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A Roadmap to Interstellar Flight (2016) (arxiv.org)
182 points by ra7 20 days ago | hide | past | favorite | 155 comments

Wow, that's a really comprehensive, deep and dense collection of information.

For anyone really interested in the science of this who doesn't necessarily want to digest the raw math, I highly suggest Isaac Arthur's Youtube channel eg [1][2].

People tend to fall into a number of traps when dipping their toes into this topic.

The first one is the trap of wishful thinking, which is the primary reason for pretty much any attempt to come up with a theory for FTL. Most of these people just don't understand what the domain of a function is. Just because you can plug a negative number into an equation that is otherwise over real numbers doesn't mean that makes sense or is valid.

Second, people oversimplify. I see on this thread and others claims like "if we just accelerate at 1G we can get pretty much anywhere relatively quickly". So that's true superficially but the energy requirements for that are so vast even if you have perfect mass to energy conversion that a significant percentage of your ship's mass is fuel.

To me, laser highways seem like the most likely form of far future interstellar travel. That's largely predicated on commercially viable fusion power however and that's not a given.

Failing that, you're talking about variations of what are essentially generational ships.

[1]: https://www.youtube.com/watch?v=wXiitWK_6Qg

[2]: https://www.youtube.com/watch?v=s6BQSgidbmc&t=1526s

If we could solve the energy problem, humans could go pretty much anywhere in the galaxy on a 1G ship. (ok there are other issues like actually building the ship, supplies (or a biosphere that can support life for decades), shielding, but that's all easier with unlimited energy).

For example, a ship could travel 1,000 light years to the Orion Nebula in only 15 years (as perceived by those on the ship). And since they are living in 1G, a lot of the detrimental effects of long-term space travel are eliminated.

Of course, to an observer on earth the trip would take over 1000 years, so don't expect to hear how the trip went.

Energy is the biggest problem, but it’s far from the only one.

Consider the ISM: at 0.086 c, ionised helium is the same thing as 14 MeV alpha radiation. Low density regions within the galaxy are 0.2–0.5 particles/cm^3; 0.2/cm^3 * 0.086 c is ~5e12 particles/m^2/second, or about 9.9 watts/m^2 if it was all He4 and therefore 14MeV/particle or about 2.5 W/m^2 for Hydrogen at the same speeds, which is bad enough given what keeping that up for years will do to any solid hull, but molecular clouds can be 10^2-10^6 particles/cm^3 and the upper bounds of that is ~10 MW/m^2.

Even without Relativity (which makes it worse), if you double the speed the kinetic energy per particle quadruples and you also double the number of particles per unit time.

ionized particle moving in your frame can be recovered as energy while the particle itself can be used as reaction mass (He) or even as fusion fuel (H).

"ionized particle moving in your frame can be recovered as energy"

You also recover it's momentum, which means you slow down. Really you are recovering your own kinetic energy, you are breaking against the interstellar medium, probably very violently and destructively.

That's like saying "a building was moving in the frame of reference of the plane, so it didn't crash - it recovered energy."

A more advanced version of this concept is known as Buzzard ramjet, and it was calculated to be energy negative


following your logic we wouldn't be making aerodynamic shapes and would be driving/flying brick shaped vessels instead.

Making something aerodynamic minimises your losses, it doesn’t “recover” energy.

If you have a more precise idea and that was an analogy, I recommend giving that idea in a more precise way — I’ve seen my own “what if” ideas wildly misunderstood by my own poor analogies.

>Making something aerodynamic minimises your losses, it doesn’t “recover” energy.

not necessarily. For some parts aerodynamic design means loss minimization, while some parts (like [ram]jet engine inlets or pretty much whole body of a scramjet plane/missile) are designed for ram effect, ie. to recover some of the kinetic energy of the incoming air as the air has velocity in the hull's frame of reference.

The same thing for interstellar medium - the "aerodynamic" design would mean either coming up with some means of smoothly redirecting the incoming medium around the ship (probably by ionization and EM field) and/or recovering the energy and collecting the mass, whereis GGP limits the consideration only to the "breaking against the interstellar medium, probably very violently and destructively."

“ram effect” isn’t something I can Google (I get pages about farming); the closest I’m familiar with given the context would be ram compression of air intakes in engines, but that’s for increasing the mass flow though the engine, not energy recovery.

The only engine I’ve heard of that attempts to use the ISM as a source of fusion fuel is the Bussard ramjet referenced by @ClumsyPilot (which uses an EM field), and as they point out, it’s been demonstrated to be a net-loss not a net-gain. An easy way to intuit this result is that the energy per particle of a fusion reaction is similar to the 14 MeV per particle (0.82-22.4 MeV for the most promising reactions) which I used as a starting point in my calculation.

You’d actually have to decelerate the ISM from the frame of reference of the ship in order to increase the rate of fusion to its maximum, but even then most nucleon impacts do not cause fusion and instead bounce, slowing you instead of helping you even if you can solve the problem of efficiently using the energy of any particles you decelerated at the front of your ship to accelerate them again at the back.

>“ram effect” isn’t something I can Google (I get pages about farming); the closest I’m familiar with given the context would be ram compression of air intakes in engines, but that’s for increasing the mass flow though the engine, not energy recovery.

ramjet inlet is designed to convert kinetic energy of incoming air into a pressure by slowing down that air, and the higher pressure results in higher efficiency of the engine. In case of scramjet a lot of the energy impacted upon the air pushed away by the hull at those speeds is concentrated in the form of high pressure of the shock wave - so the scramjet inlet is designed to scoop that high pressure air, again for high efficiency of the engine.

>the energy per particle of a fusion reaction is similar to the 14 MeV per particle

option 1. completely absorb the momentum of that particle by the hull and slow down accordingly and waste the collision energy as heat/radiation. Option 2. slow down the particle (thus still of course absorbing the whole momentum) in the EM field thus generating some useful energy and using that particle, if a suitable one, later for fusion - thus recovering significant share or even crossing into net-positive a bit (depends on efficiency of the recovery and the fusion processes)

Your option 2 does not work as anything other than a break — it brings the ISM into the same frame as the ship, slowing the ship down in the process. Really useful break, but not useful as more than a break if you can’t beam in arbitrary energy not carried with the ship and if you can beam in every you probably don’t need use the ISM as a break.

Consider: take all the energy from bringing the ISM into the same frame as the ship, use it with 90% efficiency to reaccelerate the same matter out the back of the ship in an ion drive, and you merely roughly double the velocity where drag becomes clearly dominant (I am handwaving the relativity correction as the difference between x8 and x10). 99% efficiency, you’re looking at 0.086 c * 2^2 = 0.34 c, still far short of what you need for time dilation to get you 1000 ly in 15 subjective years.

