Stephen Hawking says humanity must go to the stars. Is he right?

Great Debates
121

[QUOTE=GawnFishin’]
The people saying heavier than air flight is impossible (or practically so) are describing genuinely huge problems; Ignorance is when people wave those away and say “oh, but we’ll just solve that - I don’t know how, but I’m sure we will”

Do you doubt our ability to solve problems?
[/QUOTE]

Heavier than air flight was obviously possible because birds, even big ones exist. Gliders were built first and then it was just a case of finding an engine light and powerful enough to make it go by itself. Breaking the sound barrier was obviously possible because bullets had done it for a long. It was a matter of building the right bullet speaking metaphorically to hold one human.

Those consisted of an infinitesimally small number of challenges compared to this. I don’t think people understand the magnitude of the problems even within several orders of magnitude or you wouldn’t keep making these kinds of comparisons.

Imagine a much smaller challenge. You have to build a test case for this craft. NASA gives you and your team the resources to build a space station in earth orbit any way that is feasible that you choose. You get to grab volunteers to live on it with you. However, there are some big catches. You and tour descendants have to live on it for 500 years (this is a short term preliminary study). You have to take everything you need for the entire time including food, air, and water or the means to make it all the way down to the most basic items that a person would ever need in their lifetime including hygiene items, entertainment, clothes, and medicine. Things can be jettisoned but you can never refuel or resupply unless you can come up with a way to simulate something that will work for the real thing. Think about everything anyone on the ship could need in a time-span twice the age of the USA and figure out how to get it up, where to put it, and how to keep it working for the entire time. Those computers may not be very useful if you buy crappy displays that conk out completely in 275 years.

You and your descendants some how pull it off a some percentage of the crew makes it to 500 years. The results look a little inbred but some are workable. You call down to NASA2500 (something you can’t do after a few years in the real thing) and you tell them that it was brutal but people are ready to see their home planet for the first time. The people at NASA chuckle. You were so focused on the transportation, you forgot what the study was really about.

They tell you to start phase two “The Challenge” and pick any planet besides earth and figure out a way to live their. They the;; you that the good money is on Mars but people like to get creative sometimes. Now use your fuel they had you save for that purpose, fly to Mars, somehow get down there, and use your remaining supplies to populate the planet any way that you see fit. You can just stay with the ship for another generation or so but it will start getting haggard and running out of supplies and humans tend to get nasty with each other under those circumstances. Choice three is those Kool-Aid mixes they told you not to touch until told.

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We lack motivation. Right now it is an engineering and technical exercise. But,if we spot a comet and plot a crashing trajectory with earth ,we may suddenly find the will and the money. Then ,we see what can be done.

123

Sorry, been away for a bit.

[QUOTE=CurtC]
I’d like to see your estimate of how much food it woul take to supply 250 people for a few thousand years. Granted that you’ve stipulated that cost isn’t a factor, but there will have to be a limit when you approach what the entire population of the Earth can provide with the resources we have available, and still feed and clothe themselves.
[/QUOTE]

Well, you aren’t factoring in things like onboard hydroponics and recycling. The supply rockets are to supplement the short falls…which I estimated (roughly) as between 25-50%. Thats probably a bad estimate in the fact that I think a closed eco-system can generate more than 70% of the crews needs…but what the hell.

[QUOTE=CurtC]
Space debris is still a bigger problem that just making bigger shields will address. That adds to the weight and complexity. What about the energy requirements? I don’t think there’s enough energy available to get all that stuff in orbit and propel it to the nearest star, plus have enough fuel for them to slow down.
[/QUOTE]

Moving stuff into orbit is easy…we are moving it up over a period of decades if not centuries. We can move a LOT of stuff up…a little bit at a time. And then assemble the craft in space. As for propulsion, I already said we’d use something like the Orion concept…we’d be tossing 1MT nukes out the back to get it initially going and then to maintain and increase speed at larger and larger intervals…and once on route we’d use a modified (and upscaled) Ion drive (run off the fission plants) and a solar sail. I don’t think moving it out of orbit and on the correct vector will be a problem…an Orion NEEDS a lot of mass to work properly. Nor will we be going at relativistic speeds, so I don’t think fancy shields for small scale debris is going to be a major problem…some kind of sloped shield in front should be fine. For bigger stuff, my assumption is that the odds would be long…and I suppose the ship could be fitted with high energy lasers (run off the nuke plants) or maybe shuttles that could deal with anything like that…or just don’t worry about it as the odds of hitting something that big have got to be pretty out there.

