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Solar Sails: State of the Art 2010

Centauri Dreams readers know that I’m a great supporter of solar sailing as a technology that has interstellar ramifications as well as immediate practical value right here in the Solar System. What’s particularly appealing about the solar sail is that we’ve already shaken out many of the problems and are ready to begin testing sails in space, which is why it’s so frustrating to see NASA and ESA locked in to budgetary constraints that keep that vital next step from happening. NanoSail-D is one cheap way we might fly a sail soon, and so is The Planetary Society’s LightSail project, but as with so many aspects of the space program, we seem to be well behind earlier optimistic schedules.

In that environment, though, it’s important to keep the goal in front of us and to continue the work on solar sail theory. In June of 2007, the 1st International Symposium on Solar Sailing took place at Herrsching at Lake Ammersee, Bavaria. The 2nd in this symposium series is now scheduled for July 20-22 of this year at New York City College of Technology (City Tech) of the City University of New York. The venue is home to solar sail experts Greg Matloff and Roman Kezerashvili.

The focus in New York will be recent advances in solar sailing technologies and near-term solar sail missions. Particular attention is focused on hardware, enabling technologies, concepts, designs, dynamics, navigation, control, modeling and mission applications and programs. For more information, check the ISSS 2010 site, where you can register online. For those interested in submitting papers, abstracts are due May 15. The proceedings will be published to add to the substantial solar sail literature that continues to refine the concept.

As to The Planetary Society’s plans, LightSail-1 is to have four triangular sails constructed of 32 square meters of mylar, a configuration that will be placed 800 kilometers above the Earth to test the practicality of using sunlight as a means of propulsion. The Society talks about a launch before the end of 2010, but much depends upon the choice of launch vehicle — LightSail-1 will be flying as a secondary payload on either an American or a Russian launch. Assuming success, the next LightSail will carry a larger payload, with a third sail intended to fly on a multi-year mission that will create an early-warning station for geomagnetic storms triggered by events on the Sun.

The Planetary Society calls solar sailing “…the only known technology that might carry out practical interstellar flight, helping pave our way to the stars.” Although the language is stirring, I’ll have to disagree with the word ‘practical’ in that sentence given the times involved in a solar sail mission to a nearby star, and I’m sure fusion advocates would argue that by the time we develop laser or microwave beaming methods to boost sails to faster speeds, we’ll likely have fusion options as well.

But who can argue with the excitement of funding a solar sail mission with public and private contributions, bypassing government bureaucracies to move the state of the art forward? Meanwhile, we also have to keep an eye on Japan, where work continues on the IKAROS Project, a solar sail / ion engine hybrid whose first mission may fly this year. IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) will use thin film solar cells on the 20-meter sail membrane to power up its ion engines, with a second, larger sail envisioned that would one day target Jupiter and the Trojan asteroids. All told, 2010 looks to be a significant year for solar sail technologies.

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  • drpayton January 12, 2010, 10:39

    Tacking is a means for a sail boat to move against the wind… Couldn’t the solar sails use multiple sails positioned correctly to move the craft faster?

  • Terry Koberstein January 12, 2010, 13:19

    Would a large magnetic field generated by a space craft be equivalent to a solar sail, if the solar wind had a large enough component of charged particles?

  • Administrator January 12, 2010, 14:25

    drpayton, there are a few concepts around involving a solar sail mission with multiple sails — let me see if I can scare up a reference or two.

    Terry, a magsail is indeed an interesting possibility. The problem with the solar wind is its extreme variability, which makes navigation an interesting challenge. But yes, magsails pushed by either a particle beam or the solar wind have been under serious discussion, including the possibility of using one to decelerate at a destination star system.

  • Gary Allen January 12, 2010, 14:44

    Back in 1976, JPL had a mini-competition between two propulsion groups for the best propulsion system for a Halley’s comet rendezvous mission. The two competing technologies were solar-electric propulsion versus solar sailing. The group that I work with at the time had the contract to calculate the steering functions for the solar sail proposal. At the time I was just a snotty nosed kid with only an undergraduate degree and not allowed to participate directly in that project. However I had the opportunity to look over the player’s shoulders and photocopied everything that I could. At the time there were two major solar sailing technologies. One was the “square sail” and the other was the “heliogyro”. The square sail was very similar to the Planetary Society’s LightSail-1. For the Halley’s comet mission study, the square sail was rejected because it was believed that deployment of the square sail would be almost impossible (lots of squirrely, non-intuitive dynamic issues) . We opted instead for the “heliogyro” which was essentially a solar sail helicopter rotor. Each of the rotor blades were long sheets of Kapton/Mylar that were made rigid by centripetal acceleration. Calculating the steering function for the heliogyro was an interesting process. The steering function is essentially the control law for the solar sail’s orientation as a function of time that gets it to its destination. The steering function involves solving the Gauss Problem assuming a continuous low level thrust (almost the same problem as with an ion thruster but with the additional assumption that thrust was a function of distance from the Sun). Later in life I wanted to wrap a Ph.D. thesis around solving steering functions for solar sails but there was no research money available. Instead I was channeled into doing computational fluid dynamics (CFD) which was a better outcome since there were actually career opportunities based upon CFD while solar sails were a dead-end. The Halley’s comet activity with solar sailing ended unhappily with the solar-electric proposal beating the solar sail proposal (too risky) and then the whole Halley’s comet rendezvous mission was flushed down the toilet because there was no money for it. (this was 1976 and the Space Program was flapping around like a fish out of water due to post-Apollo trauma).

