The Limitations of Intergalactic Travel

[Ryan_m_b]

Assuming that the human mind is simply a manifestation of the laws of physics and not something supernatural, it’s just a matter of time before we can reduce it’s contents into digital information. Once that happens, the idea of moving around large masses through space (living or inanimate) will be obsolete. All you’ll need to do is send some sort of “seed” assembly plant to a distant planet, and the rest of what you need, including ourselves, we “transport” by radio.

Again, assuming that our minds are not outside of the laws of the universe, this IS NOT science fiction, but simply a matter of time AND the most practical solution.

If we do indeed have some sort of “soul” outside this plane of existence, then it’s just a matter of time before we somehow utilize that plane to travel to the stars :)

Either way, we’re going!

I strongly object to the notion that it is "simply a matter of time", especially when you make an assumption. When it comes to mind uploading I am disturbed by how prothetic and religious people become about it.

Now I do agree that the mind is an emergent property of the brain and that in principle it should be possible to replicate this through computation, even if it is revealed that human psychology is contingent on an exact replica of a brain (that includes a simulation of an exact replica) which in turn is contingent on an exact replica of a human body which in turn again is contingent on an exact replica of a habitable environment.

If we ever determine how the brain gives rise to mind
If we ever determine what the fundamental components of a brain that give rise to mind are
If we ever develop instruments capable of scanning at a resolution of these fundamental components
If we ever invent methods to simulate this process on a different substrate
If we ever build a substrate good enough to run those simulations

then yes we could in principle mind upload but it is mightily foolish to make an assumption that these things are non-trivial and will be about in "a matter of time". You're making an assumption that there will be no undiscovered show stoppers.

You also get to the problem of how do transport the necessary infrastructure to your target destination? Sure a robot body might be more durable than a human one and require less support (i.e. an ecosystem) but how do you get it to your destination? How do you transport the necessary industry?

The only problems mind-uploading solves is to do away with the need for a biosphere and to speed up transport once a base is established at a destination.

 

[Lsos]

Obviously any forecast into the future comes with assumptions, and nobody is in any position to argue what will happen in a hundred or 500 or 1000 years.

I'm basing my assumptions on the exponential speed that human progress is moving at, and on the fact that making a human mind IS possible...after all, it's already been done.

I do realize that how hard it will be to replicate and copy at will is another story altogether...

As for transporting the infrastructure, I believe by the time we jump over the mind-uploading issue (IF we jump it) then this will be the smallest problem.The technology to move large masses at ridiculous speed has already existed for decades, except the will and the economic incentive has been lacking.

 

[Ryan_m_b]

Obviously any forecast into the future comes with assumptions, and nobody is in any position to argue what will happen in a hundred or 500 or 1000 years.

Yet you are trying to do that

I'm basing my assumptions on the exponential speed that human progress is moving at, and on the fact that making a human mind IS possible...after all, it's already been done.

Contrary to popular (*cough Kurzweil) belief if you claim that technology changes exponentially you actually have to provide some evidence that this is actually occurring.

As for transporting the infrastructure, I believe by the time we jump over the mind-uploading issue (IF we jump it) then this will be the smallest problem.The technology to move large masses at ridiculous speed has already existed for decades, except the will and the economic incentive has been lacking.

What technology? Interplanetary propulsion hasn't even been viably invented yet let alone interstellar! If you are going to make claims you are going to have to back them up with evidence. This is a science forum and using the caveat "I believe" isn't good enough

 

[JaredJames]

I'm basing my assumptions on the exponential speed that human progress is moving at

That doesn't mean anything.

As for transporting the infrastructure, I believe by the time we jump over the mind-uploading issue (IF we jump it) then this will be the smallest problem.

You've taken two completely separate technologies and used them to judge the advancement of each other. This doesn't work. Even if we managed to upload a mind, that has absolutely no bearing on space travel and certainly doesn't make it "the smallest problem".

The technology to move large masses at ridiculous speed has already existed for decades

Has it really? Perhaps you'd care to share it with the rest of humanity?

 

[Zentrails]

Who get's to control this interstellar death ray? You're talking about terrawatts of power constantly focused on one little point. Beyond that it would be almost impossible for the ship to cool down, it isn't going to a perfect mirror so it is going to absorb some of the energy. It's not going to be able to radiate that heat fast enough if it's constantly being hit by a laser.

The ship itself is going to have to be tiny which is a problem, manned interstellar missions would require a habitat capable of carrying a fully sustainable ecosystem as well as the millions of people required to supply all the specialised labour a modern civilisation needs.

Designing a hypothetical 1kg Starwisp is a huge challenge, let alone some sort of manned habitat.

Yep, that would be correct for a narrow beam, but most of the solar sail designs I've seen call for a solar sail that is hundreds of miles in diameter and a beam that is much larger in diameter than that. They usually suggest the beam be generated by some kind of gigantic laser.

The biggest problem with that, IMO, is how do you attack the sail to the ship without using cables that weigh more than the ship itself?

If the beam were approximately the apparent size the sun is from our earthly vantage point, sure it would heat the heck out of anything trying to use it and wouldn't be practical for solar sail use.

I'm more interested in what such a beam would look like if it were beaming right at us from an alien solar system.
I'm guessing it would probably look like a nebula, assuming it is not laser generated light.

 

[Zentrails]

Has it really? Perhaps you'd care to share it with the rest of humanity?

