance or radiator efficiency. $\endgroup$ – Christopher James Huff Jun 16 '20 at 11:40
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Short hops cost more fuel than long hops

L. Dutch has succinctly pointed out some of the big ticket reasons why humanity is not equipped for long duration space travel yet. There is another key concept though.

To quote Douglas Adams yet again, "Space is big. Really big." Proxima Centurai is over 4 light years from Earth. To get there in even 400 years would require that the average speed during the trip was over 1% of lightspeed. We are not even close to being able to accelerate a spacecraft to that speed, yet the question assumes that most of the time would be spent settling new rocks.

Travel on Earth is fundamentally constrained by friction in a way that space travel is not. Friction (primarily drag) increases with the square of speed, so for a given shape, doubling the speed requires four times the power, tripling the speed requires nine times the power and so on. Most vehicles on Earth can reach their top speed within a few minutes at most and will remain at that speed as long as their thrust equals the drag at the speed they are travelling. So it makes sense to break up long trips into short hops - drive for a couple of hours, stop at a service station (gas station for those who don't speak Australian) for fuel and to stretch your legs, then drive for a few more hours. Repeat a dozen times with some overnight stops for a long trip. While the time spent stopped may add up, the act of stopping consumes very little fuel overall.

Space travel is completely different. Let's assume that there is a spacecraft that has made it out of Earth orbit and wants to go places with a low thrust, high efficiency ion drive or something similar. It has performance far exceeding anything currently proposed - it has enough fuel to accelerate at 1 ms^-2 for 100,000 seconds, this fuel being an insignificant portion of its total mass (yes, this is unrealistic). The spacecraft needs to reach an asteroid ten light minutes (180,000,000,000 m) away which is at rest relative to its current frame of reference. The spacecraft can either travel directly there in one hop or it can stop at a service station along the way.

  1. Express trip: The spacecraft accelerates for 50,000 seconds, reaching a speed of 50,000 m/s and covering 12,500,000,000 m. The spacecraft then cuts its engines and coasts for 3,100,000 seconds before turning end for end and decelerating for 50,000 seconds, during which it covers the remaining 12,500,000,000 m and comes to rest relative to its destination. Total travel time is 3,200,000 seconds (or about 37 days.)
  2. Pit stop trip: The spacecraft accelerates for 25,000 seconds, reaching a speed of 25,000 m/s and covering 625,000,000 m. The spacecraft then cuts its engines and coasts for 3,550,000 seconds before turning end for end and decelerating for 25,000 seconds, during which it covers the remaining 625,000,000 m and comes to rest relative to the space-going service station. Then it has to repeat the entire process again in order to reach the destination asteroid. Even assuming that the service station is the super-deluxe-instant-service version and no time is spent there, total travel time is 7,200,000 seconds (or about 83 days).

In other words, even one stop of zero duration along the way in space will more than double the travel time. Stopping ten times along the way will slow the trip by a factor of more than one thousand, even ignoring the time spent stopped. Even if humanity can build ships that can accelerate up to 1% of light speed, figure on the slow migration route taking hundreds of thousands or millions of years to reach the nearest star.

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  • $\begingroup$ Thanks for that answer indeed! If I could reach the asteroids in 37 days, I would delay the break for the Jupiter Trojans indeed. $\endgroup$ – Anderas Jun 15 '20 at 14:08
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    $\begingroup$ This seems to be assuming that you have to spread your fuel use along both acceleration-deceleration pairs, but isn't the whole point of the stop to get more fuel? $\endgroup$ – Joseph Sible-Reinstate Monica Jun 16 '20 at 1:40
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    $\begingroup$ @JosephSible-ReinstateMonica how does getting more fuel change the fact that you need as much fuel and time for decelerating as for accelerating (as long as you don’t intent to decelerate by crashing into the target)? Say you have the fuel amount x. You spend half x for accelerating, then fly with that speed until you decelerate with the other half at the target. That’s still faster than spending half x for accelerating, followed by decelerating with the other half to reach a point in the middle, get another x and accelerate again and decelerate at the target. The stop costs you time and fuel. $\endgroup$ – Holger Jun 16 '20 at 7:57
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There are many interrelated problems that we are not currently able to resolve.

Chemical propulsion limits the speed achievable regardless of the mass of the space craft to a minuscule fraction of the speed of light making journey times of the order of tens of thousands of years and stopping off at a range of locations on the way does not help.

Energy becomes an increasing problem as you move away from the sun. Beyond the orbit of Jupiter solar power is hopelessly feeble. Nuclear fusion fuel would only last a few decades and fusion power is still not with us.

Another major issue is production capability. It is one thing being able to easily produce a spacesuit or rocket engine on Earth, but it is quite something else to produce the same item in orbit, on the Moon or on some distant icy comet. This point in particular deserves to be emphasised.

