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

Great Debates
141

That should read "the energy flux of the Sun at Earth’s orbit is roughly 1.4kW/m[sup]2[/sup].

[QUOTE=lokij]
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…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.
[/QUOTE]
Good Og, 2000G? How is any useable object (much less a living payload) supposed to survive such accelerations? Heck, how would a structure of any reasonable size (including the linear accelerator itself) supposed to withstand such loads? We’re not shooting sabots of depleted uranium or tungsten here, which in any case are intended to be energetically destroyed on impact; these have to be useful objects and supplies.

As for using the energy from payloads fired off at such a speed, again, I think the advocates are at a complete loss to comprehend the magnitude of forces and speeds for this. Think of catching a cannonball with your hand; absurd, non? And this is vastly less difficult than the task described here.

I have to admit no small amount of frustration at how blase people are with throwing out accelerations in the thousands of Gs and energies at trillions of joules per kilogram as if these are piddling technical matters to be solved by tossing a few eggheads in a conference room with a computing cluster and a wheelbarrow of cash. It’s one thing to challenge the existing limits and the experts who parade them, but quite another to do so without a shread of perspective on what is even marginally plausible. Some day we may have the capability to do something like this, but not with any technology currently extant.

Stranger

142

[QUOTE=Stranger On A Train]
That should read "the energy flux of the Sun at Earth’s orbit is roughly 1.4kW/m[sup]2[/sup].

Good Og, 2000G? How is any useable object (much less a living payload) supposed to survive such accelerations? Heck, how would a structure of any reasonable size (including the linear accelerator itself) supposed to withstand such loads? We’re not shooting sabots of depleted uranium or tungsten here, which in any case are intended to be energetically destroyed on impact; these have to be useful objects and supplies.

As for using the energy from payloads fired off at such a speed, again, I think the advocates are at a complete loss to comprehend the magnitude of forces and speeds for this. Think of catching a cannonball with your hand; absurd, non? And this is vastly less difficult than the task described here.

Stranger
[/QUOTE]

Water is a usable substance, even the uranium and tungsten you talk about could be vital resources to be used by the spacecraft. Noble gases, rare earths, specially made electronics and even genetic material could all be sent this way. Encase all of these raw materials in steel capsule and off they go. This is a delivery method for raw materials, not finished goods. I’ve already discussed how to deliver them in a way that would not even remotely result in the energetic destruction of the payloads upon receipt. As for using the kinetic energy for thrust, yes.. catching a cannonball with your hand would be a good analogy.. if you were in zero G and a million ton spacecraft, if your hand were a solid metal shield meters thick and the cannonball moving a a good fraction of C. The cannonball impacts the sheild and imparts forward motion.. where is the problem?

143

[QUOTE=lokij]
if you were in zero G and a million ton spacecraft, if your hand were a solid metal shield meters thick and the cannonball moving a a good fraction of C. The cannonball impacts the sheild and imparts forward motion.. where is the problem?
[/QUOTE]
I have not the words. Clearly, no objection or problem raised, regardless of how quantitatively expressed, will dissude you from believing that this is a mere piddling issue, to be dispensed with via a wave of the hand.

Stranger

144

[QUOTE=Stranger On A Train]
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.

[/QUOTE]

Wow, nine whole kilograms of uranium. I did my own calculating and found that getting 1 kilogram up to speed would cost 1.25 gigawatt hours of energy. The state of California produced 290000 gigawatt hours of electricity in 2004. I don’t think anybody has ever said this wouldn’t be an energy intensive project and I don’t think we can visualize a project like this happening in an environment of energy scarcity, but I don’t think we’ll have that problem. Assuming we can generate lots of energy when needed via fusion or fission, or even nice eco-solar friendly photovoltaics ;>.. there is no reason why the mass driver couldn’t work. The problem I seen throughout this thread is people like myself look at these projects and think, Wow.. it’s enormous, dangerous.. expensive, very difficult but.. MAN it’s possible and others think that because it’s enormous, dangerous, expensive and very difficult that makes it impossible. The future will tell I suppose.

145

Ironically, those people pointing out problems have taken the first step to solving them.

Mankind’s destiny is in the stars.

146

For what its worth I also did 1000G’s and it worked out to only being 450,000km long, which makes it a tad past the moon from the earth in length.

I have no idea how you could do this circularly, the forces involved would be mindbogglingly immense once you got close to terminal velocity. For _20_km/s a page talks about bedrock and huge amounts of concrete to make this doable in regards to the huge forces that would be generated.