Also, any engineering solution along the lines of what you suggest would turn existing $1000 high-school-project Farnsworth fusors into useful power output fusion reactors rather than mere switchable neutron sources.

(As an aside, learning about the reality of Bussard ramscoops gave me the headcannon that the ones in Trek only refuel the ships while they park themselves in ionospheres; but Trek is big on the “space is an ocean” trope, so not that important).

Have your opened Buzzard Ramjet link? Folks competent in the area have done the math for recovering energy, mass, and using the hydrogen for fusion.

This, like many non-IT discussions on hackernews, is devolving into "How many angels can dance on the head of a pin?"

Congratulations, you just nerd-sniped me. I just had both the Good Omens quote and the knowledge of how bizarre and alien the Biblical angels are described to be merge together in my imagination as wheels-within-wheels trying to do the gavotte.

First, you have to ionize it all.

required ionization energy is dwarfed by the particle kinetic energy to be collected which is in MeV.

Fine. But how would you achieve it?

Which, to me, is why we should be focusing on new ways to produce energy. Solar and wind is nice, but we could revolutionize humanity and save the earth if we found a way to build cheap energy sources with a density of gigawatts per cubic yard.

What better way to save the earth and solve the population problem than to have the energy production capabilities to simply build e.g. ringworlds? Even that's not out of the realm of possibility with that kind of energy available.

WE already did that. Its called nuclear power. Fission energy can do almost everything fusion can do. The difference in energy density between fission and fusion is not that large compared to the energy sources we have.

With all the investment in solar and wind we could easily have built a fission based economy. The French proved that in the 70/80s and had a green grid ever since.

Had we done that collectively global warming would be a far lesser issue.

It would also help us understand neutronics and material science so we can eventually switch to fusion.

The same basic technology could also be used for space travel in the solar system and would have advanced space travel by a lot.

But of course for idiotic reasons society went away from that.

Idiotic reasons such as that nuke always turned out, on examination, to cost much more than alternatives.

Much of that high cost is often wholly-legal corruption that attaches to almost any large, centralized, hard-to-account public-money expenditure. Knowing this does not help. Thus far, solar and wind projects have mostly avoided the corruption tax by their simple accounting framework: N generating units x $C per unit = $CxN; and by their clear value proposition: $CxN is lately, and still increasingly, much less than alternatives whether those are figured with corruption tax included or not.

At this time it is cheaper to build out a new solar-and-wind farm and operate it than to continue just to operate a comparable nuke, wholly neglecting construction and decommissioning cost. We finally got the ramshackle Diablo Canyon and Indian Point contraptions shut down, but it will still cost a $billion to take them apart; or, likely, more, according to the degree of corruption tolerated.

"to cost much more than alternatives."

That's why France has cheapest electricity in Europe?

Furthemore, once you take into account damage from climate change for fossil fuels or cost of backup for renewables, their cost triples.

For example Scotland generated 100% of it's energy needs cumulatively in a year from renewables, but in reality the renewables only covered ~30% of demand because it was generating loads when none was needed, and generation little when loads was needed.

France’s reactors are all government owned, and the subsidy level is apparently hard to determine (or was in 2009, Al Gore paraphrased citation on https://en.wikipedia.org/wiki/Electricity_sector_in_France).

But this graph indicates that, unlike in 2009, nuclear is in general much more expensive than PV or wind: https://commons.wikimedia.org/wiki/File:20201019_Levelized_C...

From the cost estimates I’ve seen, PV with battery backups are cost competitive with oil and better than coal, even with pessimistic battery cycle-lifetimes. And we are building battery factories essentially as fast as we can, and batteries are still getting cheaper.

French nuclear is not just about subsidies: they actually designed a national system, where all reactors are the same, parts were mass produced, repairs were easy and workers are trained on the same thing. Nuclear fuel is reprocessed instead of being left in casts near the reactor, like is done in US.

US and many other countries have a random of assortment of one-off reactors with different safety issues and operational requirements, fuel requirements, making this very expensive to operate. This "free market" approach doesn't work for nuclear.

Regarding batteries, we will have a massive shortage for cars, let allone grid from all the forecasts I've seen.

Australia and India will be fine with solar, but Siberia has no sunlight half the year and wind is not great there either

> Regarding batteries, we will have a massive shortage for cars, let allone grid from all the forecasts I've seen.

I’ve not seen this. Do you have a link?

> Australia and India will be fine with solar, but Siberia has no sunlight half the year and wind is not great there either

Naturally. I don’t wish to suggest one-size fits all, merely what seems to be the best on average at the moment and with foreseeable trends.

Battery storage for utilities will be the high-cost alternative. Other methods are being proven at scale, and will ultimately eliminate the competition for batteries.

The most mature of these is "stored hydro", in which water is pumped up to a dam reservoir. Obviously this depends on having a dam not too far away, a ready source of water below it, and enough spare generating capacity there to take up the load when needed. Ramp-up is near instantaneous.

In places with deep mine shafts, a similar gravitational storage method is practical: suspend a heavy weight (hundreds or even thousands of tons) at the top of the shaft, attached to cables that turn a motor-generator as they unwind. This has the advantage that it may ramp up to full power in seconds, and the available energy is limited only by the depth of the shaft. Obviously this depends on availability of long vertical mineshafts, but there are very many of these worldwide. The tech involved is all 19th-century, so these only need construction. When excess / cheap power is available, the weight is hoisted back up.

Another is underground compressed air. (This may be combined with the above, pumping air into the same mineshaft, as is being done in one pilot installation.) This, also, relies only on 19th-century technology. Roughly half of the energy stored in compressed air becomes heat, so efficient use requires good heat insulation. Earth serves as excellent insulation. Power is extracted by exhaust through a turbine; existing turbines from older generation schemes may be repurposed. Ramp-up is fast. The reservoir must be charged up for some time in advance to be able to get maximum power out at sudden need. Several examples of this are being demonstrated.

Air liquification is a perhaps surprising alternative. It relies on mature 20th-century technology; there have long been numerous industrial uses for liquified air. For maximum full-cycle efficiency, heat extracted from the air is also banked. A GW-scale plant is under construction in Scotland, and a 50 MW demonstrator is breaking ground in Chile. An advantage of air liquification is that the storage capacity is limited only by the number of cheap insulated tanks built, which may be added to at leisure.

The above are the immediately available alternatives. Efficient hydrolysis of water to produce hydrogen has obvious industrial importance, and it may eventually displace other methods, as there is no upper limit to the useful amount of hydrogen that may be usefully produced from surplus generating capacity, even after local tankage is full.