    I still believe that solar sailing is an exciting technology for interplanetary travel. However the only interstellar solar sail concept that I’m familiar with was Robert Forward’s proposal involving a light beam from a laser cannon through a huge Fresnel lens to drive a solar sail. That concept has always struck as being nonsense. My main quarrel with it has been the problem of Doppler shifting as the solar sail gains speed relative to the cannon. The light momentum transfer for a solar sail is a function of the light’s wave length. As the solar sail gains speed with respect to the laser cannon, the light beam would Doppler shift to the infrared thus yielding less momentum. This loss of momentum transfer efficiency would be further aggravated by beam diffusion that would exist despite the Fresnel lens which would be almost impossible to build and control.

  • Gary Allen January 12, 2010, 14:55

    I wrote “beam diffusion” when I should have written “beam dispersion”.

  • Gary Allen January 12, 2010, 15:43

    Also it’s nonsense that the Planetary Society calls solar sailing “…the only known technology that might carry out practical interstellar flight, helping pave our way to the stars.” Inertial confinement nuclear fusion can get us to the stars. The National Ignition Facility (NIF) at Lawrence Livermore National Lab could be thought of as a starship motor prototype. Inertial confinement nuclear fusion is far more advanced than any form of solar sailing.

  • Eniac January 13, 2010, 2:06

    Terry, if I recall correctly, the solar wind is orders of magnitude weaker than the radiation pressure from sunlight, which is why we only contemplate using the latter for propulsion.

    Gary, I like your anecdotes from JPL. I also think solar sails are a long shot for interstellar travel, simply because of the low maximum velocity achievable. I would not go as far as calling the Forward proposal nonsense, though. I do not think Doppler-shifting is a big problem (If we can get to the speed where it is, we are already doing quite well…), but dispersion certainly is. The way around dispersion is to do all of the accelerating within or close to the solar system, perhaps to form a relativistic matter stream that avoids dispersion using active flight path control.

    I don’t know if you have read about Kare’s microsail concept, it it strikes me as the best idea to address these issues that I have seen, although it is also quite far from practical, at this point.

    Kare’s microsails: http://adsabs.harvard.edu/abs/2002AIPC..608..313K

    I am intrigued by the possibility that you could do away with the starship at the far end and use a steady stream of microsails like a “probe-beam” which acts as a sensor array and communication relay chain to make observations and send the data back home.

    Fusion, of course, has it’s own hurdles to overcome. Even the modest speeds envisioned by studies like Daedalus have very optimistic assumptions about the exhaust velocity that can be achieved, and even the theoretical maximum for fusion does not leave much room under the rocket equation.

  • Adam January 13, 2010, 5:06

    Gary, solar sails have been suggested as propulsion for Interstellar Medium explorers – precursor missions – not trips all the way to other stars.

  • Wedge January 13, 2010, 7:09

    @Gary (re: solar sailing being the only known technology that might carry out practical interstellar flight). There are lots of technologies with an equivalent amount of R&D as solar sails that could be used for interstellar flight, such as Orion. Other technologies such as nuclear salt water rockets, gas-dynamic mirror fusion, VASIMR and others look to be equally capable but are slightly less researched. I’d like to see more funding for solar sail R&D but it is indeed incorrect to paint it as uniquely capable of enabling interstellar missions.

  • Ron S January 13, 2010, 13:30

    Arthur C. Clarke wrote a whimsical story about a solar sailing race, with references to a variety of possible maneuvers (including tacking I think, but it’s been years since I read it). The story is called “The Wind From The Sun”, and I have it here in his short story collection “The Sentinel”.

