Project Icarus Update

by Paul Gilster on May 7, 2010

Is Jupiter the best place to collect massive amounts of helium-3? The Project Daedalus designers thought so. Back in the 1970s, members of the British Interplanetary Society set out to design a starship that would use pulsed fusion propulsion, with deuterium and helium-3 as fuel. Daedalus had mind-bending requirements, for the plan was to drive it to 12 percent of lightspeed on a flyby mission to Barnard’s Star that would take fifty years to arrive. 250 pellets of deuterium and helium-3 would be detonated every second in its combustion chamber over a thrust period of four years. That calls for a lot of fuel, and therein lies the problem.

Fueling Up the Probe

We can find deuterium (an isotope of hydrogen) right here on Earth, but helium-3 is a rarity. The Daedalus team figured it needed some 30,000 tons of helium-3, so it envisioned mining Jupiter’s atmosphere, where the stuff is plentiful. Imagine floating factories in the atmosphere of the giant planet using waste heat to generate lift, so-called ‘aerostats’ that would be serviced by orbital transports. The task of mining Daedalus’ fuel was almost as daunting as the prospect of an interstellar journey, and pointed to the need for a vast solar system-wide infrastructure to support it.

Image: Project Daedalus was the first detailed study of an interstellar probe. Project Icarus aims to reconsider Daedalus in light of new technologies. Credit: Adrian Mann.

But is Jupiter the best option? The Project Icarus team, now updating the original Project Daedalus, is asking this and many other questions about the original design, including whether or not there is a case to be made for a planet like Uranus, likewise rich in helium-3 but with a much shallower gravity well. For that matter, what about mining our own Moon, now believed to have resources of helium-3? The design team is shaking down these and other ideas in what Leonard David calls an ‘exercise in theoretical engineering to the extreme.’ David writes about Icarus in this Space.com feature.

The current terms of reference for Project Icarus are as follows:

  • 1. To design an unmanned probe that is capable of delivering useful scientific data about the target star, associated planetary bodies, solar environment and the interstellar medium.
  • 2. The spacecraft must use current or near future technology and be designed to be launched as soon as is credibly determined.
  • 3. The spacecraft must reach its stellar destination within as fast a time as possible, not exceeding a century and ideally much sooner.
  • 4. The spacecraft must be designed to allow for a variety of target stars.
  • 5. The spacecraft propulsion must be mainly fusion based (i.e. Daedalus).
  • 6. The spacecraft mission must be designed so as to allow some deceleration for increased encounter time at the destination.

Where to Send an Interstellar Probe

One current debate involves the choice of targets. When Daedalus was being designed, Barnard’s Star was thought to have planets, a finding that later turned out to be erroneous. You pick a target based on the optimum planetary findings, but just what are they in today’s exoplanetary environment? Alpha Centauri is thought to be the closest stellar system, but the WISE mission may show us a brown dwarf even closer than this, perhaps as nearby as three light years. If that happens, does Icarus consider a brown dwarf destination, or stick with larger M-dwarfs and G- and K-class stars?

Or consider Alpha Centauri itself. We have three teams now at work on radial velocity studies that should give us an answer within three years about whether there are rocky worlds around Centauri A or B. Kelvin Long, who heads up Icarus and who is coordinating the effort between the British Interplanetary Society and the Tau Zero Foundation, is wisely keeping the options open. As design work on fusion methods and numerous other components gears up, the Icarus team will hold off until 2013 before choosing its optimum target, by which time we should have some of these questions settled. The final study reports are due in 2014.

Long an admirer of the Project Daedalus effort (my copy of that team’s final report is battered, dog-eared and crammed with notes after years of use), I’m pleased not only with the quality of the Icarus team, but with the fact that many of the Project Daedalus designers are offering their insights as well. David quotes Kelvin Long on the nature of the project:

“There is a need to maintain interest in and the capability to design interstellar probes. With many of the historical leaders in this field now nearing retirement or deceased, the Project Icarus study group wants to take up the baton and keep alive the long term vision that travel to the stars will one day be possible. This is one of the reasons why over half of the team is relatively fresh out of their university studies.”

Interstellar Mission Design in Context

An exercise like Daedalus relies upon identifying the key technological markers for an interstellar attempt. That means determining where we are today with propulsion ideas like fusion, which continues to defy our best efforts to extract useful energy here on Earth. What Icarus brings to the table that Daedalus did not have is, as primary propulsion lead Richard Obousy points out, over thirty years of new data on experimental fusion, including the interesting possibility of using tiny amounts of antimatter as fusion catalysts. As Obousy tells David, “All these technologies could certainly optimize the original Daedalus design, meaning less mass for the propulsion system and more possibilities for the payload. We hope our study will result in a faster and less massive design.”

