Constraints in interstellar travel and how to overcome them.

If we want to travel to another stellar system, i.e. “traverse” light years of space more or less uninterrupted and without a timescale we’d consider reasonable and mangeable there are a series of major hurdles to overcome.

We can not make definitive statements about technologies that do not yet exist. For the same of the argument I will not “handwavium” all sorts of speculative technologies and concentrate on the “worst case scenario” that we are to travel such distances only with technologies we are currently absolutely certain of. So no elaborate nanotechnology, no energy output with “antimatter”, no forcefield or “bussard ramjets”. We stick to current technologies and technologies we can be more or less secure they are merely engineering challenges.

The question is – if technological advancement would stop today, and humanity HAD to travel to Proxima Centauri, could we? I believe the answer to be yes, and in fairly reasonable timeframe, Caveat – I am maths blind, so I will not use elaborate calculations – just simple stuff which I can ask google.

The fastest spaceship currently in existence is the Juno spacecraft which moves at 365,000 km/h. A voyage to Proximi Centaur would take more than twenty thousand years at that speed.

We face some big challenges.

  1. travel time, morality

We might in theory already be able to construct a cylindical O’Neil type habitat with artificial spin gravity and create some sort of sustainable cycle. Arguably we should be able to increase our speed well above Juno spacecraft so its safe to assume we can reduce travel times to thousands of years. If we assume normal humans as we currently are it is simply not moral to make this voyage. We can not consign human beings to (a) being severed from the rest of humanity for that long, (b) we can not guarantee that the system onboard does not break down and (c) we can not have a procreative cycle on-board and consign children on that vessel to a life of “just that vessel”. The equasion changes if we (*) can suspend life for a very long time, i.e. decades or centuries. Currently there is one major problem that difficult to overcome with existing theoretical frameworks – that is that any form of human physiology we can currently think of (and even considerably altered ones) carry trace elements that decay radioactively, such as potassium and carbon. Over years, let alone decades, damage from minute particle radiation accumulates, damaging the body from the inside. We currently don’t even have a theoretical framework to repair that damage, hence forms of “hibernation” or suspended animation (let alone “freezing” a body) anywhere over years will accumulate so much damage the patient doesn’t survive the journey.

Let me however state something else – a think city sized colonies to be a somewhat moral undertaking. I personally wouldn’t want to (or could) live outside a human community under several ten thousand human beings. I can imagine such a society to exist in a space colony type safe living environment. We know since the early 1970s such environments can be constructed at enormous economic cost. If the world collectively really wanted to, we could have an O’Neil space habitat in a stable lagrange orbit in probably under 20 years. But it would require a world war 2 mindset to do so. It is however very doubtful we could propel such a “several ten thousand population” habitat over interstellar distances in anywhere under thousands of years, probably tens of thousands of years. Juno spacecraft is a very small spacecraft in comparison. You can’t indefinitely scale up a spaceship to the size of a oil rig, or a skyscraper building, or an aircraft carrier and expect to be able to push it to the velocities we could attain with the Juno spacecraft. And if we could we can’t reasonably expect to persistently be able to feed thousands of humans for all that time and expect nothing would go wrong during the voyage. And if it would go wrong – everybody on board dies a horrible, lonely and (most poignantly) pointless death. So we have to propel the vessel faster. And if we contemplate that we quickly run in to problems

2. Propulsion systems are violent systems. An online calculator tells me that a 100.000 ton vessel (something like an aircraft carrier) that accelerates at 0.01 G (what we can fairly assume a reasonable acceleration rate) we learn that the vessel starts out as some sort of space tanker, pushing against a mountain of “fuel” weighing over 6 million ton, and the voyage is then reduced about 40 years. The trick here is to understand that in space constant acceleration eventually ends up boosting a vessel to gargantuan speed, in this case 0.2 the speed of light. Bad news is that we probably can’t do much better than that without literally melting sterilizing the vessel with radiation.

3. the next problem is debris. And this is a big one. Lets do a calculation. A one gram object hitting the space vessel at a relative speed of 0.2 C would inflict 116.499.889.321.526.300 joules energy. That’s 116 quadrillion with a q. I read somwhere else that a hiroshima style explosion does (63 trillion) joules. Oops. We can safely state that the odds of the vessel hitting a grain sized object during a several decade voyage are pretty much 100%. So we must conclude that even while we can accelerate a vessel to decent enough percentages of the speed of light we probably won’t survive the journey. So we have to travel slower most of the voyage, or we need very implausible shielding to protect us from impacts, or we need to clear the path of substantial debris, or we would need to be able to intercept minute particles from a considerable distance and somehow destroy them.