Yeah, I could use some help moving my piano. LOL

 

[Ryan_m_b]

Yep, that would be correct for a narrow beam, but most of the solar sail designs I've seen call for a solar sail that is hundreds of miles in diameter and a beam that is much larger in diameter than that. They usually suggest the beam be generated by some kind of gigantic laser.

The biggest problem with that, IMO, is how do you attack the sail to the ship without using cables that weigh more than the ship itself?

If the beam were approximately the apparent size the sun is from our earthly vantage point, sure it would heat the heck out of anything trying to use it and wouldn't be practical for solar sail use.

I'm more interested in what such a beam would look like if it were beaming right at us from an alien solar system.
I'm guessing it would probably look like a nebula, assuming it is not laser generated light.

Even if the sail is hundreds/thousands of km2 it is still going to need a fantastic amount of energy focused on it, terrawatts of it. Shine that on a planet and you can burn cities away one by one.

Heating problem still applies here, the sail cannot radiate the heat away from it so it will just cook over time.

I've yet to see any paper discussing how solar sails can be used for interstellar transport that do not hypothesise starwisp designs.

 

[Lsos]

Yet you are trying to do that

I got the impression that forecasting the future is pretty much what this thread is about, so I chimed in with my vision. I do recognize that perhaps I did overstep the boundary of how much we're allowed to speculate. My apologies.

What technology? Interplanetary propulsion hasn't even been viably invented yet let alone interstellar! If you are going to make claims you are going to have to back them up with evidence. This is a science forum and using the caveat "I believe" isn't good enough

I was talking about nuclear pulse propulsion, which was seriously being considered in the 50s and 60s. Due to the violent nature of this type of propulsion, instead of being a disadvantage, a large mass was actually a design necessity. The main reason why this method of propulsion was dropped was not an engineering, but a political one. This I'm pretty sure is within the scope of this discussion.

 

[JaredJames]

I was talking about nuclear pulse propulsion, which was seriously being considered in the 50s and 60s. Due to the violent nature of this type of propulsion, instead of being a disadvantage, a large mass was actually a design necessity. The main reason why this method of propulsion was dropped was not an engineering, but a political one. This I'm pretty sure is within the scope of this discussion.

I assume you've calculated how much nuclear fuel you'd have to carry to accelerate to a high speed (0.5c+)? I also assume you've factored in how much fuel you'd have to carry to accelerate the fuel itself?

Here's a hint, it's not as simple as people here seem to like making out when they drop this particular technology into the discussion.

Like I said, I believe ryan has pointed out on numerous occasions how much matter/antimatter it would take (that being one of the best sources) and that's not some reasonable amount.

 

[Ryan_m_b]

Like I said, I believe ryan has pointed out on numerous occasions how much matter/antimatter it would take (that being one of the best sources) and that's not some reasonable amount.

It does seem like I repeat myself on these space threads doesn't it? So once more, quoting myself...

It always boggles my mind when people express opinions suggesting that NASA worked out space travel decades ago and that all it would require is some investment and a bit of polishing off and we'll be skipping around the galaxy like true space cadets. Space travel is hard.

Antimatter/matter propulsion has the highest specific impulse that we know of. With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond. That means the ship can thrust at 1g for roughly 10 and a half days reaching a speed of ~10,000,000 mps which is 3.3% of the speed of light. To get to near 100% you would need thirty times this but remember you need to decelerate at the other end, that gives you a 60:1 ratio of fuel to ship if we use Am/M. Now Project Orion proposed using nuclear bombs but these can only match Am/M if the following few hypothetical were met;

The entire mass fissile material is converted to energy
-- It isnt, of all the uranium only ~2% undergoes fission. Of this only a half of a percent is converted to energy. Little boy, the Hiroshima bomb, contained over 60kg of uranium but only a penny's worth converted to energy. This means you need to pump up that ratio from 6:1 to 6,000-60,000:1

The bomb's mass is entirely fissile material
-- It isnt, most of the bomb is casing/primer etc. I can't find the exact figures with a brief google but it would be reasonable to assume that only 1-10% of the bomb is actually fissile. this pushes the ratio further to 60,000-600,000:1

The whole energy of the explosion hits the back of the ship
-- It won't, for a 1,400miles3 ship if we make it a cube that makes a ship ~11 miles on the side with each face 121miles2. If the explosion occurs 30 miles from the ship (about the recommended for Orion) then only 0.4% of the energy will hit the ship (the energy radiates as a sphere, the ship obscures a small part of this). This again pushes the ratio to 1,500,000-15,000,000:1

Aside from the horrendous fuel requirements there's a tendency for people to assume that all the other issues are just minor details when in actual fact all areas of space colonisation are extremely non-trivial. For an interstellar colony ship you need to;

Create a sustainable biosphere for the ship
--We have very little idea how complex ecologies work here on Earth let alone how to recreate one that is immune from ecological disaster.

Create an environment capable of growing food
--Same problem as above yet with the added problem of a ship biosphere being a small closed system. In addition a wide diversity of foods combined with the appropriate bacteria to fill up our guts (which contain 1kg of vital gut flora).

Pack a fully capable industrial system into a colony ship
--Many industrial complexes run over tens of km, add up all the wide variety of industries across the world plus the infrastructure and put it all in one place. In addition you need to redesign all of it to have near 100% recyclable capability (remember that closed system?)

