Time dilation has long been understood, even if its effects are still mind-numbing. It was in 1963 that Carl Sagan laid out the idea of exploiting relativistic effects for reaching other civilizations. In a paper called “Direct Contact Among Galactic Civilizations by Relativistic Interstellar Flight,” Sagan speculated on how humans could travel vast distances, reaching beyond the Milky Way in a single lifetime by traveling close to the speed of light. At such speeds, time for the crew slows even as the millennia pass on Earth. No going home after a journey like this, unless you want to see what happened to your remote descendants in an unimaginable future.
Before Sagan’s paper appeared (Planetary and Space Science 11, pp. 485-98), he sent a copy to Soviet astronomer and astrophysicist Iosif Shklovskii, whose book Universe, Life, Mind had been published in Moscow the previous year. The two men found much common ground in their thinking, and went on to collaborate on a translation and extended revision of the Shklovskii book that appeared as Intelligent Life in the Universe (Holden-Day, 1966).
This one should be on the shelf of anyone tracking interstellar issues. My own battered copy is still right here by my desk, and I haven’t lost the sense of wonder I felt upon reading its chapters on matters like interstellar contact by automatic probes, the distribution of technical civilizations in the galaxy, and optical communications with extraterrestrial cultures.
Much has changed since 1966, of course, and we no longer speculate, as Shklovskii did in this book, that Phobos might be hollow and conceivably of artificial origin (the chapter is, nonetheless, fascinating). But for raw excitement, ponder this Sagan passage on what possibilities open up when you travel close to lightspeed:
If for some reason we were to desire a two-way communication with the inhabitants of some nearby galaxy, we might try the transmission of electromagnetic signals, or perhaps even the launching of an automatic probe vehicle. With either method, the elapsed transit time to the galaxy would be several millions of years at least. By that time in our future, there may be no civilization left on Earth to continue the dialogue. But if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after about 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization.
The songs of distant Earth indeed! An Earth distant not only in trillions of kilometers but in time. Memories of Poul Anderson’s Leonora Christine (from the classic novel Tau Zero) come to mind, and so do Alastair Reynolds’ ‘lighthuggers.’ Could you find a crew willing to leave everything they knew behind to embark on a journey into the future? Sagan had no doubts on the matter:
Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their own world, an adventure and a challenge certainly difficult to duplicate.
But while the physics of such a journey seem sound, the problems are obvious, not the least of which is what kind of propulsion system would get you to speeds crowding the speed of light. The Bussard ramjet once seemed a candidate (and indeed, this is essentially what Anderson used in Tau Zero), but we’ve since learned that issues of drag make the concept unworkable and better suited to interstellar braking than acceleration. And then there’s the slight issue of survival, which William Edelstein (Johns Hopkins) and Arthur Edelstein (UCSF) discussed at the recent conference of the American Physical Society (abstract here). The Edelsteins worry less about propulsion and more about what happens when a relativistic rocket encounters interstellar hydrogen.
Figure two hydrogen atoms on average per cubic centimeter of interstellar space, and that average can vary wildly depending on where you are. A relativistic spacecraft encounters this hydrogen in highly compressed form. Travel at 99.999998 percent of the speed of light and the kinetic energy you encounter from hydrogen atoms reaches levels attainable on Earth only within the Large Hadron Collider, once it’s fully ramped up for service. This New Scientist article comments on the Edelstein’s presentation, noting that the crew would be exposed to a radiation dose of 10,000 sieverts within a second at such speeds. Six sieverts is considered a fatal dose.
Traveling near lightspeed seems a poor choice indeed. The Edelsteins calculate that a 10-centimeter layer of aluminum shielding would absorb less than one percent of all this energy, and of course as you add layer upon layer of further shielding, you dramatically increase the mass of the vehicle you are hoping to propel to these fantastic velocities. The increased heat load would likewise demand huge expenditures of energy to cool the ship.
If travel between the stars within human lifetimes is possible, it most likely will happen at much lower speeds. Ten percent of lightspeed gets you to the Centauri stars in forty three years, a long but perhaps feasible mission for an extraordinary crew. If we eventually find shortcuts through space (wormholes) or warp drive a la Miguel Alcubierre, so much the better, but getting too close to lightspeed itself seems a dangerous and unlikely goal.
With the intensity of radiation approximately proportional to starship speed cubed [Mauldin] as pointed out this is a serious problem in the high gamma regime. Mostly likely a combination of active and passive shield systems would be used. Leaving warp drives and wormholes aside, if the propulsion system is developed to reach such high speeds, a solution to the above may also present itself other than current methods outlined.
Cheers, Paul.
“If travel between the stars within human lifetimes is possible, it most likely will happen at much lower speeds. ”
I agree. What’s the current state of the art for suspended animation techniques?
ok everybody it just so happens that this article and hopefully your answers will be the answer to a prayer. do you know why i have been hocking about the solar system being full of sophisticated spacecraft in about 75 years? it is because i read that alot of fine minds think that light speed or better is indeed an unachieveable goal,so i was looking for something really great that imho we COULD achieve! however having said that i also see that even a percentage of light speed will be fantastic breakthrough that could easily be well used for achieveing many fine goals also! lol i am so busy with other things i almost didn’t feel that i had time to write even this reply at this time.however i hope that this will soon be posted and that MANY anwers will soon be posted below it! maybe i have cause for new hope.respectfully to one and all and i hope i will soon be reading your ideas on these subjects! i think a good time is ahead for all of us. your friend george
Mark here,
Hi Paul,thanks for the article. I recall reading Shkloviskii ‘s book back in early 1970….I found it in a bookstore in West Berlin…..the sense of possibility opened a new world…..in the fiction realm Macroscope by Piers Anthony
suggesting a novel form of inter,intra galactic internet….