If we can make it on Earth it does not mean we can (with current technology) make it elsewhere. Modern technology relies on a globe spanning network of industry which simply would not exist in the remoteness of space. If more Titanium plate was needed all manner of other technologies and materials would need to be provided. It would need cutting and forming and those machines would need repair and replacement, chlorine would be needed which in turn requires electrochemical processing and brine.

It would be no good to say that we can use other technologies instead of those I have mentioned because ultimately they all have similar issues of interconnectedness and complexity. And it would be no good to say that there is salt or frozen brine at the location, because in most cases we won’t know how much there is, and how accessible it is and what level of what other impurities are present.

An even better example might be reprocessing spent nuclear fuel rods or prospecting for Uranium. Think of the myriad of subsidiary processes required from nuclear enrichment to protective clothing, where will these come from and where will the machines that produce them come from?

Sorry to be so pessimistic I wish it were different, but such is life, perhaps in the decades and centuries to come we may slowly overcome some of these obstacles.

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  • $\begingroup$ Your point is basically that one would have to export a civilization as a whole and not a couple thousand families. Do I get that right? If yes, in a handwavy estimation which I won't use to nail you down, how many people would one need to get self sustaining? Rather 100 million or rather 1 billion? $\endgroup$ – Anderas Jun 15 '20 at 14:03
  • $\begingroup$ It’s very hard to say. As an upper bound 330 million in the US – 10% unemployed = 300 million should be plenty. Although the US relies on huge amounts of imported goods and materials, I suspect that if the rest of the world were to go away the US could manage and adapt, retool, re-urpose and adjust to be self contained. Even harder to say concerning the minimum – millions probably… $\endgroup$ – Slarty Jun 15 '20 at 15:54
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That sounds really easy and funny in your description but I don't think it is. It is not that easy to generate a self-sufficient base up in space, as you need to produce all resources, every little bit, by yourself. The first problem you will come into contact with is fuel. Our technology is mainly propulsion-based and you will not find that much materials in space you could generate acceptable fuels from. Solar sails as a drive could be a solution but our knowledge in this technology is not advanced enough at the moment. Another problem is food. We are able to grow plants in space, yes, but not on the long term. Especially soil (or fertilizer) still have to be brought up from earth and your asteroids and most of the planets/moons will not give what our plants need to grow.
These two problems alone forbid to try this with our current technology-level, still not thinking about getting the vast amount of materials and humans you need for a community living for generations (so at least a few thousand humans) up in space. And keeping them living (body degeneration in zero gravity.)

Still gave you +1 for the fresh idea of humans doing the grasshopper on a interstellar scale.

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  • $\begingroup$ Thanks for your comment! I don't think it's easy, but I tend to think that it should be possible with today's or near today's tech. The gravity is maybe the smaller problem, a rotating hammer or ring is an old concept. Stocking up energy and material, though... that's where I thought about the comets. $\endgroup$ – Anderas Jun 15 '20 at 10:55
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The key critical requirment is a clossed Life Support System. Essentially you are going to create closed ecosystems in space colonies that can last for centuries, while in the present we have no idea how to do this.

The "Biosphere 2" experiments eventually failed as the people inside the dome needed to have outside materials imported (like oxygen) as the various biomes destabilized and outpust became erratic - all in less than a year. Compounding the problem, the experiment was ended and no real follow up has ever taken place (I believe the entire Biosphere 2 compound was later sold). Some issues were not even really known at the time (the concept of "microbiomes" was barely understood in humans, much less the idea of microbiomes in plants, the soil and virtually everything else. We still don't have a clear understanding of that even today).

So with enough money, you can currently get to Mars and even live there for a while with a huge import pipeline, the knowledge to build and sustain a closed life support system is lacking. Until that cam be addressed, long term survival in space is going to be diffficult and expensive (and perhaps a workable CLSS will be equally difficult and expensive - we just don't know).

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    $\begingroup$ to be fair that was mostly due to no one paying attention to the chemistry of their building materials, the concrete kept absorbing the oxygen, and would have kept doing so for decades as it cured. $\endgroup$ – John Jun 15 '20 at 12:47
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    $\begingroup$ @John, that's exactly why a closed life-support system is a problem: we don't know what problems will crop up over time. $\endgroup$ – Mark Jun 15 '20 at 23:35
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    $\begingroup$ @Mark that is not not what happened, they could have easily predicted it if anyone had bother to even look up what the process of curing concrete is, the fact that concrete absorbed oxygen was well known. Non one on the project thought concrete was important even though a huger percentage of the structure was made of it. . $\endgroup$ – John Jun 16 '20 at 0:18
  • $\begingroup$ It was CO2 that it absorbed, but yes. The failures were predictable and the architecture and fundamental approach were not anything at all representative of a realistic life support system design. The Biosphere 2 project demonstrates only that you can't have the design and operation of closed life support systems dictated by ideology and politics instead of engineering. $\endgroup$ – Christopher James Huff Jun 16 '20 at 11:49

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