Pack a fully capable work force
--In today's high-tech and diverse society there are literally 10s-100s of thousands of different specialities. Provide enough people in the profession to staff each job plus enough to train the next generation and the total number of people in the workforce? You're looking at a figure measured in the 10s-100s of millions of people

Design a long-term stable socio-economic system
-- Societies on Earth don't exactly have a track record of long term-stability. An interstellar trip could take 100s-1000s of years. The vehicle isn't going to be analogous to a captain and his crew, it's more like rolling up an entire country's population building a wall around it and then sending it off alone. Remember a single failing point and the whole mission is gone

Solve all of those problems without invoking magic wands of super-nanotech, AI and robots and then you can play space cadet.

Sorry for the long rant but it's a pet peeve of mine when people blindly assume that manned space exploration/space colonisation is easy then pretentiously claim that it's only reason X that we can't do it.

 

[Zentrails]

Even if the sail is hundreds/thousands of km2 it is still going to need a fantastic amount of energy focused on it, terrawatts of it. Shine that on a planet and you can burn cities away one by one.

Heating problem still applies here, the sail cannot radiate the heat away from it so it will just cook over time.

I've yet to see any paper discussing how solar sails can be used for interstellar transport that do not hypothesise starwisp designs.

It's my understanding that solar sails have already been experimented with on a small scale in space (described as "partially successful") and larger solar sail experiments are in the planning stages. Not for interstellar travel, but for travel to the outer planets.

Solar sails rely on conservation of momentum, so they don't necessarily need to absorb a photon to work. It's better to use a highly reflective surface, which won't heat up as much.

The sail will certainly radiate heat away like any black body radiator. As the object gets hotter, more total energy is emitted. The question of heating up depends on whether the absorbed heat is greater than the emitted heat. I'm sure an equilibrium would be reached at a relatively low temperature, depending on the intensity of the beam, of course, otherwise the solar sail would have to be designed by a pretty lousy engineer.

Wiki has a pretty good summary of the pros and cons of solar sails:
https://secure.wikimedia.org/wikipedia/en/wiki/Solar_sail

It's probably our best option considering only today's technology for any possible interstellar travel (unmanned with exceedingly small cargo weights) assuming the Orion Propulsion system is never pursued.

 

[Cosmo Novice]

It does seem like I repeat myself on these space threads doesn't it? So once more, quoting myself...

Can I just say that rant was brilliant. It was great to read something I have been thinking for a long time when I hear these arguments.

I understand that without the magic of a "super force field and FTL drive" that space exploration on an interstellar scale may be impossible - now or in any future. Due to the many points you mentioned.

It may be that such technologies will never exist to this capability and I do not agree with the whole technological development exponentiality. The other side to that coin is that eventually the scientific developments humanity makes may be merely refinements of existing knowledge, in fact to me that stands more to reason.

As a side thought, I have always thought this is the main theoretical refutation to the Fermi Paradox, with the sideline being the assumption of radio technologies and assumed detection - assuming cosmic timescales and probably limited use, not to mention algorithms and encryption and CMB in all directions.

Thanks for the laugh and insight into the fuel consumption numbers.

 

[Ryan_m_b]

It's my understanding that solar sails have already been experimented with on a small scale in space (described as "partially successful") and larger solar sail experiments are in the planning stages. Not for interstellar travel, but for travel to the outer planets.

Solar sails rely on conservation of momentum, so they don't necessarily need to absorb a photon to work. It's better to use a highly reflective surface, which won't heat up as much.

The sail will certainly radiate heat away like any black body radiator. As the object gets hotter, more total energy is emitted. The question of heating up depends on whether the absorbed heat is greater than the emitted heat. I'm sure an equilibrium would be reached at a relatively low temperature, depending on the intensity of the beam, of course, otherwise the solar sail would have to be designed by a pretty lousy engineer.

Wiki has a pretty good summary of the pros and cons of solar sails:
https://secure.wikimedia.org/wikipedia/en/wiki/Solar_sail

It's probably our best option considering only today's technology for any possible interstellar travel (unmanned with exceedingly small cargo weights) assuming the Orion Propulsion system is never pursued.

With regard to your Project Orion claim I refer you to my above post.

IIRC Solar sails are potentially good for either supplementing normal propulsion on a probe or good for very tiny masses. The effect you allude to whereby the sail must receive less energy than it can radiate is why solar sails would be terrible mechanism for transporting anything above a starwisp and even then there are horrendous engineering difficulties. Quoting from the wiki article you linked the most efficient solar sail design yet comes from Drexler whose thesis states his design would only be 50-80 times better than existing, not a huge difference when you consider that simply enlarging the sail would produce the same effect.

Laser powered sail probes may be good when we want to send a few grams to the next system at both horrific expense and severe danger (a gigawatt laser with interstellar range pointed at your town is enough to ruin anyone's day) but they are not a practical solution to manned space exploration.

Can I just say that rant was brilliant. It was great to read something I have been thinking for a long time when I hear these arguments.

I understand that without the magic of a "super force field and FTL drive" that space exploration on an interstellar scale may be impossible - now or in any future. Due to the many points you mentioned.

It may be that such technologies will never exist to this capability and I do not agree with the whole technological development exponentiality. The other side to that coin is that eventually the scientific developments humanity makes may be merely refinements of existing knowledge, in fact to me that stands more to reason.

As a side thought, I have always thought this is the main theoretical refutation to the Fermi Paradox, with the sideline being the assumption of radio technologies and assumed detection - assuming cosmic timescales and probably limited use, not to mention algorithms and encryption and CMB in all directions.