[QUOTE=CurtC]
You use words that indicate that it’s just a question of applying more resources to the problem, but it really is a deeper problem than you’ve acknowledged. Nothing is infinite, especially not here on Earth.
[/QUOTE]

So, tell me specifically which part is impossible. Taking up the materials into orbit for a space craft several kilometers large? I don’t think that would tap out the earths resources. The materials themselves? Again, doesn’t seem that we are running out of those materials either. The supply rockets? What specifically do you think would be tapping out the resources of an entire world? What are the deeper problems that you speak of? The challenge was that there is no way with the current technology, regardless of cost or risk, to get people alive to the nearest star. Specifically, given those parameters, what do you see as the show stopper?

[QUOTE=Mangetout]
Well, there are actually several, but let’s start with this one:
-Maintaining all shipboardf systems, including working nuclear fission power plants for thousands, or tens of thousands of years. Can we do that? How?

[/QUOTE]

What is your specific issue? The fuel? I’ve already said there would be fissionable materials set out ahead of the manned craft. Maintenance? I picture using modified US Navy reactors, over engineered (i.e. there will be a lot more reactors than are needed, with many of them shut down, some idle and on standby, etc). There would be spare parts AND technicians to maintain and repair the things…or cut one loose if it started to fail. We aren’t talking about putting a bunch of bushmen from the outback on this thing and turning em loose…but highly skilled and trained crewmen and women with resources to fix, repair or replace anything that might come up…not to mention all those resupply rockets launched ahead of the ship with spare parts and other essentials on board.

Specifically, what exactly do you see as the show stopper here? Also, we aren’t talking about tens of thousands of years to get to the nearest star (though again, gods know why we’d go there). Maybe I can dig up my ancient notes on Orion and exactly how much velocity we can expect (as well as adding on that of the solar sail and Ion drive) to get some kind of ball park on how long the trip would take…but I’d be surprised if it took more than a thousand years. We ARE talking about a continuous thrust vehicle here, not one on a ballistic trajectory.

Bring out the other problems too…don’t be shy. Maybe there is a show stopper in there I haven’t seen.

[QUOTE=Otara]
XT the biggest problem with your theory is that the unmanned rockets have to be sped up to the speed the colony rocket is going. Which means you still run into the propellant problem:
[/QUOTE]

I’m unsure of what you mean with your size of the ‘fission’ tanks. You realize I’m talking about using nuclear bombs tossed out the back, right? The largest use will be the initial propulsion to get the thing out of orbit and on the correct trajectory…after that the intervals between tossing another nuke out the back will get greater and greater. And that my intention is to pick up MORE of these things enroute (probably a huge resupply ship full right here in the solar system to make up for the initial expenditure to get the craft moving)? Also, I’m talking of using a solar sail AND an Ion drive as well…and again, picking up fuel for the Ion drive enroute.

As for the resupply ships, many would be sent off ahead of the manned ship at specific velocities…and some of these would be massive. As it would take a LONG time to put together the final manned ship, some of these would be literally decades or centuries ahead of the manned ship, launched in windows so that the manned ship would encounter them at very specific points in the journey…and again, there would have to be redundancy built into this aspect too (i.e. you’d need to plan for the contingency that some of the supply ships might be lost or destroyed). Cost and risk aren’t factors, so I don’t see why this couldn’t keep the ship supplied with all the fuel and other essentials it would need to make the trip.

[QUOTE=Otara]
And we havent even successfully used this technique yet for something that weighs 100kg as far as Im aware.
[/QUOTE]

No, we haven’t…its one of the more speculative things I put on the proposed rocket, no doubt. And as Stranger says, its diminishing returns…though I think it still DOES give you returns even in interstellar space…and its continuous acceleration (though very small). The Ion drive scaled up is pretty speculative too of course, though I’d say thats a lot firmer. However, neither will have to power the craft on its own…its the combination of technologies that will keep the crafts acceleration rising (slowly). And time really isn’t that much of a factor…none of the intial crew NOR their children (etc) are going to finish the trip (unless we have some kind of longetivity breakthrough during construction).

-XT

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[QUOTE=xtisme]
[solar sails are] one of the more speculative things I put on the proposed rocket, no doubt. And as Stranger says, its diminishing returns…though I think it still DOES give you returns even in interstellar space…and its continuous acceleration (though very small).
[/QUOTE]
In interstellar space, where the Sun’s flux is reduced to nothing more than a nearby, middling bright star, the amount of thrust you’ll get is negligable; even in the outsystem you’re only going to get a small percentage of the thrust you’ll get compared to what you’ll see around Earth’s orbit. [thread=324185]Here[/thread]'s an old thread where we talk extensively about how solar sails work. And for a ship on the scale of what you’re proposing, the sails would be literally astronomical in size to get any measurable level of thrust; they’d literally have to be the size of a large moon or larger.