Does this sound absurd? It sure is. But do remember that during that voyage we have been “charging up” the vessel in the most efficient way nature allows us, with kinetic energy, worth 16 million tons, or a fraction thereoff. That reaction mass comes back to bite us in the behind so to speak, as bugs that hit or windshield hit with the energy of train wagon loads full of TNT. I.e. Ooops, and Not Good.

Now on the latter, we do have a nice verifiable track record. Right now we track a swarm of particles around Earth, some about one gram. We do so very precisely, and we are still able to manouver insane swarms of satelites through that swarm with not too much calamity. The space shuttle did get hit frequently by flake sized debris and as we have seen above we can’t afford to have even microscopic particles collide with the vessel. A flea weigh 0.01 gram and even that still impacts an object moving at 0.2C at 1.164.998.893.215.263 (1 quadrillion) joules. That’s still substantially more than a hiroshima sized explosion. The biggest nuke (50 megaton, tzarbomba) had an output of (what comes after Quadrillion?), 2100 quadrillion – just for comparison.

If we do a combination of burning particles out the way (and there are two ways to do that) in a fairly broad band of space of, let’s say about several thousand kilometers, keep doing so constantly – and we have the best radar system on the bow of the ship and the most high energy X ray laser money can buy, we still cant offer the vessel acceptable certainty. Conclusion – we HAVE to reduce the velocity of the vessel to a hundredth of C, and probably even less – unless we could more efficiently, and with more certainty guarantee the space between here and Proxima Centauri to be cleared of debris.

According to wiki the distance to proxima centauri is 268,400 AU. That’s a lot. The human mind is not capable of making any non abstract mental model. You can calculate this over and over and you will never in a lifetime attain a reasonable and realistic intuition of what that distance entails. Juno travelled to Jupiter with all kinds of cunning orbital mechanics mischief in 5 years. Jupiter is “several” AU removed from us at closest orbital distance. Thus we can safely assume that likewise a scaled up current technology human vessel could somewhat “mostly” safely make a journey of 1 AU in about a year. The faster we travel, the more unsafe it gets. Juno travels at 265541 kilometers per hour. Let’s say 30 kilometers per hour. 20% of light speed is about 60 thousand kilomers per SECOND.

There’s an interesting detail. Interstellar space is not empty. There are numerous objects there. Most these objects look a lot like Pluto, but are just smaller. They are essentially very small dwarf planets. We can likewise assume that there are also lots and lots of smaller, asteroid sized objects. Question – what is the average distance between such objects right between the Sun and Proxima centauri. The sun is surrounded by a very sparsely populated cloud of objects called the Kuyper belt, and beyond that the Oort cloud (mostly cometary debris) extends about 1 light year. Space gets very empty beyond that but I conjecture that there are numerous oumuamua like objects there, regardless. Dare I make an estimate? Let’s guestimate the average distance in a, say “several” AU wide band between the Sun and Proxima Centauri, in terms of objects like Ceres, Pluto, Sedna, Orcus, Eris, Makemake, Haumea, Qoaoar, etc. my cautious estimate is about 10 billion kilometers (preferably less), probably a bit more. In other words if I had a large piece of paper, and two dots on far end of the paper stand for the Sun and Proxima Centaur, and I drew a straight and very thin line between those two points, the line would intersect with something in the order of several ten thousand dwarf planet (in the order of several hundred kilometers in size)

What are dwarf planets?

The problem with interstellar travel is speed. And it’s debris. And it’s the required fuel. And it’s the relative frailty of matter when you inject too much energy in to heat (heat, kinetic or otherwise). You can’t make spaceships endlessly bigger. You can’t endlessly pile mountains of fuel on them. You can’t make mountain sized nuclear bunker shields on the front. You can’t make engines ever more energetic and put out more and more energy. You can’t lock people in very confined cans endlessly and you can’t do so for extremely long periods, or even generationally. There are major tolerances at play here and stuff and if we exceed these it pretty much means party is over.

But there are solutions, and given those solutions we can establish reasonable travel constraints. We can calculate what can with existing technology.