Best,
Marc
Mmmm… I wonder if the novel “The Songs of Distant Earth” by Clarke, gets its name from that phrase by Sagan…
Yeah, I think 10% of c is feasible for avoiding the excessive radiation from the impact of interstellar hydrogen. 10% c make the nearest stars about 4 decades away, which is a short enough period that you do not get too far behind if the boys back home develop hyper-drive or the wormholes.
Various speculated fusion systems can get you to 10% c.
How much simpler all these issues of radiation and time would appear if human intelligences were instantiated digitally instead of in flesh. Then we could travel not only at the speed of a beam of light, but _as_ a beam of light. There would still be risks, of course- no signal can be guaranteed to reach its destination intact.
But still, perhaps our interstellar pathmakers will be neuroscientists, not engineers.
I’ll just wait on Earth until the receiver-probe has decelerated into orbit about our target star, and only then begin my journey. I won’t dream as I travel, not even of centauri, but I think it is fair to say that my stream of bits falling out into the void, representing the state of my mind when I left, would be a hopeful one.
Hi Folks;
Perhaps magnetic fields of strengths on the order of between 100 Tesla and 10,000 Tesla can be set up around the craft either by exotic superconducting electromagnets, or perhaps by some exotic form of permanent magnet made of atomistic materials that are not of the periodic table types.
If a strong enough magnetic field can be deployed, then the interstellar hydrogen atoms will take on a dipole moment and can be directed away from the space craft and/or into a safe repository where they can be used as hydrogen fusion fuel to assist space craft acceleration. Charged matter could be diverted by a magscoup and/or an electric field scoup type of mechanism.
Then there remains the possibility that the ship could be cloaked in a matter wave cloak which would be the matter wave analogue of a negative electromagnetic index of refraction meta material cloak. However, note that the negative EM refraction index metamaterials are only operative on wavelenghts of EM radiation that are on the half order or greater magnitude larger than the active cell size of the materials, wherein the cells each consist of a conductive inductor coil like loop embedded in a dielectric base material.
Now, EM negative refraction index materials would have the peculiar property such that they are pulled forward by the impinging EM radiation, and so such materials might make an excellent pull sail that would be gainful amidst the incoming CMBR and star light instead of being push by such star light. Note that the full scope of the classical electromagnetic theory ramifications of negative EM index materials have yet to be worked out, and so classical electrodynamics has just recently received a whole new territorial field on unknowns and potentialities.
See the following link for some cool info on negative EM refraction index metamaterials.
http://people.ee.duke.edu/~drsmith/pubs_smith_group/padilla_materials_today_2006.pdf
Perhaps, the space craft shielding could simply be composed of a several kilometers to many kilometers thick frontal shield that would extent radially out beyond the foot print of the crew cabin and hull of the space craft thus enabling extreme gamma factors for a long train like space craft.
Alternatively, the shield might be composed of some yet to be devised super dense neutron crystal like materials or perhaps some sort of quarkonium matter that more or less is similar or differing from theoretical stable strangelet matter. Such a material might make an excellent shield. A 10 EXP – 15 meter thick sheet of neutron dense crystal materials would have a mass specific area of only 1 metric ton per square meter. A 10 EXP – 12 meter think sheet would have a mass of only 1,000 metric tons per square meter. Using even 1,000 square meters of such materials to shield the front of the space craft seems highly doable if such materials can be manufactured.
Note that some sources have reported that perhaps certain neutronium crystalline materials might be stable from neutron decay, even out side the environments of the extreme pressures and gravity fields of neutron stars.
Perhaps such neutron dense crystalline material, or perhaps stable strange material, somehow stable bottomonium, charmomium, or even higgsinium could be fashioned into stable matter wave cloaks, or even matter wave and EM wave pull sails or pull surfaces so as to be operative on extremely blueshifted incident matter waves and EM waves.
One way to potentially deal with hard cosmic rays relative to a ship shield using ordinary periodic table atomic and molecular matter based materials is to have a layered shield that is thick enough to cause all incoming particles to undergo collisions with sheild atoms. The massive jets of decay products, would then be diverted by electric and/or magnetic fields set up in vacancies between the shields layers. Neutral particles that were produced but which somehow passed through an initial layer would be subject to transformation into decay jets, which could include charged species, which could then be diverted by other layers, and the process could repeat itself until all of the energy was absorbed before it could irradiate the crew members or sensitive ship equipment. Nanotech self assembly repair mechanisms or other microbots could continually refashion the shielding components including field generation components to compensate for radiation damage.
A good bulk sheild material might be pure diamond, perhaps improved forms of diamond that have higher heat conductivity and better refractive properties then the best natural diamonds. Such improved diamond is theoretically possible (and has been produced in limited quantities) by altering the ratio of carbon isotopic composition of the diamond, which accordingly manifest itself in artificial diamond with improved characterists.