As for the rest of the points regarding interstellar transit, you seem to be convinced that major technical stumbling blocks are minor issues to be engineered away with sufficient money and ingenuity. With this attitude you can dismiss any objection, and having already established what I believe to be reasonable issues that cannot be conceivable resolved with the extrapolation of existing technologies, I find no value in reiteration. But in my judgement and experience as an engineer working in aerospace and rocket propulsion, I think there are both technical, logistical, and fiscal infeasibilities with your plans; even if it could be done technically with an acceptible level of risk (given that you are planning for a 1000 year journey, the risk better be pretty damn low), the expense and political will required to engage in such a sweeping project are beyond anything we have seen.

Launching millions of tons of hardware and provisions into orbit? Sending dozens or hundreds of supply rockets ahead? Building a vessel that can, away from the “free energy” of a nearby radiating sun, demonstrate “70%” efficiency in recycling and scavenging for a period of on the order of 1000 years? These would be extraordinary engineering achievements that dwarf anything practically conceived of by mankind. The problems here aren’t just big; they’re in fact too large to properly comprehend. Doing the logistics, ground support, launch operations, and risk mitigation work for a simple three stage orbital rocket with a payload of a few hundred kilograms is the work of dozens of people over several months, and you are talking a project that is many, many orders of magnitude far more complex with a vastly greater consequence of failure (and correspondingly smallar overall risk margin). Perhaps someday, when we have matured the technology to allow us to live indefinitely in space and have enhancements in propulsion technology that permit much greater impulse and efficiency that we can currently obtain, such a program will be feasible. With current technology, I couldn’t see this as anything but fanciful dreaming.

Stranger

125

[QUOTE=Stranger On A Train]
In interstellar space, where the Sun’s flux is reduced to nothing more than a nearby, middling bright star, the amount of thrust you’ll get is negligable; even in the outsystem you’re only going to get a small percentage of the thrust you’ll get compared to what you’ll see around Earth’s orbit. Here’s an old thread where we talk extensively about how solar sails work. And for a ship on the scale of what you’re proposing, the sails would be literally astronomical in size to get any measurable level of thrust; they’d literally have to be the size of a large moon or larger.
[/QUOTE]

I already said it would be diminishing returns…was that not good enough for you? How about ‘I conceed the point, in interstellar space the solar sail would not work anymore’. That do ya?

[QUOTE=Stranger On A Train]
As for the rest of the points regarding interstellar transit, you seem to be convinced that major technical stumbling blocks are minor issues to be engineered away with sufficient money and ingenuity. With this attitude you can dismiss any objection, and having already established what I believe to be reasonable issues that cannot be conceivable resolved with the extrapolation of existing technologies, I find no value in reiteration. But in my judgement and experience as an engineer working in aerospace and rocket propulsion, I think there are both technical, logistical, and fiscal infeasibilities with your plans; even if it could be done technically with an acceptible level of risk (given that you are planning for a 1000 year journey, the risk better be pretty damn low), the expense and political will required to engage in such a sweeping project are beyond anything we have seen.
[/QUOTE]

And by the same token, you seem to regard hard engineering challenges as automatically unsolvable. I never said the challenges would be minor…I conceed they would be very difficult…definitely the most challenging thing humans would have ever attempted in our history. But with unlimited budgets and no time limits I think that there is no challenge on this project that couldn’t be done. I think it shows a lack of imagination that some don’t think its possible to do this given those parameters. The TRUE impossibility (as you pointed out above and I did earlier) in all this mental excersize is that you’d never get the entire world, let alone a single country motivated to put unlimited manpower and resources on such a project for decades or centuries to do it.

[QUOTE=Stranger On A Train]
Launching millions of tons of hardware and provisions into orbit? Sending dozens or hundreds of supply rockets ahead? Building a vessel that can, away from the “free energy” of a nearby radiating sun, demonstrate “70%” efficiency in recycling and scavenging for a period of on the order of 1000 years? These would be extraordinary engineering achievements that dwarf anything practically conceived of by mankind. The problems here aren’t just big; they’re in fact too large to properly comprehend. Doing the logistics, ground support, launch operations, and risk mitigation work for a simple three stage orbital rocket with a payload of a few hundred kilograms is the work of dozens of people over several months, and you are talking a project that is many, many orders of magnitude far more complex with a vastly greater consequence of failure (and correspondingly smallar overall risk margin). Perhaps someday, when we have matured the technology to allow us to live indefinitely in space and have enhancements in propulsion technology that permit much greater impulse and efficiency that we can currently obtain, such a program will be feasible. With current technology, I couldn’t see this as anything but fanciful dreaming.
[/QUOTE]

“Building a vessel that can, away from the “free energy” of a nearby radiating sun, demonstrate “70%” efficiency in recycling and scavenging for a period of on the order of 1000 years?”