Think Ill go SOAT’s route here. This has become more about publically proclaiming ones self as an optimist to the audience than anything to do with realistic engineering.

Otara

147

Btw, using fusion is cheating.

The original argument was we could do it now with existing technology and the only barrier is the will to do it - the reasons uranium usage was mentioned is because Xt said it could be done with such.

Fusion is not currently a going concern and invoking it means we’ve already postulated the successful development of one technology not currently available to us. And Im betting a 3000km/s accelerator will need a heck of a lot more than that.

Otara

148

One thing no one has mentioned so far: On the NASA link given above they calculate the fuel needed for a 900 year journey.

My question: Who can make a spaceship that will still reliably function after 900 years?

149

For a long time to come, it will be far more efficient for us to explore the nearest stars with gigantic telescopes. Huge interferometry arrays capable of imaging continents or even smaller features, and analyzing the content of atmospheres.

If we were ever going to take a humanity-sized swing and launch a ship to another star, we’d never, ever do it until we had gleaned every shred of information we could about our nearest neighbors in every other way possible.

150

[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,
[/quote]
Why is it more practical to send it ahead? - there’s no energy saving - You have to propel it, anyway - taking the fuel with you in one ship is actually less expensive, energy-wise, than sending it ahead, because if you’re sending it ahead, it will require its own shielding etc. You seem to be acting on the assumption that breaking it up into smaller pieces will make the whole job easier or less energy-expensive in total. It won’t.

Negligibly so, as you don’t really want to be catching up with your fuel dump at a very considerable relative velocity.

You’re still talking about using nuclear fuel? Apart from the gross oversimplification of ‘toss a few…’, there’s still the problem of how you keep the fuel usable? Again, makes no difference here if you send it ahead or take it with you.

OK, here’s how:
-It uses more energy in total you simply don’t save anything by sending your fuel ahead - on the contrary, it costs a lot more (in total) to do that.
-It greatly increases the potential points of critical failure
-It doesn’t actually address any genuine problem.

151

[QUOTE=Mangetout]
You’re still talking about using nuclear fuel? Apart from the gross oversimplification of ‘toss a few…’, there’s still the problem of how you keep the fuel usable? Again, makes no difference here if you send it ahead or take it with you.
[/QUOTE]

Keep which fuel usable? The 1MT nuclear bomblets? The fission fuel for the reactors? I don’t think 1000 years is too much for stabalized fission materials to stay usable. Is it? Are you saying there is no way to keep non-active fission materials usable over that time period?

As for taking it with you…it would be less initial mass when you actually launch the manned ship of course. I’m not TRYING to save over all energy or conserve costs. As time isn’t a factor I don’t see the benifit of trying to do so…I can expend 50 times the energy that I would other wise have done if it gets the job done. Putting all the fuel and supplies, spare parts, etc on board for a thousand year journey would make the manned space craft truely massive…maybe too massive to actually get the thing out of orbit and on course. Obviously as propellent (those 1 MT nukes, the ionized gas for the Ion engine, spent fission fuel, other waste) is used up its jettisoned, making the ship less massive. Allowing the manned ship to pick up fresh fuel and supplies means I don’t have to have it all on board right off the bat. Who cares if the over all energy cost to get it out there is more in the long run…as in the short run the actually manned ship is less massive and easier to accelerate?

And as to my gross oversimplification…well yeah. Of course I’m oversimplifying. Do you WANT to get into a technical discussion of an Orion type drive? Of a scaled up Ion drive? Its not really relevant to the discussion I don’t think. FWIW I don’t think that any of these challenges would be easy to solve or trivial…I think that this would be the most difficult engineering project in the history of mankind…by several orders of magnitude in fact.

[QUOTE=Mangetout]
-It uses more energy in total you simply don’t save anything by sending your fuel ahead - on the contrary, it costs a lot more (in total) to do that.
[/QUOTE]

So what? Time and materials aren’t factors. It it takes me double, triple or more energy to send them out that way, but if I do it over decades or centuries, so what? I think sending out the fuel and other supplies ahead is a good idea to save on the final mass of the manned ship (which will be massive enough)…as it is accelerating the manned space ship will be challenge enough. It also doesn’t put all your eggs in one basket. Sending out 1000 (or 2000, or 10,000) resupply ships cuts the final mass of the manned ship by some percentage of that factor (1000, 2000, 10,000…which means less initial energy for the manned ship to get it going and keep it accelerating since there is less mass TO accelerate).