  • Curt T January 13, 2010, 14:42

    On the subject of Solar Sailing in SF, the 2nd place winner’s entry in the Heinlein Society Centennial Short Story Contest is currently posted at the Societies’ site. It deals with this subject and some here may find it entertaining; I did.

    http://www.heinleinsociety.org/Winners/index.html

  • Gary Allen January 13, 2010, 16:38

    Eniac said:
    ” Fusion, of course, has it’s own hurdles to overcome. Even the modest speeds envisioned by studies like Daedalus have very optimistic assumptions about the exhaust velocity that can be achieved, and even the theoretical maximum for fusion does not leave much room under the rocket equation.”

    Daedalus was merely the first iteration of a very long process. My main quarrel with Daedalus was the assumption of using He-3 as fuel. The ignition temperature for He-3 is so high that they might as well have selected unobtainum or fairydust as the propellant. The people who designed Daedalus were extremely bright and they understood the problems with igniting He-3. However they were too focused on the problem of energy loss due to fast neutrons. In my opinion the correct approach is to use deuterium as the propellant and absorb the resultant fast neutrons with a liquid lithium jacket surrounding the fusion combustion changer. The energy from the fast neutrons would be used as heat and extracted through Brayton cycle electrical generation. The resultant electricity would then be used for some sort of electrical propulsion scheme like VASIMR. In essence I propose a dual propulsion scheme where the bulk of the thrust comes directly from the plasma generated in the nuclear fusion detonation and electrical propulsion also used as a “topping off” process.

    Eniac’s comment about the maximum exhaust velocity is correct. I think the best maximum velocity one could hope for with nuclear fusion is a little over 1% the speed-of-light. The vehicle designers will just have to accept that interstellar transit time would be on the order of a century. As described in the Daedalus study, the starship’s autopilot would have to be self-aware and capable of directing repair of the vehicle as problems were encountered during the long voyage. The one century transit time would also mean that primary cosmic radiation is a major problem. A biological payload is out of the question and even radiation-hardened electronics would have to be heavily shielded. My approach would be to embed the autopilot within the deuterium fuel tank of the deceleration stage (use the deuterium as shielding material). Starships by necessity are huge (the product of a thriving interplanetary economy). Consequently, the autopilot could have a deuterium shielding of adequate thickness to protect it from cosmic radiation.

  • James M. Essig January 13, 2010, 18:17

    Hi Folks;

    I am reluctant to up load such a long post, but I am so intrigued by single pass sun diver space craft as of late that I felt I just had to express the following ideas which understandably come with several caveats.

    Regarding some extreme solar sail concepts, more specifically some what theoretically maximized single pass solar dive and fry craft, I had the following thoughts to consider.

    One can imagine a sail made of graphene-like material where the material(s) of composition is(are) superconducting and held in place by the Meissner effect where the sail is 99.99 percent empty space in the form of an effectively cross woven net where the net’s strings are composed of spatially separated one nanometer long sections of the above super-conducting graphene like material. The one nanometer long pieces of material(s) would be separated at uniform serial distances of about 100 nanometers and the strings in the form of the broken lines would be separated by about 200 nm. Note that graphene is a one atom thick membranous sheet of carbon and as such is about 0.1 nanometers thick with a density roughly equal to that of water. The density of this broken line based graphene like material sail would be equal to (10 EXP – 9)(10 EXP – 2) kilograms per square meter or 10 EXP – 11 kilograms per square meter.

    Assuming fraction f of the starlight is reflected straight back and the sail moves radially outward, the equation of motion is B[(1 + (B EXP 2)]dB/[(1 − B)EXP 2] {[1 − (B EXP2)] EXP 3/2} = p [(R0/x) EXP 2](dx/Ro) where B = v/c, v is the speed of the sail, x is the distance from the star, and R0 is the initial distance from the star.

    P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, M0 is its rest mass, and u0 is the energy density of starlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2]. Adopting f = 1, a value of M0/A = (10 EXP −11) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.006AU, I find P = 235.78

    This yields {[(1 − (B EXP 2)] EXP (1/2)} [7 − 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 − B ) EXP3](1 + B) = 7 + 15p = 3,543.7. With p = 235.78, the terminal velocity = 0.935693 C. This corresponds to a gamma factor of 1/{[1 – [(v/C) EXP 2]] EXP (1/2)} = 1/{[1 – [(0.935693 C /C) EXP 2]] EXP (1/2)} = 2.8343.

    One can further imagine a sail made of one atom wide chains of atomic/molecular material where the material(s) of composition is(are) superconducting and held in place by the Meissner effect where the sail is 99.999 percent empty space in the form of an effectively cross woven net where the net’s strings are composed of spatially separated one nanometer long sections of the above super-conducting one-atom-wide chains of atomic/molecular material. The one nanometer long pieces of materials would be separated at uniform serial distances of about 100 nanometers and the strings in the form of the broken lines would be separated by 200 nanometers. The mass specific capture area density of this broken-atomic-line-based, material sail would be equal to (10 EXP – 10)(10 EXP – 2) kilograms per square meter or 10 EXP – 12 kilograms per square meter.