What part is impossible? We are talking about growing food and recycling drinking water after all…with sufficient energy AND resupply while en route. Even assuming for a moment that the ship would be unable to pick up the odd ice ball or iron rich chunk of rock in the great beyond, I don’t see anything making this impossible.

“Doing the logistics, ground support, launch operations, and risk mitigation work for a simple three stage orbital rocket with a payload of a few hundred kilograms is the work of dozens of people over several months, and you are talking a project that is many, many orders of magnitude far more complex with a vastly greater consequence of failure (and correspondingly smallar overall risk margin).”

Certainly. But then, we are talking about the difference in pushing through a space program in a race with folk who have both time and monetary constraints…and also have only the resources of a single (albiet very powerful and rich) nation. I assume you see the difference.

“With current technology, I couldn’t see this as anything but fanciful dreaming.”

Only because there is no way we as a species would committ to such a venture, to pool our resources, technical knowledge, manpower, etc for such an extended period of time. My point in all this is that its not so much a matter of whether or now we could technically go…I think that given the proper world wide committment we could (with great difficulty and sacrifice) produce a craft that could get humans alive and kicking to another star. For that matter, my own scenerio doesn’t take into account technological progress, or lessons learned on such a project…IMHO just undertaking such a project would produce refinements that would make the trip quicker and better with an increase in probability of success. And of course I haven’t done aerospace engineering since college (decades ago) and was mostly talking off the top of my head (or out my ass I suppose)…someone better versed in todays technology could probably do a much better job of coming up with a workable scenerio based on the parameters of using existing or near term technology to get humans alive and kicking to another star, given unlimited budgets and having time not be a factor.

The reason all this won’t work is more a matter of politics, economics and committment IMHO than in brute technical capability/ability.

-XT

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[QUOTE=xtisme]
But with unlimited budgets and no time limits I think that there is no challenge on this project that couldn’t be done. I think it shows a lack of imagination that some don’t think its possible to do this given those parameters.
[/quote]
There is no such thing as an “unlimited budget”. If you want to posit this as an assumption for your concept then you’ve already delved way into science fiction.

[QUOTE=xtisme]
We are talking about growing food and recycling drinking water after all…with sufficient energy AND resupply while en route. Even assuming for a moment that the ship would be unable to pick up the odd ice ball or iron rich chunk of rock in the great beyond, I don’t see anything making this impossible.
[/quote]
“…pick up an odd ice ball or iron rich chunk of rock in the great beyond,…” This statement highlights everything that is wrong about your assumptions. Even if you come across such an object, you are going to be moving at such speeds (even at a fraction of a percent of c) that you won’t be able to just “pick [it] up”. The kinetic energy in the differences between objects moving at astronomical speeds are unimaginable in terrestrial terms. This isn’t like catching a baseball or even intercepting an ICBM. I think you just don’t understand the magnitude of what you are proposing, and how far beyond any extrapolation of current capability it is. This isn’t merely an extension of existing capability, writ large; this is fundamentally more complex and risky, by many orders of magnitude, than anything humanity has ever done.

[QUOTE=xtisme]
The reason all this won’t work is more a matter of politics, economics and committment IMHO than in brute technical capability/ability.
[/QUOTE]
We’re going to have to agree to disagree on this. It may someday be capable to transport people across space, and I think it’s inevitable, assuming our continued existance, that we’ll explore extrasolar space in some way (most likely by proxy, in absence of some kind of highly speculative superluminal propulsion technology) but it is so vastly beyond existing capabliities and experience that it’s not even plausible to discuss.

Stranger

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[QUOTE=Stranger On A Train]
There is no such thing as an “unlimited budget”. If you want to posit this as an assumption for your concept then you’ve already delved way into science fiction.
[/QUOTE]

:rolleyes: By taking budgetary and time constraints out of the equation we could then just talk about technology…which was the question. I conceeded that there is no such thing as unlimited budgets and manpower, as well as the fact that most human projects DO have time constraints…yet you still say the above. Either you aren’t reading what I’m saying, don’t care and simply want to soapbox to the peanut gallery or don’t understand the point I’m making.