Less massive indeed, for the original Daedalus was a 54,000 ton vehicle. If you’re interested in how the Icarus team is tackling this and other questions, have a look at the Project Icarus blog, the latest entry of which deals with measuring stellar distances and the science that an interstellar probe could return. That data return itself is a fascinating question. Do you communicate with lasers, or perhaps even something more exotic, such as a communications link based on the Sun’s gravitational lens? The energetic Icarus team is tackling these issues now and will be occupied for the next four years in refining each mission component.

A completed starship design study tells us where we are today and sparks the imagination. My own hope is that just as Icarus continues the Daedalus tradition by examining fusion alternatives, it may lead to future spinoff studies on other propulsion ideas, including beamed sail concepts. The key thing is to get serious work focused on interstellar propulsion issues so we can learn how new technologies may help us resolve seemingly intractable problems.

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{ 50 comments }

Gregory Benford May 7, 2010 at 11:31

Customary fusion with He-3 is a minimal approach. But other fusion schemes aimed at burning Boron – proton reactions at higher energy are the target of Tri Alpha Energy Corp., which is about to announce major strides forward. I worked a bit on the confinement scheme back before Tri Alpha formed, out of the plasma group at UCIrvine. Boron – proton reactions produce only alpha particles, shedding the neutron problems of He-3 fusion. Alphas can be magnetically harvested into direct streams, providing a very high Isp thrust.

Stay tuned!

John Hunt May 7, 2010 at 12:37

It is said that the moon has 1 million tons of He-3. A significant space-based infrastructure will start with development of the moon. We would anticipate that He-3 mining activities will already be ongoing for nuclear power purposes on the Earth by the time that an Ircarus would be built.. So it really comes down to an assessment of whether it will cost more to purchase from the lunar suppliers or to develop an entirely new system to extract it from an outer planet. My bet is on purchasing from existing lunar suppliers. But, if the concentration is high in the uranian atmosphere perhaps that is a cheaper source. After all, He-3 would be a bulk commodity. When in free space transportation costs are a matter of time in which craft are tied up and loss due to boil off rather than fuel lost due to friction. It would be interesting (but unlikely in my opinion) to find that uranian He-3 was less expensive than lunar He-3.

Tulse May 7, 2010 at 14:39

Echoing Gregory, there are several current active approaches exploring pB11 fusion, and some seem to be progressing very well on relatively little money (certainly compared to Big Fusion approaches). It seems to me vastly more likely that we will get pB11 fusion working, with its plentiful terrestrial fuel, than we will be able to develop the huge and fantastically expensive infrastructure necessary to harvest non-terrestrial He3. I think that any propulsion system that requires industrial level mining on the moon or gas mining of the outer planets will not be realizeable for a century or more.

Eniac May 7, 2010 at 15:16

John:

When in free space transportation costs are a matter of time in which craft are tied up and loss due to boil off rather than fuel lost due to friction.

Icarus will have to store the He3 for a century. Therefore, transportation within the solar system should not be limited by boil off.

Eniac May 7, 2010 at 15:23

Echoing Gregory, there are several current active approaches exploring pB11 fusion, and some seem to be progressing very well on relatively little money (certainly compared to Big Fusion approaches). It seems to me vastly more likely that we will get pB11 fusion working, with its plentiful terrestrial fuel, than we will be able to develop the huge and fantastically expensive infrastructure necessary to harvest non-terrestrial He3.

This would be nice. However, the reason so little money is spent on pB11 is that most mainstream researchers consider it unlikely to ever work, for fairly fundamental reasons (No, not only to save their boondoggle. Low cross-section and bremsstrahlung are the issues, as I understand). To rely on out-of-the-mainstream research to produce positive results would be foolish. We might as well plan on Warp drive or antigravity.

Enzo May 7, 2010 at 17:01

What would be the advantage of a flyby mission to a star at a significant fraction of the speed of light compared to a mission like FOCAL ?
It seems to me that FOCAL would be :
1) A lot easier and cheaper
2) Give better images
3) Longer observation time
4) A lot quicker results

Of course, if the mission could actually stop and orbit the star, then it would be different.

Also, check out this very interesting podcast about fusion on Scientific American :

A lot of practical problems are mentioned even if the National Ignition Facility breaks even, some I had never considered in detail.

Eric May 7, 2010 at 18:26

@Gregory, @Tulse

I’m happy to see your optimism here. I’ve been long disappointed by fusion being just around the corner, but it does seem that progress is real in some of these smaller-scale approaches (not at the big national labs). It would make me feel much more optimistic for the future, since otherwise, I’m pretty glum.

andy May 7, 2010 at 20:19

Talking of targets, Epsilon Eridani certainly seems like a pretty good bet right now, as it shows signs of having a young planetary system around it which may well be a fairly close analogue of our own.