We should not give up hope in the potential joys of extreme gamma factor interstellar manned travel with all the mystique and potential mysteries that such might offer as may have been overlooked in traditional Special Relativity.
Either way, I will happily settle for the 0.7 C of the ISV Venture Star bound for Pandora. They might even persuade me to undergo suspended animation, or hibernation, which are other ways of dealing with long voyage transit times. Personally, however, I rather be awake during the voyage.
The arching sky is calling
Spacemen back to their trade.
ALL HANDS! STAND BY! FREE FALLING!
And the lights below us fade.
Out ride the sons of Terra,
Far drives the thundering jet,
Up leaps a race of Earthmen,
Out, far, and onward yet —
We pray for one last landing
On the globe that gave us birth;
Let us rest our eyes on the fleecy skies
And the cool, green hills of Earth.
Robert A. Heinlein
Then why not go there in a spacecraft the size of a 50 pence piece instead? That way, even if you don’t reach your destination, you can still wander the universe until you get caught into orbit somewhere. Or fall into a black hole.
I imagine 40% of c will be the velocity used. At that speed, the gamma factor is about 1.1. Travel to Centauri would take just under 11 years from the perspective of people not on the craft, so the people onboard have to survive for a decade. Going to Eridani would take longer – 25 years – but possibly enough to make round trips worthwhile if we have life extension.
Still, I’m hoping for a combination of medium (0.4c) starcraft (ideally cyclers), life extension, and nearby brown dwarves and rogue jovians. If we have to go 1ly between stops, at 40% of c that’s still only 2.5 years. Given healthy lifespans of 150+, I’m sure people will trade.
Hi All
My first reaction to the “New Scientist” piece was to write an angry blog-post, but then I realised that such gamma-factors run up against the thermal glow of the galaxy and the CMB red-shifted into a white-hot blaze. It’s not just the proton radiation we have to worry about too. Dust, cosmic-rays and so on, all get focussed & intensified by relativistic abberation as well as the blue-shift. Essentially a “hard wall of light” forms, making such extreme speeds unhealthy. So I’m inclined to agree with the Edelstein’s, though James Essig’s suggestions of ultra-dense matter shielding may well be the ‘unobtainium’ miracle needed to ultimately achieve such.
Interestingly Alastair Reynold’s fictional “House of Suns” deep future view has maximum speeds of a mere 0.9999 c, even though they’re protected via some kind of space-time ‘interdict’ shield – though it’s hard to imagine what could power million ton starships doing 1200 gees at 0.999c…
Jefferson: suspended animation is medium-good for cells and early zygotes and embryos, that’s it so far — even if you talk to the evangelists of Alcor et al.
“Boys back home”? To say nothing of the single gender in the Heinlein ditty. Maybe the girls will have something to contribute, too, boys. Plenty of us in the neurosciences and suchlike soft stuff that may get us to Proxima and beyond, when the rah-rah methods fail or never achieve liftoff.
kurt9,thank you very much i had not to my knowledge,read before,of fusion systems getting us 10% c ! sounds real good but as always… i wonder how long it will be before this is an accomplished fact.however as i have stated above – i am pretty sure that in 75 years this will be quite possible.thus making my ideas better than even i had dreamed.we will then have both sophisticated spacecraft all over the solar system…and… a basic form of star ship! thank you very much again.your friend george
Hi Folks;
Regarding radiation shielding, all that would be necessary is a few hundred meters of Aluminum in order to adequately shield the crew from the 99.999998 C radiation.
If a ten centimeter thick shield can attenuate the radiation by one percent, then a 10 meter thick shield can attenuate the radiation to a fraction of (0.99) EXP 100 = 0.366 of its impinging intensity; a one kilometer thick Aluminum shield can attenuate the radiation to a fraction of (0.99) EXP 10,000 = 2.249 x 10 EXP – 44 of its impinging intensity, thereby making the radiation intensity problem a complete moot point.
The shield might alternatively take the form of a huge thin walled cylinder, perhaps constructed out of carbon nanotube materials, or boron nitride nanotube materials, that is filled with an atmosphere that remains compressed as the space craft accelerates, much as the atmosphere on Earth, or even as the hot atmosphere of the planet Venus, remains compressed. The atmosphere, optionally in conjunction with a mini-magnetosphere could provide shielding of the craft from impinging hydrogen atoms, helium atoms, and ions and electrons.
Perhaps a large superconducting coil electro-magnet, or highly conducting conventional conducting coil electromagnet can be deployed to produce a significant magnetic field in order to divert the incident charged plasma. Forward aimed micro-lasers or nano-lasers could ionize the interstellar and intergalactic neutral atoms thereby rendering them divertible with respect to the space craft crew quarters and electronic control systems.
Thermal energy phonon quasi-particle conductors have been proposed as theoretically possible to greatly enhance heat conduction over traditional thermally conductive materials. If the energy impinging on the front of the space craft could be conducted very rapidly away from the front of the shield and collected and exhausted out the back of the space craft such as by a photon rocket, electron rocket, ion rocket, or even a neutrino rocket whereupon, the frontal shield would have a temperature maintained below the impinging radiation temperature, perhaps even below the ambient CMBR temperature, the craft should in theory, be pulled forward by the radiation imbalance in an effectively perpetual manner so long as the process remains operative. Phonon conductor can in theory conduct heat from cold to hot material portions.