[QUOTE=Stranger On A Train]
“…pick up an odd ice ball or iron rich chunk of rock in the great beyond,…” This statement highlights everything that is wrong about your assumptions. Even if you come across such an object, you are going to be moving at such speeds (even at a fraction of a percent of c) that you won’t be able to just “pick [it] up”. The kinetic energy in the differences between objects moving at astronomical speeds are unimaginable in terrestrial terms. This isn’t like catching a baseball or even intercepting an ICBM. I think you just don’t understand the magnitude of what you are proposing, and how far beyond any extrapolation of current capability it is. This isn’t merely an extension of existing capability, writ large; this is fundamentally more complex and risky, by many orders of magnitude, than anything humanity has ever done.
[/QUOTE]

The main ship certainly could not stop once enroute…no doubt. A shuttle craft of some kind? Maybe it couldn’t stop, mine and catch back up either. Maybe it could. It doesn’t matter anyway…I just tossed that out there without really thinking that through.

I think I do have a good grasp of the magnitude in this undertaking myself…I think your grasp if imagination leave a bit to be desired. Perhaps we are both right…who knows? :stuck_out_tongue:

[QUOTE=Stranger On A Train]
We’re going to have to agree to disagree on this. It may someday be capable to transport people across space, and I think it’s inevitable, assuming our continued existance, that we’ll explore extrasolar space in some way (most likely by proxy, in absence of some kind of highly speculative superluminal propulsion technology) but it is so vastly beyond existing capabliities and experience that it’s not even plausible to discuss.
[/QUOTE]

Lets just agree to disagree then. I think it would be impossible to do this project in realistic terms, but it wouldn’t be because our current technology makes it impossible to do. You feel that it would be impossible not just from a political/economic perspective but because the fundamental technlogy isn’t there and it can’t be done. Why don’t we leave it there then? Especially since you don’t even think its worth discussing?

-XT

128

I don’t for a moment believe that we’ll be able to do it in a century (given any sort of plausible budget and motivation). In a millenium, perhaps, but I wouldn’t be surprised if we weren’t ready even then. But why think in the short term? A hundred or thousand years is nothing. Come up with a prediction of our chances in a million years, and then we’ll talk.

129

[QUOTE=GawnFishin’]
The people saying heavier than air flight is impossible (or practically so) are describing genuinely huge problems; Ignorance is when people wave those away and say “oh, but we’ll just solve that - I don’t know how, but I’m sure we will”
[/quote]
Tiresome argument by analogy, again.

I do not doubt that ability, in cases where solutions may actually exist. Do you refuse to concede that there are some problems for which no solution exists?

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[QUOTE=xtisme]
What is your specific issue? The fuel? I’ve already said there would be fissionable materials set out ahead of the manned craft.
[/QUOTE]
I’m only going to bother addressing this point, because the others are all the same; chirpy handwaving.

Here’s my issue: If you want to set extra fuel out ahead of the travellers, how are you actually going to propel it there? Your suggestion multiplies the problem, rather than solving it.

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[QUOTE=Chronos]
I don’t for a moment believe that we’ll be able to do it in a century (given any sort of plausible budget and motivation). In a millenium, perhaps, but I wouldn’t be surprised if we weren’t ready even then. But why think in the short term? A hundred or thousand years is nothing. Come up with a prediction of our chances in a million years, and then we’ll talk.
[/QUOTE]

I don’t believe we will either. I doubt that in a century we are much beyond what we can currently do. MAYBE we will have sent men to Mars or perhaps have a perminent base on the moon or in orbit…but that will probably be the extent of things in a century. In a millenium? I wouldn’t be surprised if we had a more extensive network of manned facilities throughout the solar system in that time, plus a lot of unmanned automation type facilities for mining and exploration in the outer solar system. I seriously doubt we will be sending anything but unmanned probes out to any other stars in that time either. In a million years? Well, if we haven’t left our solar system in that time, we never will…

[QUOTE=Mangetout]
I’m only going to bother addressing this point, because the others are all the same; chirpy handwaving.
[/QUOTE]

Got to love the irony of such a statement (or non-statement as it were). :stuck_out_tongue:

[QUOTE=Mangetout]
Here’s my issue: If you want to set extra fuel out ahead of the travellers, how are you actually going to propel it there? Your suggestion multiplies the problem, rather than solving it.
[/QUOTE]

I’m only going to bother addressing this point because, well, its the only one you’ve made…

How am I going to propel the resupply ships leaving earth before the manned ship? Gee…I don’t know. Maybe with the same propellent used by the manned ship perhaps, just less of it (since there will be less mass, and also since the idea is for the manned ship to eventually catch up to it)? I.E. toss a few 1MT bombs out the back, Ion drive, etc? Send them on ballistic trajectories that the manned ship will catch up too in 50 years, 100 years..500 years, a thousand years? I don’t see how this multiplies the problem to be honest…maybe you could be plain on this one point (your only so far) and just speak right out…WHY does it multiply the problem? HOW does it multiply the problem?