James M. Essig May 7, 2010 at 23:06

Hi Gregory Benford;

I like the idea of Boron Proton fusion.

My hope is that if we can reach a gamma factor of say 1.01 to 1.05 with a solar dive and fry concept, perhaps using nanometer thick class carbon nanotube based metalized sun-light sails, boron nitride analogues, or perhaps metalized graphene sails, followed by an additional increase in gamma factor of say 0.o5 to 0.1 such as might be possible with sufficient M0/M1 ratios, then we might achieve a terminal gamma factor of 1.15 which corresponds to a velocity of 0.495 C.

This is pretty good considing carried along fusion fuel and propulsion by ambient sun light.

I ran some mumbers about two years ago in my spare time and came up with an exiting velocity of about 0.251 C for a solar dive and fry space craft that approached the Sun at a distance of about 0.03 AU and which used a sail mass specific area of 10 EXP – 8 kilograms per square meter such as might be possible with a super strong Carbon Nanotube grid like sail that is 99 percent empty space and which is composed of an orgothonal cross-weave of one nanometer thick carbon fiber threads seperated by 100 nanomters. I assumed a density of the bulk sail material to be equal to that of liquid water.

As a possibility, O.495 C facilitated by Boron-Proton reactions and sun diver manuevurs might be a way to open up human star hopping flights from one star system to another, perhaps for colonization of the Milky Way Galaxy by our civilization.

Project Icarus I hope and feel will be instrumental in the development of eventual 0.5 C manned star ships, even this very Century if bold funding initiatives and a federated international alliance or global UN type of effort is undertaken for this purpose.

As a young boy, I was impressed by the huge real fire-crackers in film clips of nuclear bomb tests. Now that we seem to be drawing down our nuclear arsenals in persuit of hopefully more peacefull endeavors, I see the slow burning nuclear Roman Candles for human star flight as a far better alternative.

I think we will eventually make it to Pandora within this Century or by the middle of next Century, but definately by 2176.

Paul Titze May 7, 2010 at 23:11

Hi Paul,

Also checkout this following interesting paper from Friedwardt Winterberg on the possiblity of ultradense deuterium:

http://arxiv.org/abs/0912.5414

If this form of deuterium is found and turns out to be stable, this could be very useful as a fuel.

Cheers, Paul.

kurt9 May 8, 2010 at 1:25

Epsilon Eridani has two asteroid belts and a huge Kuiper belt that is many times larger than ours, meaning lots of resources for habitats and overall industrialization.

Adam May 8, 2010 at 9:23

Hi All

Robert Bussard looked at the deep space capabilities of p+11B in a paper that’s available at Askmar.com…

The QED Engine

…in which he examines its performance over a range of Isp. The ultimate seemed to be venting pure fusion reaction products with an Isp~1.4E6 seconds (0.0457c) which would be an improvement on “Daedalus’s” 0.0353 1st Stage maximum. The thrust would be very low and I’m not sure what the waste-heat mass-penalty would be like – probably better than the expected neutron heating that simulations of “Daedalus” pellet implosions apparently implied.

Does anyone know the details of the paper that critiques the specific pellet design for “Daedalus” ? I think Heppenheimer mentioned it in a book.

Incidentally the burn-up fraction Bond & Martin computed was ~0.156 1st Stage & ~0.118 2nd Stage. Quite low, but to go significantly higher required much higher energy inputs into each fuel pellet. They also computed there’d quite a lot of “blow-off” from the pellets. Improving on their performance will take some neat tricks in ICF design.

kurt9 May 8, 2010 at 13:01

I think all three of the B11-p fusion efforts are supposed to report results by the end of this year or early next next. In addition to Tri-Alpha, there’s EMC2 in New Mexico, who are currently doing research with their WB-8. Then, there’s Lerner’s focus fusion. I’ve always been skeptical of the latter. However, they do have defined milestones and do seem to be meeting them.

Fusion power is good for deep space propulsion. However, it will not work for orbital launch because they are all plasma systems and any launch from Earth requires a system that works in atmospheric conditions. Also, I think none of these fusion methods can give you 1+ g acceleration, which necessary for launch from Earth.

Adam May 8, 2010 at 18:39

Hi kurt9

Bussard has several papers discussing how fusion can propel launchers – not as difficult as you might think. Browse through the papers at Askmar.com to see.