BTW Haplo, the original Clarke story was published in 1959, while Sagan’s first paper was a bit later. Clarke was the inspiration if anyone.
I doubt that Ma Nature will allow a spacecraft to travel faster than 0.05 c. I also doubt that nuclear fusion will work for anything with ignition temperatures higher than deuterium-deuterium. Antimatter propulsion, worm holes, Bussard ramscoops, ruby slippers, etc. are examples of idle fantasy. So that defines the design envelope, i.e. the fuel is deuterium and the transit time is on the order of one century. Come up with a working design and let’s have a conversation (I’m optimistic that there is a solution).
well… Perhaps is soon to discard this kind of travel.
Whats the problem of the shield is to thin? It generates too much gamma radiation?
Then, separate the shield and the rest of the ship. Like a gigant pencil… if very light union is enough to prevent it lost, it could be enough far to dissipate a lot of energy (because only direct radiation would cross the same section.
Perhaps using lasers a lot of atoms could be ionized and with magnetic fields be deflected…
It could exists a lot of ideas that would prevent the problems of near-c travel. We only need one that really works.
I second Spaniard above. Separate ship and shield.
My thought is for the ship to be preceded by shields in series. Each shield is basically a solenoid. Its magnetic field will scatter a fraction of incoming ions; at the gamma factor for the speed we’re talking about (1/sqrt(1-(v/c)^2) = 5000), I’d expect the field to ionize hydrogen atoms by itself.
Remaining will be primary atoms which pass the shield, plus a cone of secondary particles: gamma rays, pions and muons, and atoms sputtered from the shield. At a distance sufficient for the cone to dilute the secondaries, we place the second shield, and the process repeats. We keep the shields separated with lasers or particle beams. If we can get the convoy up to 0.99999998 c in the first place, I presume we can expend that much extra energy.
We already know how to shield astronauts from cosmic ray impacts that range from 40% of the speed of light to 99.6% of the speed of light. All you need is a few meters of mass shielding: liquid hydrogen, polyethylene, or water. 87% of the speed of light will cut relative time an board in half. And 99.5% should cut time on board by about ten times. That should be plenty enough speed to get to a nearby star systems within 10 parsecs radius within a reasonable amount of on board time.
That should give us access to several hundred star systems. Beyond that, those future star colonist will have to make their own expeditions outward.
““Boys back home”? To say nothing of the single gender in the Heinlein ditty. Maybe the girls will have something to contribute, too, boys.”
My use of the word “boys” is colloquial, just like the word “man” in mankind or salesman. Don’t be so touchy about this. This kind of sensitivity really becomes tedious after a while.
“Plenty of us in the neurosciences and suchlike soft stuff that may get us to Proxima and beyond, when the rah-rah methods fail or never achieve liftoff.”
I have no idea if there will be any breakthroughs in propulsion. Most likely not. My point is that IF there are any such breakthrough, 40 years is not such a huge amount of time to loose if the people back home get there first even if they leave after you.
@Gary Allen
http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
http://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
http://en.wikipedia.org/wiki/Fission-fragment_rocket
Given your previously demonstrated dislike for both nuclear energy, and high power renewable energy, and now skepticism about interstellar travel, I think that you simply want the mankind to get stuck at pre-industrial level for good. I think that you are simply biased against technology en bloc, and your desired future of mankind is to return to imagined pre-lapsarian tribal past.
Kurt9, I know all about “default” terms and representations. I would be very happy not to have to waste my time and energy flagging unthinking happy obliviation – – which is not only tedious but also discouraging.
T U T thankyou very much for the listings above that contained the excellent references to spacecraft propulsion,one of my favorite subjects but below and correct me if i am wrong you seem to indicate that you think that mother nature will only allow a speed of about 5% of the speed of light,if i may ask,…why? just read and even i think mentioned in one of my responses that 10% c would be a great thing! the best way to cover really VAST distances naturally will have to be faster than c ! naturally it is a bummer that some guy,lol, i think his name was a. einstein or something,already disagrees with this concept! we need i guess to “sneak around” the concept of just going flat out faster than c by developing either warp drive or wormholes i guess! well, thank you very much your friend george
“I know all about “default” terms and representations. I would be very happy not to have to waste my time and energy flagging unthinking happy obliviation – – which is not only tedious but also discouraging.”
Whatever. If the shoe fits, wear it.
I like Athena am not optimistic about suspened animawe can option but I do think there will be sucess in pushing the lifespan to maybe 150 realtively healthy. That makes unmanned or manned probes at 10 percent light similar to what an outer solar system trip is today.
Based on the technolgies TUT has posted its really a matter of willpower and we have developed a defeatest attitude that is really really depressing
Ah, the devastating argument of “whatever”. Heh heh.
Well, there has to be a happy medium, if we spend 40 years getting to Alpha Centauri it increases the likelihood of being irradiated by high energy cosmic rays. So something faster than 10%C would be needed but less than 99.999998, giving us some room to work with.
Hi All
Gary, why do you think 0.05c is the limit? What about the pellet pusher system that Gerald Nordley has discussed in various papers and hard SF stories?
T_U_T,
The fission fragment rocket paper that you linked is interesting. I looked at Clark and Sheldon’s AIAA paper and have tried to make sense out of it. It looks like Clark first published this concept back in 1988. Right now I’m trying to figure out how this concept is different from a gas core nuclear fission rocket (the so called “light bulb” rocket). Some NASA reports on gas core nuclear fission rockets were published in the 1960-1970s. The concept didn’t get anywhere because people considered it “too hard”.