-XT

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[QUOTE=Chronos]
I don’t for a moment believe that we’ll be able to do it in a century (given any sort of plausible budget and motivation). In a millenium, perhaps, but I wouldn’t be surprised if we weren’t ready even then. But why think in the short term? A hundred or thousand years is nothing. Come up with a prediction of our chances in a million years, and then we’ll talk.
[/QUOTE]
In a million years (or even a few thousand) it may certainly be possible. But we won’t be doing it with anything like the technology we have now. Chemical rockets, Orion-type fission pulse motors, and the like are inadequate, even given an unlimited budget of materials and manpower. In ten thousand years (or perhaps a thousand, or even a few hundred) we’ll hopefully have found entirely new methods of energy production and propulsion that will make our wildest concepts today look like primitive steamboats in comparison to a hydrofoil. There’s no way to predict what that technology will be or when it will come, though. In the same time, however, I expect that we’ll alter our own species, or at least the spacefaring proxy of it, in ways that make us far more resistant to the hazards of space. The space opera notion that we’ll be flying around in big pressure hulls in hyperspace crapping, eating, and fucking is like envisioning the Internet as imagined by Vannevar Bush’s Memex or H.G. Wells World Brain.

Stranger

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[QUOTE=xtisme]
How am I going to propel the resupply ships leaving earth before the manned ship? Gee…I don’t know. Maybe with the same propellent used by the manned ship perhaps, just less of it (since there will be less mass, and also since the idea is for the manned ship to eventually catch up to it)? I.E. toss a few 1MT bombs out the back, Ion drive, etc? Send them on ballistic trajectories that the manned ship will catch up too in 50 years, 100 years..500 years, a thousand years? I don’t see how this multiplies the problem to be honest…maybe you could be plain on this one point (your only so far) and just speak right out…WHY does it multiply the problem? HOW does it multiply the problem?
[/QUOTE]
It multiplies the problem by complexity (having to arrange rendezvous between your main vessel and supply ships) without giving you anything worthwhile. You still have to expend, at a minimum, the same amount of propellent to get the vessels to the same location and velocity as the main ship. Because of the extra weight per unit of propellent (motors, tankage, et cetera) you’re going to end up requiring more propellent; and remember, every unit of propellent delivered is going to cost several orders of magnitude more just to get it in position. Assuming that the amount of total impulse you can generate is arbitrarily scalable (as it would be with an Orion) this gets you absolutely nothing except the ability to launch separate vehicles at different times, or if you have a limit to how much you can launch at any one time. (This scheme has been proposed, in various guises, for a Mars transit, but only because the amount of mass you could lift at any one time is limited by the capacity of a Saturn V or EELV.)

There’s another issue that’s barely been dealt with, but in the case of a generations-long transit through interstellar space you are going to have to provide energy. Not just energy for propulsion, lights, and the like, but energy for everything; hydroponics, material processing and recycling, et cetera ad nausum. We tend to take for granted that much of the energy we get, in the form of food, clean water, air, et cetera is free, but ultimately it comes from the Sun, which has energy to spare. Sans the massive flux of solar energy, we have to provide all of this ourselves, and most of it is not recoverable and will eventually have be radiated out into space as heat. Most science fiction authors deal with this problem by either assuming some kind of superefficient recovery technology (re: Heinlein’s direct energy converters in Orphans Of The Sky) or ignoring it entirely, but in fact, out away from a star, you’ll have to supply all of your energy yourself. This alone requires a major advance in energy technology–either scavenging fuel from the environment, or somehow carrying enough potential energy to last for generations or eons, vastly increasing the mass of provisions you have to transport.

These are not little niggling issues; these are major conceptual problems, and waving them aside as mere problems of cost or logistics, solved by applying enough money or brute force indicates a lack of comprehension of the scale of the problem. Until we have a revolutionary leap in the applicable technologies–which I think (or would like to believe) is inevitable at some eventual time–talking about travelling to other systems is an idle pastime, not a serious possibility.