    Once again, assuming fraction f of the starlight is reflected straight back and the sail moves radially outward, the equation of motion is B[(1 + (B EXP 2)]dB/[(1 − B)EXP 2] {[1 − (B EXP2)] EXP 3/2} = p [(R0/x) EXP 2](dx/Ro) where B = v/c, v is the speed of the sail, x is the distance from the star, R0 is the initial distance from the star.

    P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, m0 is its rest mass, and u0 is the energy density of starlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2]. Adopting f = 1, a value of M0/A = (10 EXP −12) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.006AU, I find P = 2357.8

    This yields {[(1 − (B EXP 2)] EXP (1/2)} [7 − 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 − B ) EXP3](1 + B) = 7 + 15p = 35,374. With p = 2357.8, the terminal velocity = 0.9735747 C. This corresponds to a gamma factor of 1/{[1 – [(v/C) EXP 2]] EXP (1/2)} = 1/{[1 – [(0 . .9735747 C /C) EXP 2]] EXP (1/2)} = 4.37888.

    The required magnetic field would need to have a very thin squashed flux distribution almost like the concept of a magnetic laser beam field so that the magnetic field energy would contribute very little to the reduction in mass specific sail area of the sail.

    Perhaps with some very low mass exotic fermionic species yet to be created that have an charge to mass ratio considerably higher than the electron, sails with a much higher mass specific capture area could be created allowing even much higher gamma factors for single pass dive and fry solar sails.

    One final note, all of us who post here at Tau Zero are part of a growing cadre of volunteers who have implicitly effectively signed on to the cause of realizing human star flight. I am always impressed by the many fine refutory and rebuttory arguments made on the many visionary threads posted and authored by Paul and his colleagues at Tau Zero Centauri Dreams.

    Folks, keep up the great efforts. Our call as the early conceptual pioneers of star flight, should such travel be realized in the future, and I believe it will be, is noble and our efforts will go down in the permanent archives from the early 21st Century of recorded history as the folks who said it could be done instead of the folks who said it couldn’t be done.

  • Eniac January 13, 2010, 21:12

    As described in the Daedalus study, the starship’s autopilot would have to be self-aware and capable of directing repair of the vehicle as problems were encountered during the long voyage.

    Self-aware as in: carrying its own blueprint and able to effect repairs. That does not mean any sort of intelligence or “consciousness”. It is much like our own bodies maintaining themselves and healing injuries without us having to spend a lot of thought on it.

    This is another reason I think interstellar probes will be self-replicating: It does not really take that much more to self-replicate than it takes to self-maintain, and self-maintenance is considered critical by most.

  • Gary Allen January 14, 2010, 1:16

    Eniac said:
    “Self-aware as in: carrying its own blueprint and able to effect repairs. That does not mean any sort of intelligence or “consciousness”. It is much like our own bodies maintaining themselves and healing injuries without us having to spend a lot of thought on it.”

    Our body’s self repair mechanism and immune system borders on being “magic” (definitely 3 1/2 billion years of natural selection on display). A starship’s self repair mechanism would have to be completely autonomous. No way it’s getting guidance from home if it’s over 2 light years away. However being able to contend with every conceivable failure mode would require self awareness and intelligence greater or equal to a skilled human technician. This same level of intelligence would also be required for exploration of the target star system.

    Eniac also said:
    “This is another reason I think interstellar probes will be self-replicating: It does not really take that much more to self-replicate than it takes to self-maintain, and self-maintenance is considered critical by most.”

    Correct! Almost by definition a practical starship would be a von Neumann machine.

    Slightly off topic: 2010 AL30 has come and gone. It’s most likely an Apollo asteroid or an old transfer stage such as the one used for ESA’s Venus Express or maybe even an old Saturn S-IVb stage. However, wouldn’t it be cool if it was an extraterrestrial artifact? Unfortunately recognizing it as artifact might be very difficult if it was over a million years old and smashed to bits by micrometeor impacts. I hope someone did a detailed radar scan of it while it was whizzing by.

  • Adam January 14, 2010, 3:21

    Hi eniac

    Rapid-prototypers make on-site manufacturing of parts a distinct possibility, but replicating a starprobe requires more than making the parts. I’m skeptical that self-replicators will be very useful for exploration – they’re more likely to self-replicate and mutate away from ‘explorer mode’ because exploration that sends signals home is less ‘advantageous’ than plain old self-replicating as soon as materials are found.