You are mistaken that I am against nuclear energy. I am very pro-nuclear energy and consider power from nuclear fission to be the best response to Peak Oil. However I am against the specific nuclear technology of liquid metal breeder reactors because of the liquid sodium / water heat exchanger safety issue. I have no basic issues with gas cooled or water cooled reactor cores. Also I am very skeptical of so called alternative “green” energy schemes where the EROEI is low and the numbers are often cooked by zealots and salespeople.
I remain guardedly optimistic that interstellar travel is possible. Unfortunately I have seen no practical scheme that can achieve velocities greater that 0.05 c. The best interstellar spacecraft proposal that I have ever read was the British Interplanetary Society Daedalus study. However that design was fatally flawed by basing its fuel cycle on He-3 which has an impossible ignition temperature. IMHO, a valid design would start with the Daedalus study and morph it into a scheme that could run on a deuterium fuel cycle. I believe(?) such a concept was proposed at Lawrence Livermore during the 1980s.
Gary, you would be not be able to get out of the bed if you declared it out of hand “too hard”. ( imagine all the stability issues with upright walking, complicated sequences of muscle contractions needed to stand up, very fast and very sophisticated feedback loops to deal with all possible contingencies, like sudden muscle cramp, slipping, stepping on something under the bed, etc… ).
And, at any rate, fission fragment rocket is something completely different from nuclear lightbulb. A nuclear lightbulb contains the fission core by physical barrier, ionized dust is contained by electromagnetic containment. nuclear lightbulb works at extreme temperatures, fission fragment rocket at moderate temperatures and low pressures. Nuclear lightbulb works as conventional rocket engine by heating thew working fluid and expelling it through the nozzle. The working fluid in the fission fragment rocket are fission fragments that leave dust particles at high velocities and are steered to the back by electromagnetic fields. The only thing that those propulsion methods share is that they use fission energy.
Modern designs use fluoride salts which are as unreactive with water as it gets, have large negative void coefficients, low vapor pressures even at high temperatures, the fluorine in them is capable to bind to all fission products, and have much higher boiling point so the reactor can work at a higher temperature.
Both medusa style nuclear pulse propulsion and fission fragment rocket can achieve velocities higher than 0.05 C. with Isp in the 1e6 range it can comfortably reach 0.1 C.
yes greg as you denote above i am positive that we DO have alot of room to work with!!! there is no doubt.thank you very much once again. your friend george
T U T,again…5% c IS quite a velocity! that we can not do it for now. – i am amazed! don’t know what else to say for now.however i am sure that these problems can be solved so check me tomorrow! respectfully george
5 % c is of course quite fast, but the real limit of fission propulsion is somewhat higher. Say 10 % c.
Kenneth
Hello everybody. A couple of observations for James Essig and Adam as well as a question for Athena and David W. First James and Adam. James your math is usually very sound and it is certainly better then mine. When I do the calculations and go through all of the engineering puts and takes of your shielding options as well as the potential mass implications within the context of both the original Edelstine calculations as well as Adam’s outstanding additional observations I come to the conclusion that about 70% light speed is doable from a protection perspective assuming that we can develope a vaiable “magic propulsion system”. In essence, it seems that about .7C is the proverbial “knee of the curve” that represents the best balance between protection and vehicle mass and therefore cost effectiveness assuming a human crewed vehicle. Is this what your math is showing or do you think that the knee in the curve can be pushed higher say to 85%-95%? Of course, much of this is assumption dependent, but it seems to me that cruising at .7C requires only 4 or 5 technical miracles that may be possible in the next ~100 years, but pushing much beyond .7C requires a true “leap of faith” unless of course there is breakthrough physics that fudamentally redefines the entire problem in ways we can’t even speculate on now.
Athena and David W. I know that Athena is skeptical about even getting to a 150 year life span for humans, at least within the next 100 years or so and you seem to imply that 150 is achievable, but much beyond that going to be a very hard push. My question is this. If you concede for the sake of argument that we will find a way to achieve 150 year life spans for Humanity, why not 200 years, 300 years or even more until you reach 1000 years where the likelihood of catastrophic accident becomes the big killer? Is there something that at least conceptually happens of significance at around 150 years that may prove to be a major limiting factor so that scaling up beyond 150 will be a project for many centuries to come?
I ask this because I once read a paper years ago by a medical researcher who believed in radical life extension, but thought that while 150 was possible and achievable that once past 150 it would be the Human Brain that would prove to be the real limiting factor on aging even in a fully rejuvanated body. Any thoughts on 150 years becoming the new “Hayflick limit” for Human Life Spans assuming of course that we can even reach the 150 mark or is 150 just an arbitrary number.
Thank You
T_U_T said: “And, at any rate, fission fragment rocket is something completely different from nuclear lightbulb. A nuclear lightbulb contains the fission core by physical barrier, ionized dust is contained by electromagnetic containment. nuclear lightbulb works at extreme temperatures, fission fragment rocket at moderate temperatures and low pressures.”