The resupply ships don’t need shielding, don’t need life support or crew space, etc. You can send out a large redundant percentage as well in case of a failure of some percentage of the ships. Velocities can be calculated so that we will know how fast the relative velocities will be for each resupply ship when the manned ship catches up. Certainly the actual capture would be a serious challenge (I was thinking of a combination of a shuttle craft and some kind of accelerating tether that can slowly take the strain of the the resupply ship while bring it up to the relative velocity of the manned ship…then reeling it in.)…and I admit, I don’t have an answer to that one myself. Doesn’t mean its impossible though.

I’m also one VERY former aero-space STUDENT (I didn’t even graduate with my degree in aero-engineering…I switched to EE and computer science). I don’t believe The World™ will be relying solely on MY plan to get us there. Given the parameters of using our current technology to get a live crew to the nearest star, with no other constraints, perhaps the bright minds WOULD choose to make one massive ship with everything they would need and then figure out how to get it out of orbit and accelerating toward its final goal. My point was that if we don’t factor in time or budget it IS possible with our current technology from a purely technological perspective. It might be highly improbable and risky in the extreme (maybe one chance in a hundred or even a thousand of the mission succeeding)…but its not equivelent to going to stone age man and asking them to produce viable jet powered flight.

Its certainly impossible from a political and economic standpoint of course because the project would cost trillions and would take decades or centuries just to launch…let alone the thousand odd years to get there.

[QUOTE=Mangetout]
-It greatly increases the potential points of critical failure
[/QUOTE]

Certainly. It also doesn’t put all your eggs in one basket if there are problems in the thousand plus year journey (like say if all the plants dies off for some reason). It also reduces the over all mass of the manned craft, since it will be able to dump useless mass (waste, spent fission fuel, etc) and pick up new mass from the resupply ships enroute, staying fairly constant.

Maybe the resupply aspect is too complex and too risky…who knows? It was just a thought I had to reduce the initial mass and also provide some redundancy and fail safe to the supply issue. As I said above, I doubt the big brains are going to come to me and slavishly follow my brilliant plan. :stuck_out_tongue:

[QUOTE=Mangetout]
-It doesn’t actually address any genuine problem.
[/QUOTE]

It addresses several. It also causes some problems as well. Ain’t engineering great?

-XT

152

[QUOTE=xtisme]
Keep which fuel usable? The 1MT nuclear bomblets? The fission fuel for the reactors? I don’t think 1000 years is too much for stabalized fission materials to stay usable. Is it? Are you saying there is no way to keep non-active fission materials usable over that time period?
[/QUOTE]
There are increasing grave concerns about the usability of our existing stockpile of boosted fission and fission-core weapons after only a few decades. Nuclear materials like [sup]235[/sup]U and [sup]239[/sup]Pu are constantly undergoing decay and producing more unstable isotopes like [sup]239[/sup]Np and [sup]240[/sup]Pu which make the device unreliable at best (prone to fizzle) and useless at worst. Other components in the device will eventually be embrittled or transmuted by neutron radiation, making it unreliable at best. 1000 years is well and beyond any usable lifetime for enriched nuclear material and bombs to be stored.

There’s another issue: you keep referring to 1MT [fission] bomblets; about the maximum theoretical yield for even a boosted fission device is about 500kT; a larger yield requires the multistage thermonuclear fusion process, and both generally require tritium, which is highly unstable (decay half-life: 12.3 years). Admittedly, you can “breed” tritium (and this is generally done in situ for the successive stages of a thermofusion device by enhancing and reflecting the initial neutron flux) but that’s an additional cost, complexity, and risk to your 1000 year mission.

The efficiency of nuclear fission pulse propulsion is quite low, too, so even with high yield devices you are going to lose the bulk of the energy to space. (The major advantage of the Orion concept is that you can achieve specific impulse levels that are unobtainable via chemical rockets, and that it scales up indefinitely, but it’s really quite primitive in propulsion terms.) I don’t think a single major authority on advanced propulsion schemas considers Orion adequate for interstellar transit.

That you aren’t aware of these issues underscores the general objection to your blase attitude; it is as if you don’t know it to be a problem, it must be a nonissue, to be dispensed with by handwaving. It’s possible–perhaps even likely–that we’ll someday leave the Solar System and explore and, in some form, colonize other systems. But not with any conventional technology; the scale of the problem is simply enormously beyond our current capabilities and experience.