Stranger

134

I have to say, the notion that we would build a starship when we see impending danger coming and evacute any significant portion of humanity is ludicrous. Even allowing that such starships could be made, the level of effort and time invested would never allow such a project to be finished in the amount of time we’d have available. And even if such a project could concievably be completed, within the short timeframe available between seeing a comet or an asteroid on a collision trajectory with Earth or whatever the danger is, we’d never evacuate. There’s not enough iron ore in the earth’s crust to build ships to send more than a tiny fraction of people away. Saving the human race through colonization might be just barely possible. Evacuation is a ridiculous idea.

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[QUOTE=appleciders]
I have to say, the notion that we would build a starship when we see impending danger coming and evacute any significant portion of humanity is ludicrous. Even allowing that such starships could be made, the level of effort and time invested would never allow such a project to be finished in the amount of time we’d have available. And even if such a project could concievably be completed, within the short timeframe available between seeing a comet or an asteroid on a collision trajectory with Earth or whatever the danger is, we’d never evacuate. There’s not enough iron ore in the earth’s crust to build ships to send more than a tiny fraction of people away. Saving the human race through colonization might be just barely possible. Evacuation is a ridiculous idea.
[/QUOTE]

Who suggested evacuation. I think I missed that post. Could you please point it out.

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[QUOTE=Stranger On A Train]
In interstellar space, where the Sun’s flux is reduced to nothing more than a nearby, middling bright star, the amount of thrust you’ll get is negligable; even in the outsystem you’re only going to get a small percentage of the thrust you’ll get compared to what you’ll see around Earth’s orbit. [thread=324185]Here[/thread]'s an old thread where we talk extensively about how solar sails work. And for a ship on the scale of what you’re proposing, the sails would be literally astronomical in size to get any measurable level of thrust; they’d literally have to be the size of a large moon or larger.

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They’d have to be pretty big but you wouldn’t necessarily have to rely just on the sun’s natural output. You could orbit a constellation of large solar powered (or even fusion or fission powered) lasers and aim them at the sail… this would give you a considerable amount of added thrust far beyond the point at which the sun would be any significant factor.

I also don’t think you’d have to launch everything from earth, in fact it would be silly to do that. Any successful interstellar mission is going to rely on industries and resources already outside of the earth’s gravity well. You launch the seed capital to start gearing up to have the industrial base necessary to engage in megascale engineering. We have to crawl before we can walk and establishing a more or less self supporting population in our own solar system, outside of earth, is necessary.

Energy needed for onboard systems could be provided by ordinary fission reactors. Reprocessing fuel, carefully designed low energy support systems and recycling could stretch a few hundred tons of fissionable material quite far, especially with a small population. I’m of the opinion that the most successful design would use the least complicated and most robust technology you could get away with.. even if other newer technologies were available. We’re seeing this somewhat now with NASA returning to the capsule design instead of the complicated shuttle for low earth orbit missions. A starship that relies on a fission reactor is a starship that could, with a decent machine tool base and the raw materials, rebuild it’s powerplant entirely enroute if necessary.. several times over .

I think the idea of sending out caches ahead of the spacecraft is interesting, you could send packets of material of relatively low volume but useful mass using very large electromagnetic linear accelerators. You couldn’t put a huge spacecraft out this way because it would weigh too much and be too fragile but reaction mass, sure… water, rare elements and possibly even luxury goods that would be too frivolous to manufacture on board (new movies ;>). If you calculate it right, aim true and launch at specified speeds and intervals, the nice little caches could sail right up to your accelerating spacecraft at a leisurely enough relative speed to scoop up right into your cargo bay. Like a motorcycle slowly pulling ahead of a huge double decker bus that’s also speeding up but more slowly than the motorcycle.. imagine somebody just leaning out of the window and plucking off the rider. :> We build linear accelerators now, for roller coasters, particle accelerators.. even in development as weapons. I see no reason why we couldn’t construct one a few dozen kilometers long to push things out of the solar system at a good fraction of light speed.

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XT it would really help if you read that link I gave. Throwing bombs out the back doesnt fix the propellant problem, a bomb is just highly energetic propellant at the end of the day.

The site specifically discussed things like antimatter and nuclear drives with far higher efficiency levels than anything we can currently make, and how even that would still need massive amounts to get a shuttleload to Alpha Centauri in 900 years. Its by Nasa, and they concluded that the only way we’re getting there is if we invent a new kind of drive that doesnt need propellant, and give examples of various theoretical drives that have been mooted.

Similar to heavier than air machines its not being said it isnt possible, its that it isnt possible with current technology. It doesnt mean it will never be possible, only that we need to come up with something new before it will be. And thats not a sure thing just because we’ve overcome previous engineering problems.