We both got it wrong. I confused the lightbulb gas core fission rocket with the vortex core gas fission rocket. The vortex core concept seems to have the possibility for a high specific impulse. The lightbulb concept seems to be a no-starter because the transparent barrier containing the transuranic fuel would ablate away. T_U_T got it wrong when he said that a fission fragment rocket had moderate temperatures. How could it possibly have moderate temperatures if it has a specific impulse of over 1,000,000 seconds? If one wants to believe the Wikipedia article (normally a mistake), one can read:
“By physically arranging the fuel such that the outermost layers of a fuel bundle will be most likely to undergo fission, the high-temperature atoms, the fragments of a nuclear reaction, can “boil” off the surface. Since they will be ionized due to the high temperatures of the reaction, they can then be handled magnetically and channeled to produce thrust. ”
What I find appealing about the fission fragment concept is in having the exhaust gas coming directly from the fission reaction (no Carnot efficiency issues). However this situation is essentially the same with a vortex core gas fission rocket. Unfortunately vortex core gas fission rockets were abandoned because they were too hard to do. A quick Google search indicates that the fission fragment concept hasn’t got past the paper study phase. This is no big surprise because the thing works by spewing out highly radioactive fission product exhaust. The only way the scheme can be tested is in deep space or in an underground cavern that’s serviced by robots.
Clark and Sheldon in their AIAA paper expressed doubts about the feasibility of the fission fragment rocket for interstellar travel when they said:
“… a 50 year trip to Alpha Centauri, 4 Ly distant, is probably not feasible, requiring a 208 GW reactor, and consuming 240 tons of fission fuel.”
I would disagree. By definition, star ships are big and require huge amounts of power. This is the most feasible concept yet that I’ve read for an interstellar propulsion scheme (again, the Daedalus propulsion concept assumed an He-3 fuel cycle which in my opinion is bogus). However this raises a red flag. Why is it that a nuclear fission propulsion concept claims it can approach 0.08 c while a nuclear fusion concept claims to have only slightly better performance? Were the guys behind the Daedalus concept too conservative or are Clark and Sheldon too optimistic? One possible explanation is the exhaust gas from a fission rocket would have a higher density for a given exhaust velocity so its momentum transfer would be greater than a propulsion scheme based purely on helium exhaust.
I’d like to see a review article on the Clark and Sheldon concept by a nuclear engineer or physicist.
Adam said: “why do you think 0.05c is the limit?”
I’ve been reviewing interstellar propulsion concepts for over 30 years. Whenever they claim velocities of greater than 0.05c, a show stopper error will pop out as soon as I do the math or review the assumptions. Also, when you get past 0.05c, relativistic effects start to get serious and the energy requirements become ridiculous. The classic example is the Bussard Ramscoop. A million years ago when I was a naive undergraduate, I discovered Bussard’s paper and thought this was ***The Answer***. I actually rederived all his algebra and could find no fault in it (Bussard was brilliant!). I then took the concept to some EE professors and they pointed out that no real material could withstand the field strengths and Bussard assumed a proton-proton fusion reaction which was a silly/asinine/dishonest assumption. Finding out that Bussard had deliberately deceived me with the proton-proton assumption was a real disappointment.
I remember growing up reading about the Daedalus study. I’m surprised that in the years since then more realistic designs for interstellar probes haven’t been developed. Can anybody point me to any more recent concepts? What is the current ‘state of the art’ for a near future interstellar probe?
Interesting post. My general impression is that interstellar travel will involve spaceships traveling at some percentage of c, or foldspace/wormholes to make large jumps. Being able to skip over large portions of space-time would be the best way of making fast interstellar trips and going further around the galaxy. Without that, the limitation of c will keep us to long-term voyages to stars within 20-40 lightyears.
I’m confident that we can achieve 10-20% c propulsion. The higher the fraction, the better, and anything in the range of 70% c would be great. As you get very close to c, physics get very difficult, so I don’t think we stake too much on getting up to 99.9% c when you can just reduce your speed by 10% and make matters far easier. The lorentz factor, while manageable in lower percentages, jumps skyhigh when you get close to c.
Long term voyages from 10-50 years are acceptable for interstellar travel, especially if we come up with hibernation/suspended animation. Any space journey lasting decade(s) would have people going into deep sleep, perhaps with rotating shifts of being awake to maintain things onboard.
Space colonization will create distances and delays between people, enforced by the hard limit of c. Even with wormholes/foldspace, each jump would be challenging and costly and not routine. But distances between people living in a space archipelago of “islands” consisting of planets and space stations is not necessarily a bad thing – it has many benefits according to my sociopolitical views, which I wont expound on now (this post is long enough lolz)
By the way, in regards to preserving our civilization – I’ve heard of M-ARC discs by the Millenniata company which, unlike most digital media, can last for a very long time – in fact theyre designed to last for 1000 years with no data loss! Some variants of the technology could probably last even longer to an undeterminable length. That could create a fine snapshot of our culture for the distant future.
Luke wrote:
Luke, since you mention Daedalus, you may be interested in its successor, the Project Icarus study now in progress:
http://www.icarusinterstellar.org/
Otherwise, the Matloff, Johnson and Vulpetti book Solar Sails: A Novel Approach to Interplanetary Travel gives a recent account of sail concepts and includes interstellar ideas. Our archives contain articles on many of these notions and more radical interstellar designs like fusion pellet ‘runways’ and particle-beam propulsion, etc.