James M. Essig May 8, 2010 at 19:05

Hi Folks;

Comet capture and conversion into a space craft might enable fusion rocket craft to obtain gamma factors close to 2 commensurate with velocities on the order of 0.867 C. A billion metric ton comet that was almost pure hydrogen ice might permit a an M0/M1 ratio of 100,000 which when substituted into the relativistic rocket equation Delta V = C Tanh [(Isp/C) ln (M0/M1)] where Isp is expressed in units of C and is assumed to be equal to a maximum of 0.119 C for fusion fuel, obtains a velocity close to 0.867 C: All this with a payload of 10,000 metric tons.

I once read a paper regarding the fusion fuel ice ball concept utilizing an M0/M1 ration being able to obtain a velocity in the range of 0.63 C. The comet like ice ball consisted of an concentric arrangement of fusion fuel ice layers wherein each layer was wrapped in some sort of insolating and containment membranous materials.

If anyone is interested in reading more about this concepts, I can try to dig up a link for the paper.

The point is, perhaps off-shoots of Project Icarus might make use of such an autophaging ice ball space craft concept.

Either way, I am encouraged by the discussion in this thread regarding how to do nuclear fusion rockets.

For starters, achieving 0.075 C to 0.1 C would be perfectly acceptable and most welcome by me. Afterall, it was about 40 years ago when the voyager space craft were launched and we would still be receiving signals from them if we could. In 40 years, a 0.1 C class space probe or manned starship could reach Alpha Centauri. This gives me much hope.

Carl May 8, 2010 at 21:43

Hello, James,

Sounds like the Enzmann starship. Your outline of onion-layering the volatile components would help mechanical integrity of the mass mounted on the fore of the ship.

This concept offers a means of refueling in the destination system.

Some Enzmann links: http://www.centauri-dreams.org/?p=1142 http://crowlspace.com/?p=589
http://enzmannstarship.com/

Eniac May 8, 2010 at 22:09

I think all three of the B11-p fusion efforts are supposed to report results by the end of this year or early next next.

“Next year” is right around the time that results are going to be reported for a great many fantastic projects. Particularly those in need of more money. For some this has been the case for decades.

Still better than the 20 years the mainstream promises, for half a century now. (Or is it?)

Seriously, I would have expected a little more healthy skepticism and a little less wishful thinking from this group.

Adam May 9, 2010 at 2:12

Taking Greg B’s & Bob Bussard’s numbers for p+11B, we get an Vex~0.045. Using the ~42 mass-ratio of “Daedalus” that’s a delta-v of 0.1682c, thus if we brake the probe and don’t use a mag-sail or similar propellantless brake, that’s a cruising speed of 0.084c for an orbital probe.

Since 83.3% of the propellant mass is solid boron – or boron+boron hydride – a lot of the difficulties of cryogenic propellants disappear. We could probably get a mass-ratio of ~100. Thus a delta-v of ~0.207c. That’s a cruising speed of 0.1035c for a deccelerating probe.

But can we do better? Boron has been studied for use as a laser-sail material. If we could shoot the propellant to our vehicle for the acceleration stage we might be able to push 0.15-0.2c cruising speed. With a magnetic sail we might push towards 0.25-0.3c.

Assume we can get 0.3c out of some mix of all the different propulsion techniques. How far can we go in 100 years and still get information back from our target in that time-frame? Trip + transmission time, assuming very high acceleration, gives a total data-return time, t, equal to x/v + x/c. Setting t = 100 years and v to 0.3c gives x = 23 ly. Within that radius is ~100 star systems, which leaves us quite spoilt for choice. Which one do we aim for?

spaceman May 9, 2010 at 8:17

Not to throw a wet blanket on the idea of using this type of spacecraft as a means of interstellar exploration, but I have a few concerns about it. One is that despite years of effort, fusion reactions have not even been accomplished here on Earth let alone on a huge spacecraft. There is still no viable fusion reactor or even a prototype. Also, it seems to me like it would be more efficient in terms of cost, time, and effort to send a bunch of smaller probes as fast or even faster– probes equiped with artificial intelligence– to explore a larger number of extrasolar planetary systems.

Tulse May 9, 2010 at 10:11

“Next year” is right around the time that results are going to be reported for a great many fantastic projects. [...] I would have expected a little more healthy skepticism and a little less wishful thinking from this group.

The main group researching the dense plasma focus approach, Lawrenceville Plasma Physics, has been very open about their recent progress, and is providing hard data from their device that suggests they are on track to demonstrating above-unity results perhaps this year. While I agree that one should remain skeptical in general about such claims, I think it is also fair to say that the “alt-fusion” crowd is currently producing more actual results than the Big Fusion devices, and that their approaches are far more affordable and practical than something like the NIF or a tokamak. To the degree that one thinks fusion of any sort can be achieved and is not just “wishful thinking”, it looks to me as if the alt-fusion folks currently have the lead.