  • Eniac January 14, 2010, 15:44

    Our body’s self repair mechanism and immune system borders on being “magic” (definitely 3 1/2 billion years of natural selection on display). A starship’s self repair mechanism would have to be completely autonomous. No way it’s getting guidance from home if it’s over 2 light years away. However being able to contend with every conceivable failure mode would require self awareness and intelligence greater or equal to a skilled human technician. This same level of intelligence would also be required for exploration of the target star system.

    Gary, I will have to respectfully disagree with this assessment. Biology is far from magic, and it demonstrates conclusively that intelligence is not a requirement for self-repair. A self-maintaining machine will need a great number of diagnostic and robotic procedures for replacing each and every part, but it will not need much in the way of “intelligence” that is beyond today’s robotic capabilities. It is the sheer number of procedures that makes this difficult, not their individual complexity. Storing all these procedures takes a lot of memory, and we only very recently reached the tipping point where sufficient memory and processing power can be included in a machine to effectively hold and execute its own blueprints.

  • Eniac January 14, 2010, 15:57

    Adam, the assumption that self-replicating machines will inevitably mutate or evolve is as common as it is false. Unless the machine is designed to mess with its own programming, such a thing is impossible. A straightforward self-replicator will have a fixed blueprint, and no procedures for changing it. Random changes caused by cosmic rays are as likely to cause a self-replicating machine to become fitter as they are of upgrading your home computer to a newer OS version overnight. If that is still too dangerous for you, you could add a procedure to have each generation do a checksum and shut down if even one bit was out of place. Or add five independent such procedures, just in case.

    The reason biological lifeforms evolve is that they wouldn’t be here if they didn’t. Machines don’t need that, as they are created by us.

  • James M. Essig January 14, 2010, 20:14

    Hi Folks;

    Regarding sails formed from reflective materials that might conceivably be composed of very low mass charged fermions, such as yet fanciful particles with a mass of a small fraction of an electron or a with a mass of a small fraction of that of an electron and a charge to mass ratio much greater than that of the electron, or perhaps even an electric charge much greater in magnitude than an electron, we will now consider sails with yet higher mass specific reflectance then the 10 EXP – 13 kilograms per square meters described above.

    We will further consider the case of a space car wherein the total rest mass of the sail and the space craft is about 1 metric ton since such as assumption puts a higher bound on the possible effective mass specific area limits of the sails. This is due to the fact that with space craft having a significantly greater mass, the capture area of the sail would need to be greater than the area of the maximum possible cross-section of the radiative flux at a distance of 0.006 AU from the Sun in order to achieve higher gamma factors than will be discussed below assumming the craft starts out with maximally expanded sails. Also, manned space craft with a mass below about 1 metric ton are really not practical in consideration of the need for shielding and life support systems.

    We consider a sail with an effective mass specific capture area of 10 EXP – 14 kilograms per square meter. A one metric ton craft sail combination would therefore have an effective capture area of {[(10 EXP – 14)(10 EXP 6)] EXP 1}(10 EXP 3) square kilometers = 10 EXP11 square kilometers and a minimum sail width of about 316,227 kilometers or a half width of about 158,113 kilometers.

    Once again, assuming fraction f of the sunlight is reflected straight back and the sail moves radially outward, the equation of motion is B[(1 + (B EXP 2)]dB/[(1 − B)EXP 2] {[1 − (B EXP2)] EXP 3/2} = p [(R0/x) EXP 2](dx/Ro) where B = v/c, v is the speed of the sail, x is the distance from the sun, and R0 is the initial distance from the sun.

    P = 2fA(u0)R0/[Mo(C EXP 2)] where A is the area of the sail, M0 is its rest mass, and u0 is the energy density of sunlight at x = R0; thus, u(x) = (u0)[(R0/x) EXP 2]. Adopting f = 1, a value of M0/A = (10 EXP −14) kg/(meter EXP 2) = the effective mass specific reflecting area of the sail craft, and u0 ~ L/[4(pi)(Ro EXP 2)C] with L the Sun’s luminosity and R0 ~ 0.006AU, I find P = 235,780.

    This yields {[(1 − (B EXP 2)] EXP (1/2)} [7 − 14B + 11 (B EXP 2) + 2(B EXP 3)]/[(1 − B ) EXP3](1 + B) = 7 + 15p = 3,536,707. With p = 235,780, the terminal velocity = 0 .9957336 C. This corresponds to a gamma factor of 1/{[1 – [(v/C) EXP 2]] EXP (1/2)} = 1/{[1 – [(0.9957336 C /C) EXP 2]] EXP (1/2)} = 10.837.