Actually, I did not get it wrong, it is the wikipedia article that is slightly misleading. Fission fragments in the core are not allowed to thermalize. The fuel grains are too small to contain them, so that the majority can escape and can be steered by the magnetic field. The fuel grains remain relatively cool all time, mostly because the fission fragments escape without imparting much of their energy to them, and because their extremely high surface area allows them to quickly emit all thermal energy away. Thus the temperature of the working fluid composed of high temperature high ionization nonequilibrium plasma is extremely high while the temperature of the core remains relatively low all the time.
Shklovskii may not have been entirely off about Phobos.
See this recent ESA post about Mars Express exploring the Martian
moon from very close distances here:
http://www.esa.int/esaCP/SEM4Q1NEG5G_index_0.html
To quote:
“Previous Mars Express flybys have already provided the most accurate mass yet for Phobos, and the High Resolution Stereo Camera (HRSC) has provided the volume. When calculating the density, this gives a surprising figure because it seems that parts of Phobos may be hollow. The science team aim to verify this preliminary conclusion.”
A good reason to get an expedition to the Mars system real soon, IMHO.
With some drilling equipment.
Earlier Adam asked: “why do you think 0.05c is the limit?”
I responded: “…when you get past 0.05c, relativistic effects start to get serious and the energy requirements become ridiculous.”
I should have responded “… when you get past 0.05c, relativistic effects coupled with the Rocket Equation start to get serious and energy requirements become ridiculous.”
A good example of this can be found in the following link (scroll down to where energy conversion fractions are related to specific impulse):
http://www.statemaster.com/encyclopedia/Relativistic-rocket
Note the standard claim that electron-positron annihilation has an Isp/c ratio of one. This is the so called “photon drive”. However this is an idealized result, i.e. all of the photons from electron-positron annihilation are sent in one direction as a laser beam. In a “practical” photon drive, a significant fraction of the electron-positron annihilation would radiate in all directions as black body radiation (the radiator would be damned hot!) or gamma radiation leaked from the combustion chamber because there really is no way annihilation energy could be perfectly transformed into a laser beam. A variation of this basic problem holds with a nuclear fusion propulsion scheme. Not only would a significant fraction of the energy be lost as black body radiation but also as fast neutrons. The fast neutron issue was why the British Interplanetary Society for the Daedalus study opted for the He-3 fusion reaction because it produces mostly charged particles despite the show-stopper ignition temperature requirement. The linked article shows an Isp/c ratio for nuclear fusion to be 0.119 versus 0.04 for nuclear fission. This brings back the earlier puzzler about the nuclear fission product propulsion claim of approaching 0.08 c. Yes, staging and a huge mass fraction helps but the number still seems too high for nuclear fission.
Should Interstellar Cyclers – https://centauri-dreams.org/?p=10891 – prove feasible, they represent a way to keep disparate isolated colonies together.
Also, assuming brown dwarfs and rogue superjovians are plentiful…
Hi Folks;
With all due respect, I think the notion that 0.05 C is the ultimate limit for a starship is complete nonsense!
Regarding matter antimatter drives not having an Isp = C, well perhaps they can have an Isp of almost C. Who says the exhaust cannot be directed in a collinear manner in a direction anti-parallel with the velocity vector of the ship! Simply produce the matter antimatter reactions within a high heat capacity bulk material and use the heat to drive turbo-electric machinery to power a photon, ion, electron, a EHPD and/or an MHPD propulsion system. The photon and/or charged particle streams could take the form of highly directed emmissions. The space craft could be highly insolated so as to prevent electromagnetic black body based losses thereby permitting more effective energy to kinetic energy conversion. Secondary and tertiary multicycle steam systems perhaps using lower phase change temperature liquids can get all the more energy out of the matter antimatter reactions.
Now, regarding the relativistic rocket equation, here are some facts:
For M0/M1 = 10,000, we have delta V = C tanh [(Isp/C) ln (10,000)] = 0.99999998, and for M0/M1 = 100,000, we have delta V = C tanh [(Isp/C) ln (100,000)] = 0.9999999998 C. Here M0/M1 is the ratio of the vehicle’s fully fueled mass to its final payload mass.
For v = .99999998, the gamma factor is 1/{1 – [(.99999998C/C) EXP 2]} EXP (1/2) = 5,000,, and for v = 0.9999999998 C, the gamma factor is 1/{1 – [(.9999999998C/C) EXP 2]} EXP (1/2) = 50,000.
Obviously this assumes an Isp virtually equal to C.
Regarding nuclear fusion rockets:
One result is delta V = C tanh [(0.119C/C) ln 1,000] = .6762 C. For M0/M1 = 10,000, we have delta V = C tanh [(0.119C/C) ln 10,000] = .79907 C, and for M0/M1 = 100,000, we have delta V = C tanh [(0.119C/C) ln 100,000] = .87870 C which corresponds to a gamma factor of about 2. This bad boy could put any star system within a 60 lightyears in range of Earth for a healthy young robust crew.
For a perhaps more reasonable, M0/M1 = 100, we have delta V = C tanh [(0.119C/C) ln 100]= .49899 C, and for a very reasonable M0/M1 = 10, we have delta V = C tanh [(0.119C/C) ln 10] = .2673 C.