Let’s assume a magnetic accelerator composed of rings. It’s a fairly robust thing, ring after ring. Each ring has a small nuclear reactor and inside the tube it’s about a meter of space for a bullet, and each element (ring) is spaces about one meter apart. I can thus envision a fairly robust steel girder structure like this. If you make one element (one meter) of this a typical modest factory can probably make about one an hour, probably a whole lot more. If we make such a linear accelerator very very long (say – the distance of the earth to the moon, or 100.000 kilometers) then we are not talking insane things. The Earth is easily crisscrossed many times over with roads, internet cables, bridges and train tracks many times more than 100.000 kilometers. If you can get in to space with a large enough sized factory you can build stuff. If you can haul the factory to one of the above KBO like objects, you do not need esoteric energies, and you probably only need a fairly modest human crew (remember, we are still talking existing technologies anno 2020, no “artificial general intelligence” just yet) to keep the factory going, tighten a few bolts every now and then, manufacture some parts for replacement. Now if we can do that we can also build very large Arecibo sized radar arrays on these objects. Once built you can abandon it, and just move on to the next one.

Ten thousand “or so” dwarf planets and moonlets and planetissimals inbetween here and Proxima Centauri. Each about several years distance travel time with a very dedicated crew. A bit like in that “remarkably realistic” movie Ad Astra. Let’s say we were to launch a structure about the size of a small oceanic ship – about 10 thousand tons. That seems to be in the ballpark for a factory where you can bootstrap whatever’s required to do the following

1 – construct a very long array of various arecibo sized radar observatories over the dwarf planets to precisely chart out all debris in the path of any starships.
2 – construct a whole bunch of observatories and scientific instruments. A bit like an antarctic base so to speak.
3 – construct a magnetic cannon capable of accelerating ferric metal objects to an extreme “interstellar travel vessel” speeds.
4 – of course – a nuclear reactor of some sort with an out put in the gigawatt range.
5 – some facilities to construct a small city, housing probably a few hundred people. I.e. again, a bit like an antarctic base.
6 – a mining facility that’s capable of outputting in the order of several tons of metallic ore and other substances – per second.

Let’s say we construct the first such (small?) outpost in a kwadrant of the sky in the general direction of alpha centauri about 200 years from now. My guess travel there could take many years, and construction would take probably in the order of a few decades. There are about 50.000 seagoing ships on Earth right now. We build most those ships in the last decades, which is quite a lot if you think about it. If we would be able to send one such vessel out in to the void every year, and every forward base were able to construct another vessel in situ about 25 years after arrival, then…

Year 1 – vessel1 launched
Year 2 – vessel2 launched
Year 3 – vessel3 launched (etc)
Year 10 – vessel1 arrives at KBO about 10 billion kilometers from the sun.
Year 20 – vessel2 arrives at KBO about 20 billion kilometers from the sun
Year 30 – vessel3 arrives at KBO about 30 billion kilometers from the sun (etc). 35 – colony1 is ready

If every such colony starts outputting their own vessels after 25 years, you enter up with a steadily expanding daisy chain of bases. Each vessel is a technologically upgraded version compared to previous versions, and they’d travel faster so the above estimates would over long times be low estimates. The expansion speed would be in the order of 1 billion kilometers per year. The vessels would literally be receiving fuel launched by magnetic accelerators in-flight. Fuel pods can be accelerated to the above speeds using the launch systems I just described, and the technology for that has existed since the 1970s. You magnetize a ring/element in the cannot, magnetize the next, and a ferric (magnetic) casing is yanked forward. The US army is experimenting with these coilguns and is capable of launching missiles at thousands of km/h speeds with far far smaller and less energetic structures.

The cannons are essentially launch systems where you precisely launch a modestly self-steering automated vessel to interstellar travel speeds. You can use these pods to (a) refuel and restock interstellar travellers with parts, raw materials, medicines, reaction mass, whatever can survive several minutes of hundreds of G acceleration. I bet you might even send simple foodstuffs in them, (b) you can send these projectiles as intercept satelites to scan the road ahead for interstellar vessels. I am sure the reader can imagine all sorts of other applications, including defensive ones, in case the aliens show up (which they probably won’t), scientific ones and on and on.

The point is, this does not require speculative and esoteric propulsion, energy carriers, impact shielding, life extension technologies, bizarre fuel loads, andsoforth.