> Idiotic reasons such as that nuke always turned out, on examination, to cost much more than alternatives.

Nonsense. In the 60 they literally developed 100 of different types of nuclear reactor often for a tiny amounts of money. Any coherent strategy over the last 50 years clearly shows that if would been cheaper to reach basically zero carbon and it could have been done far earlier and earlier with nuclear.

Navy ships in lifetime analysis show that over the ship lifetime its very cost effective. Not to mention that you can do many things you simply couldn't do otherwise.

Real private innovation and capital were never unleashed for nuclear innovation after the early explosion in the 60s. The extreme restrictions on the materials, government picking winners early and shutting out everything else. At least at that point some research was still going on, but no real effort to go beyond. After the government lost interest in the technology almost every single project was shut down and a nuclear became basically no-go when asking for funding inside the government.

And of course if you have incredibly low volume systems, with a tiny base of educated people, based on technology that is essentially from the 60s you are not gone have an effective system.

Like everything else, nuclear plants, even incredibly sub-optimal PWR designs when built in large numbers will get significantly cheaper. This is clearly evident in any analysis on nuclear economics. If you can build essentially the same reactor with many of the same project leader and teams over and again the costs go down massively. Even outside the actual parts production getting cheaper.

The cost per reactor that France achieved during their expansion is actually very low and they did it with pretty old technology that was really not at all optimized to be effectively built.

There is a gigantic design space that has not been explored that could massively reduce both the build cost and the operation cost and even decommissioning cost. Alternative reactor designs can be far smaller, far safer and require less human operation in fact a properly built reactor should have almost no human interaction ever.

This was all known in the 60s and the foundations were clear put down. In the 70s some amazing stuff was shown but at that point budget had already gone down government had already picked the commercial winners.

Check out this picture:


A design of that size would have been possible with 70 technology. Had they actually tried to innovate on technology and attempted different designs or had given real advantages to CO2 free technology at the time and let private investors take care of it, designed like this would have happened.

These main reactor vessels are literally not much bigger then a tiny ship, and could easily be manufactured in a single factory on a assembly line, transported to location and dropped into a hole filled with concrete.

Current cost actually are a result of the massively complex large civil engineering, more then the actual nuclear part themselves. That is high cost construction over a long period that has major disadvantage in financing. Having 10x smaller vessels with 100x simpler cooling system absolutely would make building reactors far cheaper and if you build many of them and have teams that are not doing for the very first time there is no reason why a nuclear plant can be build at cheaper rates then coal plant, more comparable to modern gas plants.

If you have an energy source that has that much advantage in energy density. Not taking advantage of such a dense and easy to obtain energy source is insane. Thorium mining for the whole US could have been done in one small mine for all US needs for the next 100 years or thorium extract Throium from the waste product of other mining operations. The reactor above could run on thorium that was dissolved very simply, you don't even need complex fuel production (not that that fuel production currently matters that much for overall cost of plant operation).

So you have basically zero cost of fuel, basically zero CO2, almost no mining, could have been done in the 70s, for sure in the 80s and 90s.

This is what I am talking about when I say its a missed opportunity. Even the incredibly sub-optimal route France took, lead to results that were far, far better then the route anybody else took and France did the world a huge service.

FWIW, fissible material that's used with current technology is actually running out, with a similar timeline to oil. Fission is not an infinite energy source, despite appearances to the contrary. It's also not an cheap as one might hope.

But the current technology can be improved at least 100x (fast fission) to 10000x (breeders).

This is complete nonsense. Fear mongering about Peak-this or Peak that is has proven wrong in pretty much every single case for 250 years.

If fuel were serious concern for cost, real searching for it would find much more.

And even with current technologies and reactors there are many ways to extend this, if you are willing to mix in some more highly enriched materials.

Eating up the absurd amount of nukes the US still has for example would be helpful.

And of course no society that actually bet on nuclear would still be using that fuel cycle in any serious way.

This is just the typical fear mongering that people use for all technologies they don't like, "Not enough lithium for EV".

Those numbers are for once-through fuel cycle used by US, which only burns 3% of U235.

Fuel reprocessing employed by France uses all of U235 + generates and burns plutonium. Fast reactors, which exist in small numbers in Russia, etc, take it up by another two orders of magnitude

I think there is one more step change theoretically possible in terms of energy output which is based on anti-quark collisions. We need to understand QCD better. Its hard to know when breakthroughs will happen. Maybe its already happened and we just havent heard about it yet!

We also dont know how to model quantum gravity still. That seems pretty fundamental.

Im optimistic there will be physicists who find something new and whacky that no one could predict.

Yes, fine, lets research that. However here is the thing. We figured out that fission thing in the 50s and still fail to use it.

All this cheap energy eventually turns into heat. The only way to get rid of excess heat is to radiate it into space, and the only way to radiate _more_ heat into space is with an increased temperature.

I love this somewhat tangentially related article: https://dothemath.ucsd.edu/2012/04/economist-meets-physicist...

In a world with hard-capped energy use the incentives to expand into space would be enormous, I don't think it's unrealistic at that point to think the growth will continue outwards.

In particular one can imagine outsourcing every high-entropy activity to space and leaving Earth for low-entropy high-value activities.

Unless the cheap energy isn't being burned inside the atmosphere.

That might be a blessing on mars.

I’m a little confused. Are you proposing to Save this world, or build/settle others?

I'm proposing we could build other worlds, such as ring worlds, so that the population and industry of the human race can be moved off-world.

That means we'd be able to treat the Earth as a great big park, with little-to-no human inhabitants.

Fanciful, but still possible, given such an energy breakthrough. Even if it takes a couple centuries, genetic records and some terraforming (feasible with that kind of energy available) would probably allow us to recover from the damage incurred by the Earth in the time it takes to develop the tech.

>I'm proposing we could build other worlds, such as ring worlds, so that the population and industry of the human race can be moved off-world.

>That means we'd be able to treat the Earth as a great big park, with little-to-no human inhabitants.

That's a nice idea, however the resources required to move enough people off the Earth just to keep the population from rising, let alone moving the bulk of the population off the planet, would be astronomical and definitely impractical.

Consider the data:

1. Population: ~7.75 billion.

2. Deaths per year: ~60 million

3. Births per year: ~140 million

4. Annual population growth: ~80 million

Let's start with the idea of keeping the population the same, rather than moving 7 billion humans off the planet.

In order to move 80 million people just into orbit, let alone have some place for them to go would require that we launch (rockets? catapults? space elevators?) about 220,000 people, along with whatever cargo is required (we'll ignore the cargo here) every. single. day.