Now for a maximum Isp of 0.04C for nuclear fission fuel, and a very reasonable M0/M1 = 10, we have delta V = C tanh [(0.04C/C) ln 10] = 0.0918 C. For M0/M1 = 100, we have delta V = C tanh [(0.04C/C) ln 100] = .1821C. For M0/M1 = 1,000 we have delta V = C tanh [(0.04C/C) ln 1,000] = .2694 C. For M0/M1 = 10,000 we have delta V = C tanh [(0.04C/C) ln 10,000] = .35259 C, and for a still plausible M0/M1 = 100,000 we have delta V = C tanh [(0.04C/C) ln 100,000] = .4305 C.
It really does not matter whether or not the Isp values are off by 5 to 10 percent from the above quoted examples, velocities much greater than 0.05 C will still be obtainable, and for matter antimatter propulsion systems, gamma factors much higher than the LHC particle beams will be obtainable.
I will go one step further and state my hope that wormholes, anti-gravity, and superluminal travel will one day prove possible.
We are all familiar with the mass energy conversion relation of E = M[C EXP 2], and the general relativistic equivalence of accelerated reference frames and gravitation. Well perhaps there is also an equivalence principle operative between space time and mass energy, whereby a space craft could be converted into a distance location of space time, and then be reconverted back into its original form once it arrived instantaneously or nearly so at is final destination. Better and perhaps safer yet, would be the ability to convert the interveining space time between a space craft and its destination in piecewise mode or in one feld swoop into space craft kinetic energy, energy to power gravatic or anti-gravatic propulsion systems, wormhole production systems and/or the like .
Note that I am not referring to zero point energy extraction here, which I am also a fan of, but rather the concept of interconversion of space time and mass-energy.
Hi Folks;
What is even more silly is putting artificial limits on gamma factors achievable via the assumption of technological artifacts as they currently exist.
We will be able to produce continuous magnetic fields of field strengths much greater than the current record of 45 Tesla and likely develope high temperature superconductors with very high Tc, Ic, and Mc values. I will hazard the guess that neutron density solid materials will be developed at some point. New methods and forms of fusion reactors will be developed. Materials such as phonon conductors I believe will come to fruition. Matter wave cloaks are theoretically possible, and certianly EM wave (including visible light and UV-A, UV-B, and UV-C) negative refractive index, EM field cloaks are being studied. DARPA is pouring 10s of millions of dollars into such EM materials at least in part for the possibilities offered by invisibility cloaks. I will even hazard a guess that the venerable Interstellar Ramjet will be ressurected in new yet to be concieved forms, and the host of could be’s and/or likely be’s could not be contained in the totality of the books contained in the Library of Congress.
We should not assume velocity limits of 0.05 C based on craft operational parameters using currently available materials, energy production apparatus, and even currently concieved relativistic space craft systems and sub-systems. I hope and believe that velocities virtually equal to C will be obtained for human crewed space craft at some point, and I will not let naysayers dashed such hopes. The human race deserves better.
Opinion Ivan Korznikova about these issues:
http://go2starss.narod.ru/pub/E009_RMP.html
Kenneth,
James Essig,
I could not agree more with your logic train since people forget how primitive we still are and how much we are likely to advance in the next ~100 years. The idea that there is some sort of practical limit at .05C based on what we kow today is short sighted to say the least. My question to you is the same as above. Your math is better then mine, but when I go through all of the puts and takes of system engineering based on known physics (always a dangerous limitation, but one has to bound the problem somehow) and do the calculations I keep coming back to about 70% C as being the “knee in the curve” for achievable and practical in the next ~100 years (assuming all goes well). What I mean by this is that when one factors in propulsion requirements, the Edelstein calculations, our local insterstellar medium which may be rather thicker then normal, and then you extrapolate from what DARPA is doing in things like Meta Materials and related magnetic field research for protection along with the mass implications there of you begin to see how with a few technology miracles we can get to about 70%C. Beyond about 70%C we move from the realm of the very hard, but doable in technical and engineering terms to the realm of several physics miracles to make things much more practical. Therefore, it seems that there might be a practical engineering limit around 70%C beyond which new physics is required to make things more cost effective or we simply “break the bank” as we push beyond about 70%C.
For example, if you run the Edlestine calculations the protection requirements start to rise dramatically right around 70%C. By 80%C you start shaking your head about all of the Gamma Ray exposure to protect against assuming a Human Crew. The same seems to be the case in the propulsion domain. Getting to around 70%C is very hard, but at least one can see a path. The energy requirements beyond about 70%C become almost unfathomable, and when you get to about 90%C they are truly daunting. However, I am not sure about my mathematical calculations. Are you seeing the same knee in the curve around about 70%C for both energy and protection requirements that I am seeing, where things move from the very hard but achievable to the brutal? Do you agree that relatively speaking the technical “low hanging fruit is picked at around 70%C or do you think that the practical as oppossed to the theoretical bar can be pushed higher to say 90%C without to much added trouble?
I like the idea of the lithium/deuterium fusion drive. It addresses several problems: 1) The easiest of the fusion reactions (D-T) can be used, 2) the dreaded neutrons are not lost but captured to breed the tritium, 3) the fuel is very stable, 4) the fuel can be arrayed ahead of the craft for shielding without any mass fraction penalty, and 5) most of the structural mass can also be fuel, giving a good head start on the rocket equation.
Is there anything seriously wrong with this concept, compared to others?