    Note that it is conceivable that one space car massed pod after another could be serially launched by a dive a fry maneuver wherein a fusion rocket propulsion systems attached to the space pods would permit trailing pods to catch up to the pods in front of them wherein a 1,000 metric tons or perhaps even a 100,000 metric tons rest massed space craft could be assembled enroute that would travel at a gamma factor of about 10.8 thus permitting the craft to visit any star systems within a 500 light-year radius of Earth in one contemporary human lifetime productive working period ship time. Human life span augmentation and/or suspended animation or human hibernation might in theory extend the travel distances to indefinite times both background and ship based reference frames.

  • Ron S January 14, 2010, 21:03

    When a component fails, it could very likely be that ‘growing’ a replacement will be more certain to correct the failure than an attempt to troubleshoot and repair the problem. Humans aren’t very good at growing new fingers so we focus on repairing injuries, but in the technology business it is usually cheaper and more effective to replace a component or subsystem. You also don’t need a greatly skilled service agent. So take along the instructions to build and replace components.

    This assumes a built-in factory of some type and that the failure is in a micro-component, which are the hardest to troubleshoot and the most susceptible to degradation and radiation damage.

  • James M. Essig January 14, 2010, 22:22

    Now regarding the effects on apparent sunlight black body temperature and frequency on exiting solar dive and fry sails, the following formula describes the apparent relative fractional temperature of the solar radiation on such sails where T’ is the actual black body temperature of the sun or star and T is the apparent black body temperature of the sun relative to the reference frame of the space craft:

    T’ = T{{1/{[1 – [(v/C) EXP 2]] EXP (1/2)}}{ 1 – [(v/C) cos (theta)]} -1} where theta is the angle between the velocity vector of the exiting space craft and the direction of propagation of the emitted sun light with respect to the space craft sail.
    For the case of a space craft moving directly away from the Sun, the formula is reduced to T’ = T{{[[1 + (v/C)]/[1- (v/C)]] EXP (1/2)} – 1 }. A black body having an actual temperature, T’, that is receding with velocity v appears to have a spectrum identical to a stationary black body at temperature T.

    The effective relative fractional power received by the exiting relativistic sun-diver space craft sail is equal to:
    Prec = (sigma)[T EXP 4][(4)(pi)(R EXP 2)(A) = (sigma) {T’{{{1/{[1 – [(v/C) EXP 2]] EXP (1/2)}}{ 1 – [(v/C) cos (theta)]} – 1} EXP – 1} EXP 4}[(4)(pi)(R EXP 2)](A), where T’ is the actual black body equivalent temperature of the Sun, theta is the angle between the velocity vector of the exiting space craft and the direction of propagation of the emitted sun light with respect to the space craft sail, sigma is the Stefan Boltzmann Constant, and A is the area of the space craft sail. This reduces to Prec = (sigma)[T EXP 4][(4)(pi)(R EXP 2)(A) = (sigma){ T’{{{[(1 + v/C)/(1 – v/C)] EXP (1/2)}- 1} EXP – 1} EXP 4}[(4)(pi)(R EXP 2)](A) for space craft traveling directly away from the sun. Note that v is the velocity of the space craft with respect to Sun. Also note that sigma = [2(pi EXP 5)(k EXP 4)]/[15 (C EXP 2)(h EXP 3)] = [5.670400 x (10 EXP – 8)](Joule)(second EXP – 1)(meter EXP – 2)(K exponent – 4).

    Now, for a Sun Diver space craft whose sail area increases at a rate proportional to f = (T EXP 4)(R EXP 2) where R is the distance of the space craft from the Sun, the effective power received by the sail remains constant as the space craft travels away from the sun, but then so within limits as eventually, the width of a circular sail as such would need to grow at an impossibly fast rate, in fact for some scenarios, at the speed greater than C and regardless, the limiting condition of the sail occupying a hemisphere with the Sun at its center would maximally bound the limits of gainful solar flux path angular subtendance. As long as at least the following conditions are met: 1)The relative area expansion of the sail is less than or equal to [(Fractional Delta T) EXP – 4][(Fractional Delta R) EXP 2] [Z EXP 2]; 2) The spacing between the line segments of the broken grid lines can be increased but is not so much so that poor reflectivity results; 3) The linear expansion of the maximum width of the sail does not occur faster than C.;4)The kinetic energy imparted to the expanding sail elements relative to the point of sail deployment can occur in a manner such that the space craft can utilize its total energy gain for net positive acceleration. ; 5) The induced drag force on the space craft is lower in magnitude than the sun-light driving force.; 6) The effects of special relativistic reference frame rotation and distortion do not result in drive inefficiencies that cause the magnitude of the drive force or be less than the magnitude of the drag forces imposed by the interplanetary medium.; 7) The magnetic field mechanism holding the sail elements in place is sufficiently concentrated or low enough in total field energy such that the effective inertia of the space craft, the material sail elements, and the magnetic field still permits a net positive acceleration of the entire space craft system, the space craft should continue to be able to accelerate at meaningful levels.