Thanks for the laugh and insight into the fuel consumption numbers.

No problem it's a pet peeve of mine. I find it fascinating to discuss space travel more so when we stick to reality than when we start with the proposition "if we had magic technology X". It clashes with my other pet peeve, Ray Kurzweil and his exponential change arguments. It really is disturbing when intelligent people propose that we will have an extraordinary technology by X year and base their reasoning on, what is essentially, Moore's law. It really is staggering.

As for the Fermi paradox it always struck me that there are three conclusions;

1)We lack the capability to detect interstellar civlisations
2)No other tool using species has evolved in our galaxy
3)There is something prohibitive about interstellar travel

Option one is impossible to falsify without discovering a civlisation so we can leave it out, option two is interesting because it points to the rarity of such phenomenon. Option three is the only one we can actually investigate and it really is fascinating. Aside from the huge engineering hurdles to overcome there's also the little matter of the devastating potential of an interstellar war fought with relativistic weapons...

 

[Cosmo Novice]

As for the Fermi paradox it always struck me that there are three conclusions;

1)We lack the capability to detect interstellar civlisations
2)No other tool using species has evolved in our galaxy
3)There is something prohibitive about interstellar travel

Option one is impossible to falsify without discovering a civlisation so we can leave it out, option two is interesting because it points to the rarity of such phenomenon. Option three is the only one we can actually investigate and it really is fascinating. Aside from the huge engineering hurdles to overcome there's also the little matter of the devastating potential of an interstellar war fought with relativistic weapons...

Couldnt agree more.

1) Probably true.
3) Probably resource/technology/total non human rational that does not envisage space travel (this is not assuming a primitive race) etc etc..
2) *Negated by 1 and 3.

*Although I think given the vastness of our galaxy their are probably pockets of life, a very small percentage per star, and then a small percentage of those whose environment is suitable for multi-cellular development, and then a very small percentage of those developing into societal analogues, then developing similar radio technology to humans and probably (on comsological timescales) existing only for a small amount of time.

Well to be fair the dinosaurs did OK. Personally I think they will prove to be more the more successful species on a history of our planet in the far future. All highly speculative of course :)

 

[Zentrails]

With regard to your Project Orion claim I refer you to my above post.

IIRC Solar sails are potentially good for either supplementing normal propulsion on a probe or good for very tiny masses. The effect you allude to whereby the sail must receive less energy than it can radiate is why solar sails would be terrible mechanism for transporting anything above a starwisp and even then there are horrendous engineering difficulties. Quoting from the wiki article you linked the most efficient solar sail design yet comes from Drexler whose thesis states his design would only be 50-80 times better than existing, not a huge difference when you consider that simply enlarging the sail would produce the same effect.

Laser powered sail probes may be good when we want to send a few grams to the next system at both horrific expense and severe danger (a gigawatt laser with interstellar range pointed at your town is enough to ruin anyone's day) but they are not a practical solution to manned space exploration.

No problem it's a pet peeve of mine. I find it fascinating to discuss space travel more so when we stick to reality than when we start with the proposition "if we had magic technology X". It clashes with my other pet peeve, Ray Kurzweil and his exponential change arguments. It really is disturbing when intelligent people propose that we will have an extraordinary technology by X year and base their reasoning on, what is essentially, Moore's law. It really is staggering.

As for the Fermi paradox it always struck me that there are three conclusions;

1)We lack the capability to detect interstellar civlisations
2)No other tool using species has evolved in our galaxy
3)There is something prohibitive about interstellar travel

Option one is impossible to falsify without discovering a civlisation so we can leave it out, option two is interesting because it points to the rarity of such phenomenon. Option three is the only one we can actually investigate and it really is fascinating. Aside from the huge engineering hurdles to overcome there's also the little matter of the devastating potential of an interstellar war fought with relativistic weapons...

I agree with you about the looney Orion Propulsion idea, although it resulted in a damn good book written by Freeman Dyson's tree-house living, canoe-building son.

Solar sails are a very tough engineering problem, but at least they would get a constant acceleration from a reasonably non-divergent light beam. I think the heat problem would be insignificant as long as it doesn't melt the sail. The heat would radiate out in both directions, so the net momentum changes would cancel out.

What I envision is a space based extremely powerful EM rail gun for initial propulsion, with the solar sails unfolding and used after that. Like you, I don't think it is possible, with today's technology, to use it with anything but very small, unmanned probes.

Enlarging the sail would require more (and longer) tether lines, so there would be diminishing returns from that. If the ship has a supplementary on board propulsion system that would require the tether lines to be non-compressible as well as having good tensile properties.

That's also something that would be required by a "space elevator" cable. You won't get that using carbon nano-tubes as the cable raw material, which seems to be the consensus material of choice by "space elevator" believers.

Your 3 conclusions are all negatives and negatives cannot be proven, but are probably all true, but I would suggest that:

1) We probably can detect an alien civilization, but only if that alien species is purposely trying to contact us by sending a signal they know we will detect.

2) There may be intelligent species in our solar system that don't require tools and/or could be so different from us that we don't even recognize them as living beings.

3) Probably true, but that may only apply to manned interstellar travel.

There is one more possible way to achieve manned interstellar travel, that I've thought of:

Use spacecraft made of huge chunks of ice that feed water into a nuclear reactor heat exchanger, heating the water to insane temperatures, then letting it blow out of a rocket-like nozzle.