Stranger

153

[QUOTE=Stranger On A Train]
That you aren’t aware of these issues underscores the general objection to your blase attitude; it is as if you don’t know it to be a problem, it must be a nonissue, to be dispensed with by handwaving. It’s possible–perhaps even likely–that we’ll someday leave the Solar System and explore and, in some form, colonize other systems. But not with any conventional technology; the scale of the problem is simply enormously beyond our current capabilities and experience.
[/QUOTE]

Too right…because you know, if I can’t come up with a plausable way, and if I don’t know how to solve all the problems or am aware of the underlieing issues, then that should close the case. and of course, if YOU can’t either…well, its demonstrably impossible then. :stuck_out_tongue:

You win Stranger. Because a mid-40’s aged network engineer can’t give you a plausable scenerio and since you can’t think of one yourself, its impossible with todays technology regardless of the level of effort, time and economic resources thrown at it. Guess we might as well head back toward those cave men 20k years ago and see how they are doing on jet powered flight…

-XT

154

I don’t see why humanity must go to the stars in order to survive. There’s a lot of middle ground between languishing on a ticking time bomb and colonizing a completely different Solar System, isn’t there? The problem with all this star talk, IMO, is that we spend a lot of time talking about technology that only exists in science fiction instead of looking at other, more practical strategies for extending the life expectancy of our race. It would be unfortunate if we spent billions of dollars and manhours trying to create a warp speed flux capacitor-type gizmo so that mankind could someday reach Alpha Centauri, and then have a Doomsday asteroid take us out before we even got around to doing something constructive with the moon.

But maybe I’m talking out of place, as this thread makes me feel like I just walked into a Trekkie convention without paying the registration fee.

155

[QUOTE=xtisme]
Too right…because you know, if I can’t come up with a plausable way, and if I don’t know how to solve all the problems or am aware of the underlieing issues, then that should close the case. and of course, if YOU can’t either…well, its demonstrably impossible then. :stuck_out_tongue:
[/QUOTE]
XT, this whole sub-discussion was started because you asserted that interstellar human spaceflight is possible with today’s technology. We’ve asked you to demonstrate your assertion, only to get your simplistic answer that all it takes is enough money and time. You’ve been presented with real technical roadblocks, especially by Stranger, and now you’re switching to the victim-talk. You made the assertion, it’s up to you to demonstrate it. It’s obvious to everyone now that you can’t.

156

[QUOTE=you with the face]
I don’t see why humanity must go to the stars in order to survive. There’s a lot of middle ground between languishing on a ticking time bomb and colonizing a completely different Solar System, isn’t there? The problem with all this star talk, IMO, is that we spend a lot of time talking about technology that only exists in science fiction instead of looking at other, more practical strategies for extending the life expectancy of our race. It would be unfortunate if we spent billions of dollars and manhours trying to create a warp speed flux capacitor-type gizmo so that mankind could someday reach Alpha Centauri, and then have a Doomsday asteroid take us out before we even got around to doing something constructive with the moon.

But maybe I’m talking out of place, as this thread makes me feel like I just walked into a Trekkie convention without paying the registration fee.
[/QUOTE]

Overall, what you say is 100% correct. The only thing you are missing is the conversation went from some ideas over someday getting to the stars to a more specific debate over the ability to do it now. I think most of us in this thread agree that we can and should colonize the solar system. This would be a first step and that colonizing out of system would come later. Once we learn to live in near space, we will have eliminated many of the technical hurdles to a generation ship. What we need is fusion power or some similar very efficient power system and a propulsion system that can get us up to some decent percentage of the speed of light. Many think we are far from this and some think we may be closer. I seem to be in the middle myself.

The biggest wildcard is that we cannot predict the technologies that will actually be used. We are still learning much about physics and might luck into a pleasant surprise or two.

Jim

157

[QUOTE=CurtC]
XT, this whole sub-discussion was started because you asserted that interstellar human spaceflight is possible with today’s technology. We’ve asked you to demonstrate your assertion, only to get your simplistic answer that all it takes is enough money and time. You’ve been presented with real technical roadblocks, especially by Stranger, and now you’re switching to the victim-talk. You made the assertion, it’s up to you to demonstrate it. It’s obvious to everyone now that you can’t.
[/QUOTE]

:stuck_out_tongue: Right…'cause if I can’t back it up, it must be impossible. I’m not crying victim here Curt…I’m laughing at you guys to be honest. Because a network engineer with some half forgotten engineering from 20 years ago can’t solve it, then its impossible. Because you guys can tear holes in something I reeled off the top of my head one night, this makes you feel good. Well good! I’m happy for ya.