    Note that the relative terminal gamma factor increases achievable from such area expanding solar sails are small compared to the examples of calculated gamma factors I have included in my above previous post. However, collimated BB sources and even more so tightly focused laser or monochromatic microwave or RF beams can be captured by area expanding sails to enable very high relative increases in gamma factors relative to the use of non-expanding single pass solar diver sails, and expandable sails are especially technologically applicable to space craft that must be accelerated over interstellar distances by such collimated or collimated and monochromatic light sources due to beam divergence.

  • Eniac January 15, 2010, 0:27

    Ron,

    You are right, except I would call it manufacture, not grow. When a self-replicating machine (SRM) is developed, it will probably turn out to be advantageous to minimize specialized “repair” procedures, and simply swap out malfunctioning modules for newly assembled ones. There may be recycling procedures for broken parts, or not, depending on the availability of raw materials.

    Of course there is a factory, by definition an SRM is a factory, one able to produce and assemble all of its own parts. Almost as a side effect, any factory able to do that will also be able to produce and assemble other parts of the same general kind, meaning it can produce a much wider range of machines besides itself, which brings us to spacecraft, communication systems, and habitats.

    The most critical part of the SRM, that which should be developed first, is a manipulator that will assemble the parts (some sort of robot arm capable of using tools and moving parts around, including other manipulators), and the physical matrix in which it is embedded (some sort of scaffolding, preferably three-dimensional, think Meccano). The first significant proof of principle would have a bunch of manipulators positioned in the matrix do three things in an automated fashion: 1) Move manipulators and tools around to configure workstations and assembly lines, 2) Assemble and install a functional manipulator from smaller modules, and 3) extend the matrix in any direction to grow the system. With these three capabilities, we already have an SRM, although its feedstock is complex parts, which we have to provide externally. All further development radiates out from there, assembling the tools and complex parts from smaller parts, forming elementary parts from materials, refining materials from raw materials, breaking down rock and dirt into raw materials, and finally gathering the rock and dirt from a natural environment, such as desert soil or lunar regolith. The scope is monumental, but the principles and individual procedures are all rather tractable, I think. Much of it can be adapted from well-established industrial processes. The preferred materials would be those whose constituent elements are found in the natural environment, i.e. iron, aluminum, titanium, silicon, ceramics (metal oxides) and glass (silicon oxide). Organics would be avoided, ideally not used at all. Copper would be replaced by aluminum or calcium in its use as conductor, plastics would be replaced by woven glass or mineral fiber in many applications.

    In practice, most of the benefits of an SRM can be obtained without complete “closure”, i.e. leaving some “vitamins” to be supplied. Those would be things that are needed in small quantity, but are very hard to produce. They could range from rare elements to microchips. Rare elements can hopefully be optimized out of the processes, but microchips can’t, obviously.

    Consider, though, that the availability of even an incomplete SRM makes it possible to produce things by programming alone. Thus, the workshop makes way for a computer workstation, and an SRM greatly helps in its own development. This leads to an autocatalytic effect that counteracts the diminishing returns of achieving complete closure.

    An interstellar probe would not carry a complete machine. It would carry a seed, i.e. a selected minimal set of hardware and supplies that is able to maintain itself for the duration of the journey and bootstrap into a full SRM when it finds fertile ground in the form of a rocky asteroid. For a variety of reasons, not the least of which is to keep the mass of a seed low, the machine would be heavily miniaturized, think toys or watches.

    Pheew, I get way too excited about this issue, please let me know before I am boring you to death.

  • Ric Capucho January 15, 2010, 5:41

    Has anyone produced a self-replicating machine yet? No matter how simple, crude, and of limited (or none at all) utility; other than replicating itself to prove the concept, of course?