In order to reduce the amount of nuclear fuel you would need for the manned craft, you could first launch many similar, but much smaller unmanned craft with a nuclear propulsion module, giving them an initial velocity using space based EM rail guns.

You have them programed to burn just enough fuel to get to a particular velocity, then shut everything down. The unmanned crafts would have no radiation shielding and just enough electronics to control the reactor and navigation system.

After all the unmanned crafts have launched, launch a manned craft using the same rail guns to get the thing going initially, but unlike the unmanned craft, at accelerations that humans can survive.

Then, the manned craft "catches up" with each unmanned craft one by one, with the unmanned craft accelerating to make the manned craft's velocity. Then the manned vehicle rendezvouses with the unmanned craft and scavenges the remaining fuel and ice from them - or maybe just switch reactor modules.
It would still have to go significantly slower than the speed of light to keep the crew alive.

The crew would have to be in suspended animation for most of the voyage.
There have been some animal studies that suggest you can do that using H2S, IIRC.

I agree with you about Ray Kurzweil, although he seems to be right about a lot of things.
He has been quoted as saying that if he can live 50 more years, he expects aging to be completely eliminated, resulting in nearly limited life-spans.
(We can't get past that "heat death" thing, though, without moving to another universe.)

 

[mjacobsca]

Previous posts have discussed "mind uploading". I think it is more likely that we will send cryogenically frozen embryos across the vastness of space, and "grow" them when they reach a suitable destination, then feed them knowledge from endless video feeds when they wake. We'll have to keep them asleep while they grow, except for language lessons. We already freeze embryos today, so I see no reason why this couldn't be done, in theory. We'd have to protect the biological organisms from harmful radiation during the journey, we'd have to supply them with seedlings to grow food, and we'd have to build robots that could do construction work to build habitats and start crop growth. There are nearly as many challenges here as there are in building colony ships that travel for centuries with populations on board, but when the Earth becomes uninhabitable, we might resort to anything to save our species. Personally, I'd sign up for a colony ship with a 100,000 year journey ahead of it, but that's just me.

 

[Zentrails]

Previous posts have discussed "mind uploading". I think it is more likely that we will send cryogenically frozen embryos across the vastness of space, and "grow" them when they reach a suitable destination, then feed them knowledge from endless video feeds when they wake. We'll have to keep them asleep while they grow, except for language lessons. We already freeze embryos today, so I see no reason why this couldn't be done, in theory. We'd have to protect the biological organisms from harmful radiation during the journey, we'd have to supply them with seedlings to grow food, and we'd have to build robots that could do construction work to build habitats and start crop growth. There are nearly as many challenges here as there are in building colony ships that travel for centuries with populations on board, but when the Earth becomes uninhabitable, we might resort to anything to save our species. Personally, I'd sign up for a colony ship with a 100,000 year journey ahead of it, but that's just me.

Better yet, why not just send a DNA synthesis machine (programed to synthesize various genomes) to an alien planet that has the necessary raw materials to construct viable human DNA, then add it to embryos that had the DNA removed from the start?

That way you avoid the DNA damage that likely would occur from long term interstellar travel. You'd also have to figure out a way to do that with the mitochondrial DNA.

 

[Ryan_m_b]

The idea of a Von Neumann probe capable of building and raising humans counts as a magic technology in my books, as I said earlier it's more interesting when we leave out AI because it completely changes what we are talking about. If you posit strong AI and self replicating tools there's very little that can't be done.

As for cryogenic freezing we are still a long way off that being possible (if it is for long periods of time). Though I will admit that studies in that field are getting better, but its a world away from freezing a dog for a few hours to freezing a human for hundreds of thousands of years. Then you still have the problem of building a vehicle that can last, needing the technology to build an ecosystem from scratch, packing an industry onto the ship etc etc

 

[sophiecentaur]

Had there been an Internet at the time, I could imagine that discussions about the Philosophers' Stone would have gone along the above lines.
Would we not be fundamentally limited by human nature, at least as much as by Physics? Few governments can get voted in on the basis of projects taking more than a decade at the most. Just Who is likely to want to put off their present enjoyment in order to fund a project taking hundreds or thousands of years? And who would it all be for? Great great great grandkids?
Even Wormholes and the like are not going to let bodies through. Possibly communications though.

 

[Cosmo Novice]

The ideas of Von Neumann probes are interesting for our cosmic exploratory development. Tehnologically advanced genocidal Von Neumann probes may also be the answer to the Fermi paradox - which I find an amusing idea.

As this thread seems open to speculation I will throw in my two penneth. *IF* we are to ever actualise interstellar travel then it may be entirely necessary for technology to provide an extended degree of control on the physical Universe. Such as the ideas of advanced Alcubierre Drives, contained singularities and post physical evolution. All of which are highly speculative and may be impossible to realize.

I do not think anything we can currently develop or technically create (such as solar sails, ion propulsion etc) or even when these technologies have been refined, that they will provide realistic interstellar travel.

 

[sophiecentaur]

The VN Probe sounds a bit like inventing a new bacterium. This could evolve, all on its own, and decide to put an end to Humanity, on the grounds that we are an absolute shower and a blot on the Galaxy. Shooting ourselves in the foot or what?

 

[Ryan_m_b]

The thing is if we ever invent VN probes we would drastically change the parameters of what we are talking about.