Denver May 9, 2010 at 10:35

Eniac May 7, 2010 at 15:23

Echoing Gregory, there are several current active approaches exploring pB11 fusion, and some seem to be progressing very well on relatively little money (certainly compared to Big Fusion approaches). It seems to me vastly more likely that we will get pB11 fusion working, with its plentiful terrestrial fuel, than we will be able to develop the huge and fantastically expensive infrastructure necessary to harvest non-terrestrial He3.

This would be nice. However, the reason so little money is spent on pB11 is that most mainstream researchers consider it unlikely to ever work, for fairly fundamental reasons (No, not only to save their boondoggle. Low cross-section and bremsstrahlung are the issues, as I understand). To rely on out-of-the-mainstream research to produce positive results would be foolish. We might as well plan on Warp drive or antigravity.

Bussard’s Polywell, for the win.

James M. Essig May 9, 2010 at 11:08

Hi Carl;

Thanks for providing the links on the Enzmann Starship.

The graphical depictions and art work on the third link are outstanding.

One benificial thing about fusion powered starships is that we have so many fusion sequences to chose from. This includes any fusion methods that might involve combining interstellar gas with carried on board fusion fuels and the like.

Now it would be nice if the concept of the ISR could be ressurected in some usefull form wherein lossy issues of astrodynamic drag could be reduced to the point where the ship could reach extreme gamma factors.

NS May 9, 2010 at 14:37

I have nothing against starships, but I suspect that for the foreseeable future it will be infinitely more practical to build space-based telescopes of various kinds to observe stellar systems, rather than trying to travel to them. For all we know this is the approach other civilizations (if there are any) have taken. It may even be easier to build large artificial habitats in the solar system than it is to colonize planets around other stars. At my age (mid-50s) I frankly don’t expect to see a starship launched in my lifetime.

Henry May 9, 2010 at 17:08

Question:

All this talk of how a .1c or what-have-you starship would be significantly hampered by doing a very brief fly by.

Why couldn’t the starship just enter some kind of orbit around the star, a big planet, etc.? Heck in the case of Alpha Centauri you have three stars if one wasn’t enough gravity to stop you.

Maybe orbit is impractical at that kind of speed, but do a layperson like me I don’t see why a relativistic mission to a nearby star would have to be a flyby.

Administrator May 9, 2010 at 18:07

It’s an excellent question, Henry, and the answer is that at the kind of speeds we’re talking about — well over 10 percent of lightspeed — the amount of fuel it takes to decelerate is absolutely off the charts. Remember that every bit of fuel you need to slow down has to be carried with you in the first place, which jacks up the fuel needed for the journey out, and when you run the numbers, they defy description. This is why slowing down through other means, such as magsail braking, becomes an interesting option.

Wikipedia has a math-heavy article on the rocket equation:

http://en.wikipedia.org/wiki/Rocket_equation

but here’s a more general quote from my own Centauri Dreams book:

“Even a far more efficient ion engine would need more than 500 propellant takes the size of supertankers to complete an Alpha Centauri flyby within a century, according to NASA physicist Marc Millis…. And the problems are only just beginning. Slowing down takes as much propellant as speeding up; we must, therefore, push that much more fuel. If we wanted our spacecraft to stop once it reached Alpha Centauri, those five hundred supertankers would need to be supplemented by another three hundred million supertankers to make the 100-year journey and stop! As you keep adding more propellant to push still more propellant, the ratio of fuel to payload simply goes off the chart, an unfortunate fact that grows out of the equations that relate mass to velocity change…”

Eniac May 9, 2010 at 18:26

Henry: At the speeds we are talking about, gravity would make hardly a dent in the trajectory even if we were to graze the central star. Capture is entirely out of the question, at best we could aim for the next system if we can find one that lies almost exactly on the line of sight from Earth.

Eniac May 9, 2010 at 18:42

Microwave sails come to my mind, which have of course been discussed a lot. What has perhaps not been discussed, yet, is the possibility of using a gigantic yet simple phased array for beam generation instead of a high powered maser and gigantic lens. The advantage would be that the array would be composed of millions (or billions?) of very simple solar powered transmitters, mass produced and thus cheap. There would be no need for a central power source. The size of the array (and thus the beam focus) would only be limited by production capacity and lifetime of the devices.

The other critical issue is absorption at the sail. I understand Forward envisioned superconducting mesh that would have lossless reflection, which would be perfect. However, it is said that this later was shown infeasible. Is this latter argument really conclusive? Does anyone have a reference (Landis, was it?)? Perhaps todays High Tc superconductors would work better?

Administrator May 9, 2010 at 20:34

Eniac, I can dig up the reference re the problem with the Starwisp microwave sail, but yes, it was Geoff Landis who showed that the design wouldn’t work, and Forward agreed to Landis’ findings.