How many rockets/space elevator cars/catapult ships or whatever would we need just to move that 80 million off the planet?

Let's use space elevators (SE) here since we can (theoretically at least) make space elevator platforms arbitrarily large.

Let's say we use an SE car that can carry 1,000 people and can travel up the elevator at 1000 km/hour.

In order to just keep the population from growing, we'd need to launch 220 SE cars a day.

Unfortunately, a round trip to/from geostationary orbit[1] (yes, rockets/catapults could deliver payloads in much less time to low Earth orbit. But such payloads would likely be an order of magnitude less than a space elevator) at 1000 km/hour would take ~70 hours.

Which means we'd need ~650 SE cars in service at any given moment.

Loading and launching 220 SE cars a day would require enormous logistical effort:

We'd need to launch ~10 cars per hour (220 per day).

How can we load and launch 1000 people times 10 in an hour, every hour?

I suppose we could build ten space elevators, then each site could just load and launch one an hour. Which seems doable.

But where will people stay while waiting to board a car?

Just expending the resources to build (and maintain) the infrastructure for such an enterprise would be enormous. And without some serious energy (commercial fusion on a massive scale), just running these elevators would quite possibly dwarf the total power output of the planet.

And that's just to keep the population flat. Attempting to remove the bulk of Earth's population would require raw materials, manufacturing, construction and maintenance several orders of magnitude larger than the example above.

So no. We're not going to solve any population issues, let alone removing the bulk of the population, by sending folks off-world.

[0] https://ourworldindata.org/births-and-deaths

[1] https://www.esa.int/Enabling_Support/Space_Transportation/Ty...

The starting assumption is an astronomical increase in our access to energy, if you recall (and I also mentioned that energy being "cheap"). With that energy, we can:

- cheaply access space

- cheaply access cubic miles of asteroid iron

- build whole automated production industries in space, to build however many spaceships we want

- automate the construction of spaceships and that ringwood

- use those ships and tools to automate the construction of more production facilities

You're imagining a single transport into space, but that's a mode of thinking still constrained by expensive and inefficient energy. Imagine hundreds, thousands, or tens of thousands of autonomously constructed ships all available for that population transfer.

That energy is the catalyst for an exponential growth in our ability as a species to move ourselves and control our environment. It just takes some imagination to see outside our current constraints.

It's not the ai singularity, but I think our ability to build stuff cheaply, in space, with materials obtained in the quantity they're available in space, will represent a singularity-esque leap forward.

Yes? If the latter is accomplished, the former becomes trivial.

What?! Why?

How will settling Mars or any planet save the biosphere?

We have a few decades to not kill the planet. Settling another planet would take centuries.

I think the idea they are suggesting is that if we can figure out how to terraform Mars and create a livable atmosphere from scratch, the tech and domain knowledge created along the way means cleaning up our atmosphere should be easy in comparison.

Kind of like Olympic gymnasts lift weights and work on flexibility and they want to exceed what their performances require such that the performance is easy and not pushing them to the limits of their ability.

Ie "You have to crawl before you can walk. If you learn to run, walking becomes trivial in comparison."

If that makes sense.

This analogy only makes sense if the world’s problems were that of technology. We have ample evidence that it is not.

There exists technology that can transport people over short to medium distances really efficiently and without much greenhouse gas emissions. All it would take would be for policy makers to allocate some funding to build out the infrastructure required, yet they don’t.

There exists technology to transport and distribute food wherever there is hunger. Even technology to grow food more efficiently and without additional greenhouse gas emissions. Yet we don’t apply that.

Most countries continue to pour money into their most devastating government institutions (the military and the police) that not only cause a world of societal problems on their own, but also pollute a bunch in the meantime for everybody else. At the same time they could be using that money to build infrastructure that would allow us to live a more sustainable lives. But they don’t.

If the technology existed that would take people to Proxima Centauri in 10 years, and we invented a bunch of good technology that would help us make our current world better. I bet this new technology would be used equally sparingly as our current technology.

I don't actually disagree with you. But I also know that a lot of tech in use today was born of our efforts to solve problems in space exploration, so I also don't entirely disagree with the line of reasoning that space exploration requires us to meet such a high bar that inventions that grow out of it end up being essentially trivial to implement here on earth.

I'm personally heavily invested in the pieces of the puzzle that tech, per se, cannot solve. My work in that regard gets little in the way of attention and people have long attacked me as a nutter, etc.

I run a citizen planners forum on Reddit. I try to write about local community development at eclogiselle.com. Sometimes something I wrote gets a few thousand page views, but most of what I write gets very little traffic and that seems to be generally trending down, not up.

And I have mixed feelings about that because I have actively sought to ditch traffic rooted in lurid interest in me, so that's sort of "huzzah. I win? I guess."

I would like to see more focus on passive solar design. I would like to see more development of missing middle housing. I would like to see more walkable, bikable communities where Americans can actually live without a car.

I would like to see social change of the sort that's needed to actually solve these problems with the currently available tech. The problem I see is that tends to require a charismatic leader of the sort that historically founded various religions and I see problems with that approach.

I think it's inherently problematic to just take someone's word for it and do as you are told because they said so and you basically worship them. People need to think for themselves, not dutifully do as they were told.

And I don't know how you put out good info to foster the right kind of change in the amount needed etc and do so in a way that sidesteps the tendency for leaders of any sort to dictate what others should do.

So I have kept my footprint intentionally small in some sense while I figure out best practices. "If you don't have time to do it right the first time, when are you going to find time to do it over?"


Precisely! We have the technology, but I think people do not realize we have the technology. They think to tackle climate change, we have to reduce our ambitions for the future (which isn't true) or that human life depends essentially on fossil fuels. There are no fossil fuels in space. That is why space travel and tackling climate change go hand in hand: space travel is tangible evidence of civilizational capacity, that we can do ambitious things even harder than fighting climate change (and we don't even have to spend THAT much to do those things) and we don't need fossil fuels to do it. It also provides an essential kind of perspective that I really think is essential to broad-based environmental consciousness. In the words of Carl Sagan: https://www.youtube.com/watch?v=8Xtly-dpBeA

We can start it in a few years. And it can assist in part by just showing that we have a societal capacity to do great things. In the words of Carl Sagan: https://www.youtube.com/watch?v=8Xtly-dpBeA

(And we don't need to and probably shouldn't spend a lot on this. What we currently spend on space exploration/settlement through agencies like NASA is sufficient for this. And we also get the spin-off technology advantages.)