VN probes, by definition are self-replicating machines. If we still wanted to colonise space we could send some to the moons of a gas giant and get them to dismantle all of them before using the mass to build millions of O'Neill cylinders. Into all of these cylinders we put different ecologies and study them to discover which one works best. Using this super-experiment we could crack the problem of building a sustainable environment by observing what works and what doesn't (without danger to human life or Earth's ecosystem)

There's really no need to go interstellar from that point because we can just live in millions of habitats orbiting the sun, the increased surface area allows populations so large we'd have to use standard form.

VNs seem a silly idea for space colonisation because you don't actually get to colonise anywhere, nobody leaves your planet you just make another planet full of humans. Though even if you did manage to build some sort of fantastical universal constructor capable of being packed into a small enough mass to be sent interstellar you would still have to crack the problem of designing an AI to raise the children on the other end. Children do not develop from passive media (i.e TV). They need interaction, specifically human interaction. If we ever overcome the hard problem of consciousness we might begin to see how we could go about making an AI but until then we're stuck where we are.

 

[Zentrails]

I do not think anything we can currently develop or technically create (such as solar sails, ion propulsion etc) or even when these technologies have been refined, that they will provide realistic interstellar travel.

Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.

 

[Ryan_m_b]

Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.

Very impractical. Looking at the numbers on the Spacecraft Propulsion article of wikipedia current solar sails produce 9 Newtons per thousand square metres at a distance of 1 AU (~300watts per metre). Working off of those numbers to propel a 1 tonne probe at 1g would require a sail 1km2.

As I said a suitable investment in resources might help us send interstellar probes via beamrider but no manned.

 

[Cosmo Novice]

Impractical might be a better word than unrealistic or maybe economically/politically unfeasible.

Socio and economic feasibility are generally what I was referring to. I stand by unrealistic - the reason I stand by unrealistic as opposed to impractical are that impracticalities automatically assume possibility, I refute that the technological examples I gave (Solar sail, ion propulsion etc) will realize interstellar capability. If interstaller technologies are ever theoretically proven (technologies beyond what currently exists) then I am happy to refute my comment and agree with impracticality.

Although really this is just semantics :)

 

[Ryan_m_b]

the reason I stand by unrealistic as opposed to impractical are that impracticalities automatically assume possibility

Totally agree. Something can be technologically impractical whilst still being totally unrealistic

 

[Lsos]

It always boggles my mind when people express opinions suggesting that NASA worked out space travel decades ago and that all it would require is some investment and a bit of polishing off and we'll be skipping around the galaxy like true space cadets. Space travel is hard.

Antimatter/matter propulsion has the highest specific impulse that we know of. With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond. That means the ship can thrust at 1g for roughly 10 and a half days reaching a speed of ~10,000,000 mps which is 3.3% of the speed of light. To get to near 100% you would need thirty times this but remember you need to decelerate at the other end, that gives you a 60:1 ratio of fuel to ship if we use Am/M. Now Project Orion proposed using nuclear bombs but these can only match Am/M if the following few hypothetical were met;

The entire mass fissile material is converted to energy
-- It isnt, of all the uranium only ~2% undergoes fission. Of this only a half of a percent is converted to energy. Little boy, the Hiroshima bomb, contained over 60kg of uranium but only a penny's worth converted to energy. This means you need to pump up that ratio from 6:1 to 6,000-60,000:1

The bomb's mass is entirely fissile material
-- It isnt, most of the bomb is casing/primer etc. I can't find the exact figures with a brief google but it would be reasonable to assume that only 1-10% of the bomb is actually fissile. this pushes the ratio further to 60,000-600,000:1

The whole energy of the explosion hits the back of the ship
-- It won't, for a 1,400miles3 ship if we make it a cube that makes a ship ~11 miles on the side with each face 121miles2. If the explosion occurs 30 miles from the ship (about the recommended for Orion) then only 0.4% of the energy will hit the ship (the energy radiates as a sphere, the ship obscures a small part of this). This again pushes the ratio to 1,500,000-15,000,000:1

Aside from the horrendous fuel requirements there's a tendency for people to assume that all the other issues are just minor details when in actual fact all areas of space colonisation are extremely non-trivial. For an interstellar colony ship you need to;

Create a sustainable biosphere for the ship
--We have very little idea how complex ecologies work here on Earth let alone how to recreate one that is immune from ecological disaster.

Create an environment capable of growing food
--Same problem as above yet with the added problem of a ship biosphere being a small closed system. In addition a wide diversity of foods combined with the appropriate bacteria to fill up our guts (which contain 1kg of vital gut flora).

Pack a fully capable industrial system into a colony ship
--Many industrial complexes run over tens of km, add up all the wide variety of industries across the world plus the infrastructure and put it all in one place. In addition you need to redesign all of it to have near 100% recyclable capability (remember that closed system?)

Pack a fully capable work force
--In today's high-tech and diverse society there are literally 10s-100s of thousands of different specialities. Provide enough people in the profession to staff each job plus enough to train the next generation and the total number of people in the workforce? You're looking at a figure measured in the 10s-100s of millions of people

Design a long-term stable socio-economic system
-- Societies on Earth don't exactly have a track record of long term-stability. An interstellar trip could take 100s-1000s of years. The vehicle isn't going to be analogous to a captain and his crew, it's more like rolling up an entire country's population building a wall around it and then sending it off alone. Remember a single failing point and the whole mission is gone

Solve all of those problems without invoking magic wands of super-nanotech, AI and robots and then you can play space cadet.