Automated systems can do this. You should visualize cargo arriving as a very gentle and seamless process, say, a pod every few minutes, and then for years and decades, and probably for centuries. Sedated humans in impact gel containment could probably survive in the order of 10G. If you then mount a propulsion system on ‘the ordnance fired’ you could launch the components of a larger propulsion stack in consecuetive launches and literally assemble a higher output transfer vessel from parts launched. I don’t think humans could then every catch up with a vessel travelling at 0,01 light speed (3000 kilometers per second, or about 100 times the speed of Juno) but I am positive sperm and genetic material and eggs could. And I am absolutely certain scientific advances could be implemented on a small colonial habitat literally ‘on the fly’.

These projects are frightfully long, and require very stable political system and infrastructures. But the construction of such an interstellar highway could have started in the order of a few centuries. It would take grotesquely long to travel to Proxima Centauri given current constraints painted above. Many centuries.

But we now see a totally new mental image that veers sharply away from the “Starship Enterprise” model of interstellar travel. Instead we see basically a very large, very long (spindly) container vessel with very very robust shielding that looks more like an industrial sea refinery rig than a space ship from science fiction. It’s huge, in the order of a nimitz class flight carrier, and very robust. Set with scientific instruments extending in booms from the central structure, and with an active and engaged crew. Children will be born, be raised, be educated, grow old and die. The vessel will be entercepted by cargo from reloading structure and will be in constant radio and reloading contact with the outpost(s) before and after the vessel. A factory will be constantly churning processing metallic iron cannisters into reaction mass for main propulsion and resources need for life aboard the ship.

The end requirement of building a literally uninterrupted sequence of thousands, or ten thousands of waystations with the effect of literally “outsourcing” propulsion to moonlets along the way, would not be dissimilar to the requirements we have seen in Colonial Earth’s past. People were born in quaint little ports along the way of ocean going sialing vessels, somewhere on a coastal region on the way to a far-off destination. Children would be born off the coast of, say, Zanzibar, and grow up, grow old and be buried there, only to hear stories of far away homeland but never to actually see it.

Construction of this interstellar highway to Proxima Centauri would take very very long in human terms. If each outposts netts a persistent one settlement vessel every ten or so years after a few decades, then you quickly hit maximum saturation in an exponential number of settler vessels moving at a very gradual pace, the speed of “several times” that of the Juno mission, i.e. something like over a hundred kilometers per second. Let’s assume we can construct a base every ten years, indefinitely, then after a few centuries you’d see fast interstellar vessels use reload mass to decccelerate from interstellar velocities and proceed to construct new colonies at the advance front of progress. An alien civilization observing quitely from some distance would see a line of faint glowing lights gradually extend outward from the sol system towards Proxima Centauri.

How long does it take to roll out such a daisy chain of settlements? Like I said before, thousands of years. But you would then be able to literally accelerate ever faster interstellar vessels to the maximum speed, propulsion, shielding, logistics and human crew would muster. At any point the vessels headed for Proxima Centauri would be less than a light day apart, maintaining steady and uninterrupted radio contact with outposts and each other.

Yes, you can create interstellar colonies. Yes the voyages take very long, literally centuries. But the image of a lonely Aurora type vessel that’s far from home kind of becomes invalid. Instead, the travellers would be rolling out massive sweeping scientific, technological, industrial infrastructures as a tsunami across interstellar space.

My guess how long it would take to roll out this interstellar highway completely? Actually not much longer as the actual voyages. Maybe three times? That’s less than a thousand years at 0.01 C.

And we would be doing this not with only Proxima Centauri. There would be interstellar highways reaching out to all nearby stars rolling out at the same time. Do realise – the galaxy is billions of years old, and to literally wave a three-dimensional web would take, assuming nothing much more advanced than currently existing technologies anno 2020, not much more than a hundred million years, at best.

Now…… if you start adding human cloning, long stay hibernation, cryonic suspension, cybernetics, genetic modifications of humans, mind-machine interfaces, artificial general intelligences, uploading, robust nanotechnology, etc etc. to the mix then all this would be able to roll out much much much faster.


Yes my maths abilities are bad bad bad. I am maths blind, mostly. Readers of this article may do the actual legwork in making meaningful calculations of all aspects of article. I will add any (if any) submissions or updates under the main article.