We have about a decade to start taking “saving the earth” (from CO2 in particular) seriously, not to complete the task.

I’m moderately optimistic.

If you take your whole civilization with you, it doesn't matter how fast or slow you go, you are already where you need to be.

We are there right now, zipping through space together.

We have some social problems and some HVAC problems. But the vehicle is already underway.

You might enjoy this video on terraforming Venus, https://www.youtube.com/watch?v=G-WO-z-QuWI

I think we should consider the whole solar system our galactic ship while making sure to keep the current habitation quarters in top notch shape.

Civilization is a believed concept. Physically taking all of your people with you does not equate to maintaining civilization.

This thread is reminding me of one of my favorite Clarke books, which partially explores this theme: The Songs of Distant Earth [1]. Is there such thing as a singular human civilization, once it become split by great distances and millennia?

1: https://en.wikipedia.org/wiki/The_Songs_of_Distant_Earth

Humanity will diverge as soon as we have off Earth reproduction.

It’s an excellent book. Mike Oldfield made an excellent soundtrack for it, and Clarke wrote the insert for the disk.

I think some of the people responding to the comment about solving the energy problem are missing the point. There's solving the energy problem in the sense of finding an energy source that can power human civilization (preferably without melting the planet I live on, but that's a discussion for another thread). Then there's solving the energy problem in the sense of accelerating a spaceship to ultrarelativistic speeds. The latter is a very hard problem.

A good rule of thumb to remember is that if you're going fast enough to experience large time dilation, then your kinetic energy is to be large compared to your rest mass. It's the same factor of γ=1/√(1-v²/c²) either way. So if you're traveling 1000 years in 15 years ship time, you've got γ=67 and your kinetic energy is 66 times your rest mass. Ouch! (And here I'm making the incredibly optimistic assumption that you aren't carrying your fuel with you, because if you are then you also have to expend energy to accelerate your fuel, and to accelerate the fuel you use to accelerate your fuel, etc.)

If the rest mass of your ship is about the same as that of an aircraft carrier (the USS Enterprise, CVN-65, seems appropriate), then 66 × 86000000kg × (3×10×⁸ m/s)² = 5×10²⁶J. That's a lot.

Nuclear energy in the ordinary sense is nowhere close to enough. This is thousands of times more energy than we could get with all the U-235 on Earth. Or we could imagine getting this energy from fusion, using the big fusion reactor located a cozy 8.5 light-minutes away from us. Earth gets about 123000TW from the Sun, so another way of looking at this number is that if we captured 100% of that energy, it would take us 130 years to get enough for the ship's kinetic energy.

Accelerating big things to ultrarelativistic speeds isn't easy.

Not true. Generating energy is only part - propulsion is hard too. Even if you had a bunch of anti-matter, how do you build a spacecraft that can safely transport humans for years with a propulsion and storage system that uses it? How do you use M-AM annihilation usefully? It’s not like Star Trek.

Just having a chunk of anti-matter isn't really "solving the energy problem", obviously you need to harvest the energy, but I think that given nearly unlimited energy, propulsion is a much easier problem to solve.

One possibility is:


With antimatter propulsion you'd want to use it as its generated rather than the messy business of trying to store it.

How much energy would that be for that trip, for a reasonably sized space ship to take care of a crew for that long?

Are we talking miniature fusion drive or antimatter type energy density required?

I haven't done the calculations (which would be hard to do without knowing how big/heavy the ship needs to be), but I'd assume it's going to need something well beyond our current technology.

An aircraft carrier is powered through the water (horizontally, of course) for 25 years on nuclear power.

Now imagine the power it would take to make it hover for 25 years, and that's your 1G ship.

Assuming perfect conversion of mass to energy you will still need a staggering amount of fuel for each kg of payload. Goggle the relativistic rocket equation and you should find a page with some figures for example scenarios.

You could do the "spinning drum" kind of 1G gravity, and save fuel that way.

Without the 1G of acceleration, you're limiting how far you can go in a single generation. At 0.1g, that 1000 light year trip would take 90 years (to those on the ship). The beauty of 1G is that it provides normal gravity and you can go great distances in a human lifespan. Of course a 2G trip will get you there in half the time, but it's harder for humans to live at 2G.

Though maybe the time to the destination is not a problem if you don't mind a generational ship where generations of humans live (and die) on the journey. It doesn't matter to those on earth, since the trip will still take over 1000 years in their timeframe, so whether it's 1015 years, 1100 years or 2000 years probably doesn't really matter to them.

"The beauty of 1G is that it provides normal gravity and you can go great distances in a human lifespan."

It like unicorns - beatifull and entirely fancifull. Fusion or fission cant do it, even a ship with pure antimatter drive cannot accelerate like that for decades

We will have hibernation before we have practical antimatter, and lets you travel intersteller without wasting enegy

> even a ship with pure antimatter drive cannot accelerate like that for decades

Sure it can. You just need a decently-sized asteroid and an equally-sized antimatter asteroid for fuel, and you can do it for decades, even with the price you pay early to accelerate the fuel you will use later.

What are you trying to do with an asteroid, and why does it need to be decently sized? Neither is relevant.

You calculate mass fraction, i.e. for a ship to get to orbit using kerosene, the ship must be 97% made of fuel my mass.

If you have an antimatter ship, if you can produce, store, and 100% efficiently use antimatter and carry nothing else as your reaction mass and don't have issues with waste heat or radiation, your ship will need to be over 90% fuel by mass to maintain 1G acceleration for years.

> What are you trying to do with an asteroid, and why does it need to be decently sized? Neither is relevant.

I'm using the asteroid for mass, to react with the antimatter. So, yeah, it's relevant.

> If you have an antimatter ship, if you can produce, store, and 100% efficiently use antimatter and carry nothing else as your reaction mass and don't have issues with waste heat or radiation, your ship will need to be over 90% fuel by mass to maintain 1G acceleration for years.

Yup. And it's exponential - if the non-fuel mass is 10% to be able to do 10 years, then to do 20 years, the non-fuel mass can only be 1%, and for 30 years it can only be 0.1%. That's why you need the asteroid.

Taking antimatter fuel out of your carefully designed, failsafe magnetic containment tanks and putting it into a highly-volatile asteroid of doom (attached with what? Antimatter-immune rope that can pull a billion tonns at 1G?) does not mean the rocket equasion gives you a free pass.

Its still the same formula, nothing has changed. Its combined weight of your ship and it's asteroids. All you are saying is "you need a big fuel tank"

I'm saying approximately how big a fuel tank you need to refute your "you can't do it for decades" statement.