Sorry for the long rant but it's a pet peeve of mine when people blindly assume that manned space exploration/space colonisation is easy then pretentiously claim that it's only reason X that we can't do it.

I don’t understand how everyone can so easily dismiss the nuclear-pulse propulsion idea, especially in this thread which is obviously open to some off-the-wall concepts.

ryan_m_b, you’re the first source I have ever met that for one reason or another doesn’t accept the Orion Propulsion idea. No offence, but I’m sure you understand that to me, all the other sources are going to be more credible than you.

I am left with simply digesting the above dissertation you made. Again, no offense, but it appears like both an incomplete AND cherry-picked collection of data designed purposely to maul nuclear-pulse propulsion, but not necessarily reflect reality.

For example,

“of all the uranium only ~2% undergoes fission”. This is perhaps true for the Hiroshima bomb…the first ever bomb of its type not only used, but tested. The second bomb used was 10x more efficient, and modern bombs, boosted by fusion, are much more efficient than that.

“only 1-10% of the bomb is actually fissile”. Again this is perhaps true for the very oldest designs, but I’m sure modern ones are much better designed than that. Just a few years after Hiroshima they could make bombs two orders of magnitude lighter with the same yield.

“If the explosion occurs 30 miles from the ship (about the recommended for Orion) then only 0.4% of the energy will hit the ship (the energy radiates as a sphere, the ship obscures a small part of this).” ~30 meters was the recommended for Orion. The bombs were shaped charges which directed almost all the available energy at the pusher plate.

“With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond”. This part I can’t don’t understand because I have no idea how this alleged matter/ antimatter propulsion system is supposed to work. The only thing I can figure out is that it’s incredibly inefficient, as matter/antimatter annihilation produces enough energy which, if fully harnessed, could move a 1:1 ship not much slower than the speed of light. This matter/antimatter propulsion harnesses only a small fraction of the available energy. This leaves many possibilities, including that it’s more inefficient, or perhaps similarly to the Orion concept, and therefore it’s likely that it has the same sources of inefficiencies (if not more) as those outlined above. This would mean that they were calculated twice.

And then, of course, nobody says that the spaceship has to be 1:1 fuel to payload. 1:1 is damn good. Hell, some modern commercial jet liners do that.

I don’t have time to go through all the numbers and see for myself if Orion is feasible, but I hope you can understand how an armchair space cadet such as myself will, for now, continue to take their word for it, and not yours.

 

[Ryan_m_b]

“of all the uranium only ~2% undergoes fission”. This is perhaps true for the Hiroshima bomb…the first ever bomb of its type not only used, but tested. The second bomb used was 10x more efficient, and modern bombs, boosted by fusion, are much more efficient than that.

I hardly cherry picked data, I used what I had. Even if it was 10 times more efficient we still aren't talking on the order of 100% efficiency.

“only 1-10% of the bomb is actually fissile”. Again this is perhaps true for the very oldest designs, but I’m sure modern ones are much better designed than that. Just a few years after Hiroshima they could make bombs two orders of magnitude lighter with the same yield.

Again I am unaware of a nuclear device where the fissile material accounts for the majority of the mass.

~30 meters was the recommended for Orion. The bombs were shaped charges which directed almost all the available energy at the pusher plate.

Are you kidding? A nuclear explosion 30 metres away? Do you have any links for that? How small would the explosion have to be not to destroy the ship/flood it with radiation and yet provide useful thrust?

“With a 1:1 ratio of fuel (itself a 1:1 mix of antimatter+matter) to ship we get a specific impulse of a megasecond”. This part I can’t don’t understand because I have no idea how this alleged matter/ antimatter propulsion system is supposed to work.

Here's the wiki article on http://en.wikipedia.org/wiki/Antimatter_rocket" outlining the Isp

The only thing I can figure out is that it’s incredibly inefficient, as matter/antimatter annihilation produces enough energy which, if fully harnessed, could move a 1:1 ship not much slower than the speed of light. This matter/antimatter propulsion harnesses only a small fraction of the available energy. This leaves many possibilities, including that it’s more inefficient, or perhaps similarly to the Orion concept, and therefore it’s likely that it has the same sources of inefficiencies (if not more) as those outlined above. This would mean that they were calculated twice.

Sorry but how did you work any of that out? A moment ago you mentioned not knowing anything about how antimatter propulsion would work yet now you are claiming that the specific impulse of antimatter is somewhere close to 30megaseconds (close to what you would need to get "not much slower than the speed of light"

And then, of course, nobody says that the spaceship has to be 1:1 fuel to payload. 1:1 is damn good. Hell, some modern commercial jet liners do that.

Of course it doesn't have to be 1:1 but that's a good reference to the efficiency of a propulsion system hence why specific impulse assumes it. You may wish to still believe in Orion but you could at least look into it yourself, especially with the things I've discussed that are nothing to do with propulsion.

 

[JaredJames]

I don’t have time to go through all the numbers and see for myself if Orion is feasible, but I hope you can understand how an armchair space cadet such as myself will, for now, continue to take their word for it, and not yours.

Recommend you read through this: http://en.wikipedia.org/wiki/Project_Orion

The numbers just aren't enticing.

 

[Lsos]

Are you kidding? A nuclear explosion 30 metres away? Do you have any links for that? How small would the explosion have to be not to destroy the ship/flood it with radiation and yet provide useful thrust?"