I remain unconvinced that with a maximum effort that it is ‘impossible’ to get a live crew to the nearest star. To me, the impossibility is not the technology but to very real economic and political constraints. I remain unconvinced that the analogy between a cave man 20,000 years ago trying to build jet powered flight corresponds to a group today attempting to build an interstellar space craft. The fact that you can tear my own sad (and simple as you pointed out) concept to shreds has not convinced me of anything more than I shouldn’t be on the design team when the unified earth exploration vehicle is designed and implimented. Though of course if you wish to call it XT’s Folly, feel free. :wink:

I started a GQ thread on the question to see if any of the big brains in there agree that, given the (unrealistic) constraints it really is impossible. I suppose if everyone in the thread over there comes back and says ‘nope, not possible mate’ then I will provisionally conceed defeat and slink off. Fair enough?

-XT

158

Stranger has done a far more compentent job of addressing your points than I could (although I would be trying to make exactly the same points - the ones you keep trying to brush off), but:
[QUOTE=xtisme]
The resupply ships don’t need shielding,
[/QUOTE]
So it won’t matter if they smash into something along the way?

159

While I generally agree with what Stranger on a Train is saying, I can see other ways humanity could expand to other stars.

For example, rather than building a ship in orbit from material shipped up from Earth or the moon, why not just dig in to an asteroid? An asteroid like Eros, 21 miles long and a few miles thick. Hollow out the center, and create a gigantic ecosystem maybe 100 cubic miles in size. Heat it and light it with nuclear power, and build a civilization inside it.

This civilization would live in the solar system for hundreds of years, proving out the ability to sustain itself, repair its own machines, etc. And eventually, if it decided to depart for another star system, it could use its own mass as reaction fuel, heating it to a plasma with a nuclear engine or throwing it backwards after accelerating it to a significant fraction of the speed of light, perhaps with an accelerator hundreds of miles long either built out the back of the asteroid or coiled around it. Acceleration would be very, very slow. Given the amount of mass available, it might take thousands of years to get anywhere. So you’d need a generation ship so big that it could house tens of thousands of people and enough parts and raw materials to sustain itself essentially forever. People would consider the asteroid itself to be ‘home’. Some might not even care if they ever arrive somewhere else, at least within the constraints of their nuclear fuel running out.

I would assume that such a trip would only be made under the most dire of circumstances, such as the impending nova of the sun, or evidence that some other calamity was going to strike the entire solar system. Because otherwise, why would you leave? Science Fiction aside, what is there to gain from travelling to another star system that you can’t get right here by terraforming the other planets, given that the effort involved is so tremendous? If you went to the trouble of creating a whole new ecosystem inside an asteroid, why not just continue doing that?

Another option is to send our seed to another star system. If we’re considering a multi-generation ship, and the people who leave Earth would never see the other star system anyway, why not just freeze thousands of fertilized human eggs and send them, along with automated facilities to revive them, hatch the kids, and raise them? Now you don’t need all the consumables, and your ship is much smaller. Furthermore, you can wait until your ship arrives at the destination and surveys it for habitable planets before the kids are uncorked. In fact, maybe they’re only uncorked ON a planet. If no planet suitable for colonizing is found, the probe never uncorks the eggs, and no one has to die or be marooned in another star system. And if the probe fails along the way or hits some interstellar debris, no people are killed.

The requirements for a probe like that would be orders of magnitude smaller. You could send dozens of them to the best candidate systems, hoping for a success. And you could take your time, leaving almost all the equipment shut down until arrival so it doesn’t wear out.

160

[QUOTE=Sam Stone]
The requirements for a probe like that would be orders of magnitude smaller. You could send dozens of them to the best candidate systems, hoping for a success. And you could take your time, leaving almost all the equipment shut down until arrival so it doesn’t wear out.
[/QUOTE]

Seed ships are interesting. It will take some enormous increases in robotics and failsafe computer redundancies for us to not only make the delivery but then raise and educate these kids without any adults around. I am not sure this would even be fair to unleash on the universe. :wink:

I like the idea of an asteroid ship. Start them as space colonies and build in the plans the ability to go interstellar. I am not sure how humanity would come together to build such ships.

For the first few centuries would they be good platforms for industry and yet still be safely habitable? Could they act as a giant apartment complex with Zero-G labs and factories in near orbits with them. This would allow the asteroid to spin to provide partial G and yet still gain the advantages of Zero-G.

Jim