Later: found it. Check this:

Landis, Geoffrey A. “Microwave Pushed Interstellar Sail: Starwisp Revisited” (paper AIAA-2000-3337, presented at the AIAA 36th Joint Propulsion Conference and Exhibit, Huntsville AL, July 17-19 2000).

Eniac May 9, 2010 at 22:50

Thanks, Paul.

Unfortunately, I was only able to obtain the first page of this. There it says:

It was realized that, in the absence of superconducting meshes, the performance of lightsails may not be as good as that originally calculated by Forward, and the extremely thin sails proposed will absorb, rather than reflect, microwaves.

Now, as I read this, it leaves open the possibility of superconducting meshes that reflect microwaves 100%, but then, the body of the paper may say something different.

Of course, the possibility of a phased array is fully embraced by Landis pretty much the way I imagined (“perhaps not been discussed”, yeah, right…). I find this one of the most doable means of interstellar propulsion and think it should be considered for Icarus.

Adam May 10, 2010 at 5:24

Hi All

It’s rather funny to be quoted as a reference on the Enzmann starship – I’d like more information myself.

Incidentally the artist of that Enzmann starship painting disowns it because it makes several errors – he painted it many years ago while renting a room from Enzmann himself. I’ve been told that he wishes it’d been destroyed.

Adam May 10, 2010 at 6:27

Hi All

Heinrich Hora is a fusion researcher at UNSW here in Oz who works with George Miley, Leif Holmlid and other alternative fusion researchers. He has several very interesting papers on using lasers to induce “fast ignition” of solid boron-hydride. Fast ignition is when a sufficiently sharp energy pulse is used to set off a self-powering fusion detonation wave in the target, and a recent advance in producing very narrow pulses of laser energy has raised the prospect of igniting p+11B fusion with an energy input just x10 that needed for D+T. That’s really good news and Hora is obviously very enthusiastic about the idea.

Here’s his web-page… Heinrich Hora
And a representative paper… Laser-optical path to nuclear energy without radioactivity: Fusion of hydrogen–boron by nonlinear force driven plasma blocks

…so fast laser ignition will be worth investigating for “Project Icarus” too.

Tulse May 10, 2010 at 11:07

the reason so little money is spent on pB11 is that most mainstream researchers consider it unlikely to ever work, for fairly fundamental reasons
[...]
Bussard’s Polywell, for the win.

But Polywell is pB11 fusion, at least in its most commonly researched form.

Henry May 10, 2010 at 12:12

Thank you for your answers Eniac and Administrator. I now see why gravitational capture, reverse-gravity-slingshot (or, uhh, whatever it is called) and of course lugging all that extra fuel is out of the question.

However, that will not stop me from dreaming about a probe which could take its time and lounge about in the Centauri system doing science! Hmm, direct impact with a rocky planet or moon?

:)

Adam May 11, 2010 at 5:50

Hi Tulse

Polywell is being trialed with deuterium in its early fusion tests, before they scale up to p+11B. The advantages of D+D fusion are manifold for terrestrial applications since the neutron flux can be used to heat water directly and the pre-existing steam-plant at current coal-fired stations can be used directly.

Tulse May 11, 2010 at 14:05

Polywell is being trialed with deuterium in its early fusion tests, before they scale up to p+11B. The advantages of D+D fusion are manifold for terrestrial applications since the neutron flux can be used to heat water directly and the pre-existing steam-plant at current coal-fired stations can be used directly.

But those advantages go away when generating electricity directly as in pB11 fusion. In other words, the only reason to use deuterium and steam-plant technology (which is a very indirect way of getting electricity) is because one can’t actually do what Polywell (and Focus Fusion, and Tri-Alpha) are attempting to do with pB11.

I agree that there are other industrial uses for heat, such as district heating, but I really doubt that achieving that with energetic neutrons is the best approach.

Adam May 12, 2010 at 5:08

Hi Tulse

Deuterium fusion is easier to do and the fuel available is near limitless. 11B isn’t so easy to come by. I think using D+D reactions makes a lot of sense, though it might be sub-optimal in a Polywell, eventually a means will be found to efficiently thermalise the neutron flux and get a good energy return from it. In pure DD reactions some 62% of energy is charged particles, so that’s already a bonus. Efficient energy conversion technologies can extract ~50% of the energy from the neutron heat, so assuming 80% efficient direct conversion from the charged particle flux means ~69% efficiency overall. Nothing to sneeze at. On a cold planet, like Titan, the heat is an advantage and the higher heat-sink differential means even higher efficiencies of conversion. If we try really hard then energy extraction from the heat can go even higher, a good thing for all concerned.