A fraction of a kiloton was my understanding. The wiki page provides a range of numbers ranging from 25-60 meters, as well as pretty much every other link I've seen. I'd be interested in where you got the miles figure from...would an atomic bomb do anything more than give you a sunburn from 30 miles away?

Sorry but how did you work any of that out? A moment ago you mentioned not knowing anything about how antimatter propulsion would work yet now you are claiming that the specific impulse of antimatter is somewhere close to 30megaseconds (close to what you would need to get "not much slower than the speed of light"

I just used E=mc^2 and then calculated velocity from the resulting energy. Of course I realize that neutrinos and gamma rays and the whole "action-reaction" thing will make the whole process inefficient. The point I'm making is that the antimatter rocket in question already took these inefficiencies and more into account, perhaps overlapping or completely encompassing the sources of inefficiencies which you went over again with Orion. And perhaps it would have it's own problems that Orion wouldn't. The links you showed don't lead me to believe otherwise.

Of course it doesn't have to be 1:1 but that's a good reference to the efficiency of a propulsion system hence why specific impulse assumes it. You may wish to still believe in Orion but you could at least look into it yourself, especially with the things I've discussed that are nothing to do with propulsion.

Of course. I just brought that up to underline the fact that Orion was never meant to be 1:1.

JaredJames, I have looked at that link as well as many others. For 60s technology using fission, I'm still impressed, and I still haven't seen anything that makes me think it wouldn't work.

 

[Ryan_m_b]

Well I'm happy to change my stance on the basis of the distance. I can't actually find the link I got mine from.

What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.

The fact still remains that project Orion (and for that matter Daedalus) were both concepts, not fully worked blueprints. They little more bearing as a realisable product as Da Vinci's drawings of a helicopter. Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.

 

[qraal]

Well I'm happy to change my stance on the basis of the distance. I can't actually find the link I got mine from.

What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.

The fact still remains that project Orion (and for that matter Daedalus) were both concepts, not fully worked blueprints. They little more bearing as a realisable product as Da Vinci's drawings of a helicopter. Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.

Orion is described in great detail, sufficient to begin construction. They fully intended to build it, but the Test Ban and Outer Space Treaty ended hope of using it in a civilian role. But the fission version was insufficiently energetic for an interstellar mission. Dyson did a sketch of that in a 1968 paper, but that's as far as that got.

Daedalus was more than a concept. The ignition system is described in great detail in the original reports and the rest of the vehicle was deliberately designed using known or near-term technology. The only "futuristic" parts were the computer system and the need for gas-mining Jupiter via gas-core nuclear rockets.

Alan Bond & Tony Martin, who led the Daedalus study, went on to design World-ships for interstellar colonization. They would've been immense, with cruise speeds of just 0.005c, but propelled by gigaton nuclear pulse units ignited by accelerator driven ignition units. Gargantuan but not inconceivable if O'Neill-style space colonies became the normal habitats of much of humanity. Definitely not "near-term" but not a big techno-stretch.

 

[sophiecentaur]

I think you are taking the term "detailed design" here a bit more literally than can be justified.
The very best one can hope for here would be broad feasibility studies as no enough is known of the practicalities.
Aamof, there are two ways in which 'efficiency' affects design. The amount of energy actually involved in refining 'fuels' and building the unit is highly relevant and should not be dismissed when considering feasibility.

 

[Lsos]

What do you mean by calculating the resultant velocity from E=mc2? Are you trying to go directly from mass -> energy -> momentum? I still fail to see why you think an antimatter rocket would be less efficient than Orion.

Yes, that's what I did...mass-> energy -> velocity...if just to establish a theoretical limit and compare it to the figure given in the article. What I gathered is that the article is being conservative.

Why would an antimatter rocket be less efficient? I'm not saying it would be less efficient, just that it might have similar (or more) sources of inefficiency. The article explained nothing about their antimatter concept, other than that antimatter could be used as a fuel. This leaves us to speculate all we want. First thing that comes to mind is that simply containing the antimatter could be a 1x - ?100000x larger pain in the *** than encasing some uranium. For all we know they're just using the antimatter to heat water and throw it out the back. They really gave us nothing to work with. The article wasn't informative at all and seemed like they shot from the hip to arrive at the specific impulse figure. Not only that, but their very own estimate for antimatter specific impulse was only 100x better than fission. It seemed designed for nothing more than to incite interest, and certainly not as a foundation from which to invalidate Orion.

Note that I'm not saying that nuclear fission/fusion are not potentially good propulsion technologies, I'm objecting to the notion that we've got it pretty much all figured out.

Ok I'll go with that. It seemed like you were dismissing the very concept as hogwash, but maybe it was necessary to balance out my overly optimistic vision.

 

[Ryan_m_b]

Ok I'll go with that. It seemed like you were dismissing the very concept as hogwash

Oh no no no, far from it. In my view nuclear propulsion could be a good way of achieving interplanetary travel, however there are potentially better technologies (e.g VASIMR) than lobbing a radiation inducing, EMP producing nuclear bomb out of the back of your ship.

My peeve is the idea that there are firm blueprints to how to build these things, lot's of work has been done on the idea but its nowhere near the stage where we have a "Project Orion kit, just add money!".

On the subject of colonising off Earth we have the collection of troubles I outlined that are nothing to do with propulsion (i.e establishing a biosphere, industry, society) etc. There is a perception I regularly come across that all we need to live in space is better rocketry, but there's so much more left to do!

but maybe it was necessary to balance out my overly optimistic vision.

Maybe you can counteract my pessimism