So how big is it? Is a "decently sized asteroid" 50 meters across? 5,000? 500,000? Thats 4 orders of magnitude range.

Like I know what's a decently sized car, or a house, or a mountain

If you could somehow accelerate at 1G indefinitely you can get anywhere in the universe in a human lifespan.

Of course to accelerate a 1000 ton spaceship you'd need about 4 quadrillion tons of fuel.

The 1G in this case is the acceleration towards your target star system at 10m/s^2 until you are half way, then decelerating at the same rate until you arrive.

The fact that this also produces liveable conditions is just a nice side effect of a 1G ship.

But does that get you to the Orion Nebula in 15 years? Or do you have to accelerate all the way to do that? (And then you get to wave briefly at the Nebula as it goes cruising by at a significant fraction of the speed of light?)

Or do you have to accelerate all the way to do that

That's what "1G" means in this concept, you're always accelerating at 1G (9.8 m/sec^2)

On the first half of the trip you're accelerating toward the destination, on the second half, you're decelerating at 1G.


The graphic in that article is what I wanted.

My point was that, due to the highly non-linear time cost, it takes much less time to travel from Earth to X if you accelerate the whole way, rather than accelerate half the way and decelerate the other half. I wanted to know which way gorgoiler used to compute the subjective time.

And, thanks to your link, now I know.

Subjectively in decades due to relativity by constantly accelerating, objectively in a millennia or worse. Wonder how to get around interstellar obstacles though, changing courses would be plain impossible

Large interstellar objects are, well, large, so you can see them well in advance and just steer around them.

changing courses would be plain impossible

You can steer by vectoring your acceleration. It would take about 3 days for a 1G ship to travel from the Earth to the Sun, so it wouldn't add much time to the journey to steer around an object the size of the sun. Ideally you'd identify those objects so far in advance that it would take little course correction to go around them.

Smaller objects like asteroids that are too small to see in time to avoid would need to be absorbed by the shielding or destroyed or pushed away by some other method. At near relativistic speeds, this would obviously be a non-trivial problem to solve, but hey, you have unlimited energy to solve it.

But the faster you go, the more the galaxy is Lorentz contracted from your frame of reference, and therefore the denser the stars are, so dodging gets harder. Also, they're coming at you faster, so you have less time to dodge.

> and therefore the denser the stars are, so dodging gets harder.

Not sure this is true, since the stars also get "smaller" and therefore harder to hit. Also, while you have less time to dodge you also don't have to dodge "as much" since your vast speed means you move through any zone quite quickly and therefore don't have much time to heat up/get irradiated. You can probably pass by a star relatively closely as long as you are moving fast enough. (From a heat/radiation perspective, solar wind particles might still mess you up good)

The stars only get smaller in the direction you're traveling. Perpendicular to that (that is, in the direction you have to dodge), they don't change no matter what your velocity is.

But is hitting a star (or a planet) head on a realistic risk?

Here are my preemptive prayers to lost billion colonists aboard the ships that hit stray planetary bodies between 3450 - 4550 A.D.

Your genes and spirits will be carried on by the quadrillions that made it, but for now, may you rest in peace.

Relativity means that by the time your space kids are on to the second generation of space labrador, you’ll be there.

To a static observer, 1000 generations of labrador will have been bred back on Earth.

You still need something like the 1G acceleration, though, for the travel part. If you want it to happen within a single human lifespan.

If the rocket equation still applies (if a practical reactionless drive has not been invented), then you're talking about mass-of-the-galaxy size spaceships to accelerate at 1G for years on end.

The linked paper describes a laser propulsion system by means of which no reaction mass need be stored on the spacecraft whatsoever.

Which is impractical because you don't have a way to slow down at the end. Their big plan was to accelerate a camera up to 1/4c and have it snap pictures as it whizzes past the solar system.

It's also far less than 1G acceleration. You don't get big acceleration figures like 1G from laser pumped solar sails.

>"If we could solve the energy problem"

Energy is a fundamental currency of the universe, up to heat death. It's not something to be casually 'solved'.

We are more likely to 'solve' hibernation, aging and death, and then we can travel to the stars with ease - current tech can get us there in a thousands years, and that isn't much in the grand scheme of things.

Current tech is not durable or reliable enough to last anywhere near thousands of years without frequent maintenance and a supply of replacement parts. Even simple things like analog electric circuits with no moving parts degrade over time.

Laws of physics place hard limit on your speed, they don't do that for lifespan of your engine. It's an fundamentally easier category of problem. We have wooden ships that are hindreds of years old, and space doesnt have corrosion, erosion, saltwater, etc.

Also you wouldn't send one guy in a box without spare parts, would you?

> Laws of physics place hard limit on your speed, they don't do that for lifespan of your engine.

That cannot be true. Nothing escapes the laws of physics. It obviously depends on the engine, but for example current electronics have a lifespan of only a few decades. Even the half-life of DNA might be a problem, depending on the hibernation method.

Can someone do the math to calculate the energy needed for a ship like this to accelerate near the speed of light? I suspect the amount of energy may exceed what's available even if we were to capture all energy generated from the sun.

It's not even the energy that is a problem, it is the mass. The rocket equation is a harsh mistress.

But more energy is the only thing that is needed when mass is increased, so not sure why it wouldn't all boil down to energy?

Unless you're talking about escaping Earth specific gravitation.

Are you talking about kicking your reactant so hard that it picks up significant amounts of relativistic mass?

Regardless of what you use for power a rocket has to eject mass from the rear to propel you forward. Newton's second law.

The problem is that as you increase burn times (like years at 1G), you need to increase the amount of mass you are pushing out the back. But to do this you increase the mass of your rocket which means you need to push even more mass out and it becomes a vicious circle of exponentially increasing rocket mass.

The idea of accelerating at 1G for the entire trip only works if you have a source of propulsion that doesn't obey Newton's second law. Otherwise there literally isn't enough mass in the galaxy to make it work. The caveat being that if you can somehow exploit relativistic mass to effectively multiply your fuel it might be possible, but my math isn't good enough to work it out. My gut feeling is that it would require an unreasonable amount of power however, even for an extremely powerful nuclear power plant.

Haha no but I'm not assuming the rocket equation is applied in Earth's gravitation, but maybe the spacecraft could be built far enough from any plants to be impacted by an initial gravitational field.

If nuclear energy is used, it can be increased in less than linear change in mass.

> a lot of the detrimental effects of long-term space travel are eliminated

Apart from the fact that everyone you know is now dead and if you returned home you'd basically be a living caveman museum piece.

It's obviously a one way trip, no one would expect to return home (at least, not to the same home that they left, in 2000 years, human civilization may not even exist when they return, they could return to a planet colonized by Apes... hey, they would make a great movie!).