Adam May 12, 2010 at 5:18

Hi again

The other point is practicality – neutrons, being uncharged, escape the potential well of a Polywell directly, removing the need for pulsed power modes or similar engineering work-arounds possibly needed to extract useful energy from a Q > 1 Polywell fusor. Continuous operation has a lot of advantages and may well be preferred in the early days. Bussard was keen on p+11B for aerospace – specifically for powering QED plasma rockets & ramjets – but fixed power may need something else.

Gregory Benford May 13, 2010 at 0:47

James M. Essig:

Capturing comets for fusion fuel — yes, could work. David Brin and I planned a sequel to our novel HEART OF THE COMET, with just that happening — but haven’t written it. The basic trouble with interstellar is the fuel mass.

To solve that…

Must admit I still long for the fabled Bussard ramscoop. There’s been a lot of calculation about whether it is dynamically feasible, now to ionize the hydrogen before it, etc — can someone summarize the state of knowledge in this area?

Administrator May 13, 2010 at 7:51

I bet Adam Crowl has some thoughts on that, Gregory. The biggest issue I’ve seen with the Bussard ramscoop is what Zubrin and Andrews found when they looked at the drag situation — the scoop seems to act more effectively as a brake than anything else, thus setting up scenarios for slowing an inbound spacecraft. I haven’t seen anything that gets around the drag issue, but let’s see what Adam comes up with, as he’s deeply involved with Icarus.

Adam May 13, 2010 at 19:17

Hi Greg & Paul

Must admit I am behind on the current literature of ramscoops – that’s where the most work has been done, saving the hard problem of fusing protons for another time. AFAIK Daniel Whitmire’s CNO catalytic ramscoop is still viable, though the effective ramscoop diameter may be ~2,000 km, according to Matloff & Mallove’s “Starflight Handbook”, a result that hasn’t changed in ~22 years. Laser efficiency is improving all the time and Whitmire’s preionization approach may be viable, but all the really juicy UV laser work is DoD research and I’m not sure how much is open access. Free-Electron Lasers are the big hope for high power laser systems, but diode lasers are advancing in leaps and bounds too.

One thing that gives me hope is that Tom Ligon says Bussard believed his basic idea would work, in time, using CNO catalysis, and should be able to push towards v ~0.999999c (“six nines” as Bussard called it.) That’s something Bussard believed until his death in 2007, as Tom has noted.

IMO a fully developed ramjet needs a total annihilation drive to work sufficiently well. Louis Crane’s artificial black hole work is the only way currently known of achieving that with at least some hint of how to engineer it – assuming the gamma-ray control/reflection engineering physics becomes possible down the track. More speculative is Frank Tipler’s macroscopic sphalerons – a low probability, but high pay-off prospect. To work at the low energies he imagines a superposition of particle energies across the Multiverse has to be achieved… somehow!

But if Doc Bussard thought CNO was enough, then I’m inclined to agree. Greg, you used that for the “Lancer” in the “Galactic Core Saga” didn’t you?

Gregory Benford May 14, 2010 at 1:36

Adam & Paul:

Yes, used that CNO method in #2 of the Galactic Center series, ie, ACROSS THE SEA OF SUNS.

I’m working on a Really Big Object novel with Larry Niven, which opens on a ramscoop. So wondering about how ramscoop physics looks for the really far future engineering. Thanks for these tips. I’ll use it to make the opening all the more credible.

Aside from that, it’s good to know there’s some home for ramscoop technology. Still, every such advance opens further the eternal Fermi question, where are they? But we do live on the borderlands, so maybe we’re not interesting enough so far to visit? One hopes.

Adam May 15, 2010 at 0:43

Hi Greg & Paul

IMHO the ETI quest can only presently proceed by assuming we know what at least some aliens will do, because physics/economics tells us they could do it. “What’s not forbidden is compulsory” – with caveats. Also it seems clear that no one in this Galaxy has gone in for astrophysical engineering in a really big way and that’s kind of worrying – at least it creates a data void we inhabit with dark imaginings from our own experiences. Self-annihilation, astrophysical perils, failure to thrive… bogeys rise up to taunt us at every point, every blank on the Galactic metro-map.

What if we’re wrong about us being latecomers – what if this is the early days of cosmic history, once all the Galaxy-sterilising events have settled down long enough, once all the heavy elements have grown sufficiently abundant and once planets have had long enough to cook-up Intelligences with itchy-feet that want to roam. We’re right before the phase-transition to a Galaxy filled with Intelligence – and the rest of the Universe looks that way because we’re looking further and further back in Time as we look further out.