Otara

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“you could send packets of material of relatively low volume but useful mass using very large electromagnetic linear accelerators”

Ok someone else will have to do the sums in more detail but this doesnt sound remotely possible to me. To even get a mass to say 1% of lightspeed would take massive amounts of energy and involve an accelerator length that would be mindbogglingly long, you’re talking getting up to 3000km/s to manage even that.

If you were accelerating at 100g the whole way which is rather heavy, you’re talking 3000 seconds to get there, ie 50 minutes. Ive had a go at figuring the length and as far as I can tell it would need to be about 4.5 million km long, but its been while since I figured that so feel free to correct my calcs.

And your payload would take 300 years to get to 3/4 of the way to Alpha Centauri with that, and all of this assumes perfect aim or mass wasted on maneuvering ability, a .001% error would translate to huge distances at the other end.

Otara

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[QUOTE=Otara]
“you could send packets of material of relatively low volume but useful mass using very large electromagnetic linear accelerators”

Ok someone else will have to do the sums in more detail but this doesnt sound remotely possible to me. To even get a mass to say 1% of lightspeed would take massive amounts of energy and involve an accelerator length that would be mindbogglingly long, you’re talking getting up to 3000km/s to manage even that.

If you were accelerating at 100g the whole way which is rather heavy, you’re talking 3000 seconds to get there, ie 50 minutes. Ive had a go at figuring the length and as far as I can tell it would need to be about 4.5 million km long, but its been while since I figured that so feel free to correct my calcs.

And your payload would take 300 years to get to 3/4 of the way to Alpha Centauri with that, and all of this assumes perfect aim or mass wasted on maneuvering ability, a .001% error would translate to huge distances at the other end.

Otara
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I am curious as to why you think 100g of acceleration is the upward limit, I believe we have already made coilgun drivers that can accelerate at almost 2000g. If arbitrary length is a problem for you, use a very large loop design instead of a staight line, perhaps around the equator of a small moon. Do remember the mass driver is for enroute resupply of a generation ship, to help account for lost efficiencies in the habitat in interstellar space, not for transporting people. I do wonder if you could maybe use the kinetic energy of these payloads on impact to provide forward thrust for your ship.. it would save on carrying those Orion style nukes onboard. As for aiming, make the packages self guiding to some extent, once you have them underway. Surely it wouldn’t be too difficult to make the payload container have a directional thruster and use it to correct for deviation. The US navy is researching coilguns and rail guns for use as kinetic energy weapons and aircraft catapults, I believe they want ‘smart’ projectiles for those weapons too for precision striking capability. If this is in development today, I would bet the technology, when mature, could be adapted for use in space travel.

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[QUOTE=Otara]
Ok someone else will have to do the sums in more detail but this doesnt sound remotely possible to me. To even get a mass to say 1% of lightspeed would take massive amounts of energy and involve an accelerator length that would be mindbogglingly long, you’re talking getting up to 3000km/s to manage even that.
[/QUOTE]
The total impulse required to get 1 kilogram going at 0.01c would be 3·10[sup]6[/sup] N·s. Assuming no losses, and your linear accelerator mounted to an effectively fixed mass (so that you don’t lose anything by momentum transfer to the accelerator), this requires 4.5·10[sup]12[/sup] N·m or 4.5 terajoules per kilogram of payload. To put that in perspective, the energy flux of the Sun is roughly 1.4kW/m[sup]2[/sup]. If you had completely efficient conversion of solar energy this would require 11.5 million square kilometers (slightly more than the solar-facing aspect of the Moon) one hour to collect that amount of energy. (In reality, the best theoretical efficiencies for photovoltaic conversion hover around 30%, with 15% being the current capability.) Complete fission of pure [sup]235[/sup]U gives about 77 TJ/kg, assuming you can reprocess it indefinitely to use every last bit of it. More realistic values for current fission reactor designs are somewhere on the order of 67 GJ/kg of natural ore, so we’re assuming roughly 9 kg of uranium for every kilo of payload launched.

And remember, we’re going to have to decelerate this somehow at the other end, using some kind of thrust propulsion that will be vastly inefficient in comparison to our linear accelerator. Also, assuming an average acceleration of 10G and an exit speed of 3000 km/sec, it’s going to take 8.3 hours to accelerate your payload (by which time your human payload would certainly be dead) and a distances of 45 million kilometers (120 times the average distance from the Earth to the Moon). You could make your linear accelerator circular and use magnets on top to hold the payload down while you’re spinning it up, but the loads and energy would be enormous, magnitudes beyond the tensile strength of any known material.

It’s all well and good to talk about unlimited budgets, but in fact there are limits beyond fiscal perserverance; in terms of today’s technology and capabilities, this is well and beyond feasibility.

Stranger