But one-way trips have been proposed for planetary colonization as well, and there are a lot of people that would be willing to make that trip.

Time traveling and seeing what Earth looks like 1000 years from now by spending 30 years in a metal can (and expending an astronomical amount of energy) is a trade I would consider.

Just solve the aging problem first. That seems to be pretty close at hand anyhow.

It seems to me the most likely scenario is that we perfect the ability to copy minds into an electronic computer system. Such a digital mind would no longer need most of the resources a human body would and could potentially last nigh-indefinitely. It would also be able to take advantage of both >1G (no flabby meat body) and <1G (no aging) constant accelerations. Further, a colonization effort by digital beings would only require raw resources to build more electronics and starlight for power, which expands the number of habitable places from "planets with human survivable conditions" to "basically anywhere near a star".

It seems trivializing and like an existential crisis to have all of a humans being encoded in a computer. Once you have that, why even keep it running for all time as it is? Its just a version of software you could make changes to. We are probably not as glorious as we could be to become immortal as we are.

Why does this seem ‘most likely’ to you? I don’t see current tech that points this way at all.

We don’t even have broad agreement on what a mind is beyond the the everyday folk notion. What are we to copy?

Are there any software, hardware or cloud providers currently existing that you would trust to run your consciousness?

I am not convinced there is any solution to aging. When I was a child, I read headlines that claimed a solution was close at hand. We seem only marginally closer to eliminating aging.

Importantly, how many memories do you think can fit into a human brain? If a 1,000 year old man cannot remember all of his life, has aging been solved?

Does one need to remember all of their life to be happy?

Maybe the details just get spottier with bigger gaps, but you still remember the big events and when you need more details your Google neural implant tells you more. I'm far from 1,000 but I can't remember every detail of my life, but I don't think it interferes with my quality of life.

No, one does not need a perfect memory to be happy.

But I’m not convinced that somehow expanding brain sizes to accommodate immortality means we have solved aging for humans.

I think we’ll probably be having subspecies of humanity like Homo Sapiens Destinationshipnamehullnumberius

Getting there is "easy". But if the wafer is speeding at c/4 past the exoplanet, it will not be able to take any measurements.

4.2 Braking to Enter Orbit on Arrival - A very difficult challenge is to slow the spacecraft to typical planetary orbital speeds to enable orbital capture once arriving. This task is difficult as the initial entry speeds are so high (~ c) and the orbital speeds are so low (~ 10-4 c). Dissipating this much energy is challenging. We have considered using the stars photon pressure, the stellar wind (assuming it is like our own solar system), using the magnetic coupling to the exo solar system plasma. None of these techniques appears to be obviously able to accomplish this task and much more work and simulation is needed. A simple fly-by mission is clearly the first type of mission to explore in any case to assess the environment in a given system to design (if possible) an optimized braking strategy.

The big problem with near-light speed travel like the one suggested by Lubin is the potentially fatal interaction of the spacecraft with interstellar dust. Even considering their faint density, the long journey would make the spacecraft interact with far too many high-speed dust particles, which would likely cause significant damage.

I do not know if somebody devised a solution to this fundamental problem; if not, I am extremely skeptical about highly-accelerated spacecrafts.

People really don’t get this, unfortunately.

I always think that if f=ma, then as velocity approaches light speed, the deceleration of the grain of sand as it hits the hull approaches infinity. So even with a small mass, the force is huge.

Yes, being a high-speed particle particle you should use relativistic momentum to describe the hit, but the idea is the same.

The usual sci-fi novel approach is to construct a thick ice shield on the bow of the ship. Not sure if that would really work.

Send a few thousand ships to clear the way and find the safest path first?

Unfortunately there is no safest path. In free space, like in my home, dust is everywhere.

I love the idea of the wafersats

> Thus a 1 kg spacecraft going at 0.3 c will have an effective "yield" of 1 MT or roughly that of a large strategic thermonuclear weapon.

Lets not accidentally wipe out a neighbors city and start an interstellar war.

A tungsten rod placed in an eliptical polar orbit can be pointed to any point on earth fired without anyone detecting it, and have total plausible deniability when wiping a city off a planet.

We're far more likely to have destroyed ourselves long before we get a chance to destroy any other civilisation. If we can travel intersteller, we can destroy a planet.

Your quote does remind me of a line from Independence Day I like

"Los Angelinos are asked not to fire their guns at the visiting spacecraft, you may inadvertently start an intersteller war"

Any fractional-c attack will be either impossible to detect, or impossible to prevent....

Related: https://hub.jhu.edu/magazine/2021/spring/apl-interstellar-pr...

(Disclaimer, I work at APL, but not on this)

Little toy tool I made to sim out various known propulsion systems for a crewed interstellar mission: https://redskyforge.com/interstellar/

Only nuclear is really interesting. Chemical rockets are useless at this scale.

Beamed propulsion of some sort (not necessarily using lasers) relying primarily on solar energy is the best option. Nuclear is useful for ancillary propulsion. Chemical not useful except for launching all the stuff around the solar system to build such a system.

But how do you stop?

Braking against the interstellar medium (and possibly the stellar wind as you get closer) using magnetosail-like devices… between the stars isn’t completely empty but has a diffuse plasma that can be pushed against magnetically or electrostatically. This is how you’d slow down in pretty much any scenario as it is “propellantless.” It’s a big challenge to get the superconducting coils light enough, but it is a critical piece to interstellar flight, particularly if crewed.

I actually ran numbers on electrosails to decelerate, they do not work, at all - many orders of magnitude. Email me for spreadsheet. Pulsed nuclear propulsion is the only tech we have that works

Electrosails aren’t as good for this as magnetosails; I just included electrosails for completeness.

Could a black hole be used?

There is apparently a BH with 3x solar mass about 1,500LY away - escape velocity at 830 radius (about 820km from event horizon) is about 0.1c.

Well... I like the beamed energy idea. It works to get you out of the solar system. But how do you continue to accelerate from other stars? You fire ahead of you a construction kit for a solar-powered beam-generating station. That beam station can be used either for acceleration or for braking.

But how do you get the kit to decelerate? (Remember, you fired it ahead of the ship.) And this problem gets worse as the velocity of the ship goes up. If, say, your construction kit enters a solar system at 0.1 c, it's going to cross it in maybe twenty days. I don't think that's enough time for a solar sail to effectively decelerate it. It's probably going too fast for using planets' gravity to slow it down. And using atmospheric drag is an explosively bad idea.

I don't have an answer...