The Galaxy looks empty because it’s yet to be sown with Intelligence. But how close are we to the phase-transition? Are we doomed statistical freaks? Or one of “the Progenitors” of a whole family of intelligent species yet to appear on the Galactic stage? Important questions and a good motivator for efforts like “Icarus”, TZF, CD and the BIS. We won’t ascend to that next level if we never try.

Duncan Ivry May 17, 2010 at 17:09

Adam: “… it creates a data void we inhabit with dark imaginings from our own experiences.”
“What if …”
I have one more “what if” for you:
What if travelling through space, performing spectacular tasks there, and filling the galaxy with intelligence is just much more difficult than you and others think? Based on our current scientific knowledge — what else –, it could very well be as simple and banal as that. And I know this would probably exclude the “next level” you are talking about.

Adam May 18, 2010 at 3:31

Hi Duncan
Maybe so. Don’t know till we try.

Ronald May 18, 2010 at 9:27

Adam May 9, 2010 at 2:12: “Within that radius (23 ly) is ~100 star systems, which leaves us quite spoilt for choice. Which one do we aim for?”

No, not too spoilt I am afraid. With all due respect, but 100 star systems is not a great deal of choice, mainly M dwarfs.

Which ones to aim for? Within a 30 ly radius I would suggest, apart, of course, from Alpha Centauri, Tau Ceti and Epsilon Eridani:
40 Eridani
Sigma Draconis
Eta Cassiopeiae A
82 Eridani
Delta Pavonis
107 Piscium
Beta Canum Venaticorum
61 Virginis
Zeta Tucanae
Chi1 Orionis A
Gliese 785
Beta Comae Berenices
Gliese 442 A

And if you can wait another 3 ly of travel distance: Alpha Mensae.

I consider Beta Canum Venaticorum, 61 Virginis and Alpha Mensae the most promising within 33 ly with regard to the prospect of habitable planets.

Ronald May 18, 2010 at 9:52

Adam May 15, 2010 at 0:43: “what if this is the early days of cosmic history”, “The Galaxy looks empty because it’s yet to be sown with Intelligence”.

I actually think that there’s a very good chance that you hit the nail right on its head, that our MW galaxy and our universe are in fact amazingly young, i.e. still in its infancy of (advanced) life-bearing.

There was at least one interesting article by astronomer Mario Livio (“HOW RARE ARE EXTRATERRESTRIAL CIVILIZATIONS, AND WHEN DID THEY EMERGE?”, Astrophysical Journal 1999), suggesting that the universe just became mature enough for life production, particularly sufficient carbon accumulation inside massive stars, after about 8 – 10 gy.

Combine that with the added facts that a sunlike star requires sufficient time to get past its UV aggressive youth phase, an earthlike planet requires sufficient time to build up an adequate atmosphere and life itself takes a few gy’s to develop, and you get to approximately our age, maybe a little less, for advanced life to emerge.

Furthermore, according to Lineweaver’s well-known work (“An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect”, Astrophysical Journal 2001; “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way”, Science 2004), sufficiently old sunlike stars with sufficient (but not too high) metallicity (which in turn is strongly age-related) in a relatively supernova-free zone, defined as the Galactic Habitable Zone, are a prerequisite for life-bearing terrestrial planets.
This strongly selects for sunlike stars between 4 and 8 gy old. Not surprisingly, we are smack in that population (though many sunlike stars are somewhat older than our sun).

We may be the early arrivals indeed.

Eniac May 19, 2010 at 0:17

Perhaps, then, we are the first and only ones in the Galaxy, for the very same reasons that made us alone here on Earth: Fast spread and suppression of competition.

Ronald May 19, 2010 at 10:49

Eniac: maybe I just misunderstand you, but ‘Fast spread and suppression of competition’ ???
We haven’t gotten very far in our galaxy yet.

Early appearance, yes, but that is not a matter of rapid dispersal through the galaxy but rather an evolutionary matter, both biological and stellar evolution.

Eniac May 19, 2010 at 23:41

Eniac: maybe I just misunderstand you, but ‘Fast spread and suppression of competition’ ???
We haven’t gotten very far in our galaxy yet.

No, that is just what I meant. If fast spread and suppression of competition is a common characteristics of intelligent species, there will be one and only one such species in the galaxy, because any putative second will be suppressed. This is exactly what happened on Earth with the hominids. There were many candidate species on the march towards domination of the Earth. There was room for only one, the others had to go. Most notably the Neanderthals, but likely many others.

If there is only one, then we must be it. In other words, the fact that we are first is no coincidence, because there is no second. Except, perhaps, if we are the Neanderthals…

Eniac May 20, 2010 at 9:10

Note that “suppress” does not necessarily mean fight and exterminate. “Taking care of nature” such as in hunting grounds, farms, zoos, and national parks is just as effective in suppressing the rise of competing species.

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