Your braking laser kit is fired out 1000 years ago.... and you hope it makes it there on time

A lot of this sounds like the Breakthrough Starshot initiative ideas. I could not find out if the author affiliated with said initiative.


Let me correct myself. Phil Lubin (the author) is on the Starshot Advisory Committee. He is so listed in the Bidder Briefing slides for the Starshot Sail RFP.

One of my favorite ideas is having a pair of mirrors, one on the spaceship and another on Earth. Then shine a laser on the ship, it will bounce back and forth, and eventually all the laser's energy will go to speeding up the ship. (Well, the beam will get wider, but you could probably get thousands of bounces.)

A funny twist on that idea just came to my mind: if you're flying past a black hole, having just one mirror on the spaceship is enough. Shine a laser from the ship toward the edge of the black hole just right, it will curve around and come back to your mirror, then reflect and come back again, and so on. Needs some tricky positioning of the mirror, but in principle it lets you "push" off the black hole without using rocketry.

> and eventually all the laser's energy will go to speeding up the ship.

Half to the ship, and half to the Earth in the other direction, right?

Oh. Of course you're right.

sounds a bit like Starshot project funded by Yuri Milner https://en.wikipedia.org/wiki/Breakthrough_Starshot

"The Starshot concept capable of making the journey to the Alpha Centauri star system 4.37 light-years away. It envisions launching a "mothership" carrying about a thousand tiny spacecraft (on the scale of centimeters) to a high-altitude Earth orbit for deployment. A phased array of ground-based lasers would then focus a light beam on the crafts' sails to accelerate them one by one to the target speed. At a speed between 15% and 20% of the speed of light, it would take between twenty and thirty years to complete the journey, and approximately four years for a return message from the starship to Earth."

how does it slow itself down for entry into the alpha centauri system? seems like it would just blow past it

It doesn't. It snaps some pics, sends them back and moves on.

Actually, the Wikipedia page for the project notes in one of the subsections:

> The travel times are for the spacecraft to travel to the star and then enter orbit around the star (using photon pressure in maneuvers similar to aerobraking).

Not sure how trustworthy that information is but this kind of "aerobraking" at least seems possible in principle.

To be honest this is one of the more real proposals I've seen for an Interstellar probe, and there are still many problems to be worked out as both the paper and the comments here point out.

I admire the optimism in the paper though, even if its misplaced. Maybe pooling together resources in the scientific and engineering communities to develop an interstellar probe system could give a working result, but there are some significant hurdles to overcome first. It would be a big project, taking many years to see fruition, but at the same time it could also be a game-changing development and a hugely inspiring feat.

Could we see probes in other star systems by 2075? I think its not an unreasonable suggestion.

Interstellar flight is an insane challenge. It’s one of the hardest things humankind can attempt yet still not beyond the laws of physics. That means you have to approach things from first principles and often have to invert the problem (“assume we manage interstellar flight by 2075. How did we do it?”, etc). It’s intellectually seductive in the best way.

I think it’s good to attempt such challenges, even if just conceptually, as it makes lots of other things seem a lot more achievable in contrast, sometimes almost laughably so.

I've always thought interstellar probes were kind of pointless. Even if the probe could average 10% of the speed of light, the amount of time it would take for a probe to reach its destination (all but the closest stars) and then for data to return at the speed of light is greater than the lifetime of anyone who wants to study. You'd have to send out the probe, hoping that your grandchildren wait around for the response.

Interstellar space exploration will likely be manned-only. As other threads point out, with sufficient acceleration, due to time dilation, humans can reach distant stars within their own lifetimes (at the expense of thousands of years passing on Earth).

Disagree. At 0.1c, it takes 42 years to reach Proxima Centauri (and 4 more years to get data back). The Voyager 1 and 2 probes are still operating and transmitting scientific data that people are interested in (the missions are still funded) and were launched 44 years ago and are likely to continue a few more years. So we have existence proof that people would still be interested in it over that kind of timeline.

And I know you excluded the “closest stars,” but I don’t understand why, except that it undermines your point. There are at least 3 nearby stars that could be reached by probes in a researcher’s (or at least their funding institution’s) lifetime vs just our one star system.

(Besides the 3 stars in the Proxima/Alpha Centauri system there are also at least 11 additional stars—including fusion-capable brown dwarfs—within 10 light years of the Sun.)

Humans have planned and executed multi-generational projects before, even if that scale of project seems out of fashion right now.

This article (Starships & Cathedrals) explains the ideas better than I can in a short comment: https://www.centauri-dreams.org/2020/07/17/the-cathedral-and...

I disagree. It the same principle as sending something to the outer solar system. Many of the scientist who propose those things and work on the instruments and so on have long left the field by the time the missions arrive and many will soon after. Before the mission ends, a whole new group of people are working on it.

This is the same idea, you send a new an improved probe every 10-20 years and eventually you get data back and over time you have constant data stream from all kinds of different places.

> You'd have to send out the probe, hoping that your grandchildren wait around for the response.

Why would our grandchildren be less interested in interstellar exploration than we are? We need ways to organize (parts of) society for the long term, that seems way easier than inventing magical "accelerate at 1g for a decade" tech.

maybe a reason similar to whatever explains the drop in oceanic exploration among the peoples of south-east asia and polynesia several thousand years ago?

Doesn't look so bad once a program is established. It's like aging cheese - get one batch started while another is finishing up. The turnover time is just 50 years instead of 2.

Propulsion aside, I really wonder what the human species turns out to be like in say a couple of hundred years.

Hopefully our solar system isn't the only place where human DNA is found; that said, I'm curious what'll be left of humans as we know today.

With tools like CRISPR accelerating adoption to new environments and increased resilience towards hostile situations (zero-g, radiation etc)

Unless we embrace nuclear energy for space flight this would never happen.

Looks like NASA is developing thermal nuclear rocket https://www.nasa.gov/directorates/spacetech/nuclear-propulsi...

Thermal fusion rockets, meanwhile, languish. The Princeton group got some money, but under current plans cannot hope to fly a test vehicle before 2035. Yet another victim of the Tokamak mirage.

Field-reversed configuration, D-H3, aneutronic.

I think the best chance for large projects is a fleet of drone factories that can replicate themselves and extract resources from asteroids. Millions of tonnes of fuel and other resources may not be a huge problem if you factor geometric growth of population of drones.

The benefits on earth would be pretty significant too. Any drone swarm sophisticated enough to do this, given inputs it could custom order, could set up productive farming on any reasonable plot of land on earth with a few exceptions (e.g glaciers).

The exoplanet mission from Civilization 6 in real life - what a fascinating read!

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