    Ric

  • drpayton January 15, 2010, 10:57

    Would tandem usage of tethers, solar sales and fusion engines increase the velocity of a craft beyond the individual capabilities of each engine? And another question… It is my understanding that basically a fusion engine is a nanostar. Wouldn’t this cause gravity? And if it does couldn’t this actually be a stepping stone to a gravity well? Which might actually be used to increase speed even further. Even if these engines were used in stages it could help with energy constraints. Using the solar sail to give the craft a push until the solar “lift” was negligible and then deploying the tether to gather energy to ignite the fusion reactor. It occurred to me that it might be more beneficial to compound these ideas as opposed to compairing. It seems obvious to me that all of these ideas are good ones, and are even better when placed together . Of course I understand that this idea is not unique, but I’m not seeing where it is really being discussed at length. It seems that most of the focus is on trying to find the better engine when it could be that the engines that have been conceptualized are are all correct and could be used together to massively increase velocity… That is of course if u can add velocity beyond the capabilities of the engine with the most thrust by creating seperate thrust from another engine. Using the solar sails and tethers after the initial push and after the detonation of the fusion engine to “tack” for vector change and to use the oposing cosmic “wind” to actually increase velocity sounds promising and not too complicated. It would look strange compared to classicly designed space craft with it’s huge sails and long snakelike tethers glowing with gathered electrons and photons, not to mention the huge ball of sunlike fire seemingly unatatched to the parabolic laser array positioned opposite of the direction of the craft… But it piques the imagination to say “no” it isn’t possible.

  • Eniac January 15, 2010, 13:30

    There have been a number of efforts, you can find out about them at http://www.molecularassembler.com/KSRM.htm. Make no mistake, this is a humongous task and will probably need national scale funding. However, I do think that a proof of principle as outlined above is within the capabilities of a dedicated academic research group or a well-funded startup company.

    Keep in mind, also, that information systems have only fairly recently reached the needed density of storage and processing power, so there was not much time for a real, practical effort. And now there is nanotechnology, which has diverted much of the talent (think Eric Drexler and Robert Freitas) to a much more difficult goal.

  • Gary Allen January 15, 2010, 15:16

    Ric Capucho asked:
    “Has anyone produced a self-replicating machine yet? No matter how simple, crude, and of limited (or none at all) utility; other than replicating itself to prove the concept, of course?”

    Yes. An ordinary machine shop lathe can replicate itself. Of course a lathe needs a human operator but a pair of human operators can also replicate themselves.

    A better question: Does a self-replicating machine exist where it could be placed next to raw materials and be able to fully replicate itself without human intervention? This is essentially a form of artificial life. Has anyone yet constructed a completely artificial bacteria?

  • Terraformer (a.k.a Tobias Holbrook) January 15, 2010, 16:45

    //Has anyone produced a self-replicating machine yet? No matter how simple, crude, and of limited (or none at all) utility; other than replicating itself to prove the concept, of course?//

    Well, there *are* those guys who are trying to perform biogenesis in a Lab….

    Can’t the self replicating machine be a modified biologic organism? Dyson trees are one example of self replicating space habitats, and I’m sure they could be engineered to produce all that is required for humans to live, even medicines. I’m confident an animal-plant hybrid creature could be developed that would form Dyson trees under some circumstances, and a starship (using solar sails) under others, and landing on a planet would cause it to develop along the pathway appropriate for terraforming. Giving it a compliment of bacteria would be a good idea, engineered to be radiation resistent.

  • Ron S January 15, 2010, 17:32

    Eniac,

    I put ‘growing’ in quotes to avoid the very criticism you raised. I don’t disagree with you, but I did want to leave open the method of manufacture, whether it is more conventional (as you write) or some macro/nano-hybrid that (big maybe) could become practical some day in the future.

    While there will be a need for manipulators for some repair and maintenance tasks, it is not always the best solution. For example, if some electronics dies, build the replacement and plug it in anywhere convenient within the reach of the craft’s internal network. It’ll then connect with other components over that network. That reduces the need for a (risky) manipulator task. This of course is not something that applies to a component that has physical proximity or integration as critical to its function.

    And, no, I at least am not bored by your enthusiasm for this topic!

  • Eniac January 17, 2010, 2:41

    Has anyone yet constructed a completely artificial bacteria?

    Craig Venter claims to be real close, and knowing him I would bet he is serious about it, and probably right. See, e.g., http://www.wired.com/science/discoveries/commentary/dissection/2008/01/dissection_0125

    While there will be a need for manipulators for some repair and maintenance tasks, it is not always the best solution.

    Manipulators are definitely needed, simply for putting parts together. Once they are needed for one thing they are available for everything else. The questions then is not “Could this be done without manipulators?”, but rather “Could this be done with manipulators?”.

    For example, if some electronics dies, build the replacement and plug it in anywhere convenient within the reach of the craft’s internal network.

    Yes, but how do you plug it in without manipulators? And you cannot just plug it in right where it comes off the production line, it would block the line. You have to move it at least a little bit, why not move it where it really belongs?

    Once you see things in the way of the second question above (“Could this be done with manipulators?”), you can go quite far. Transportation of parts can be done entirely by manipulators, in the form of bucket brigades. Manipulators could be mobile without a separate mobility mechanism: they could be uninstalled, moved by bucket brigade to where they need to go, and reinstalled there. Even storage could be done by manipulators simply holding things, although in this case efficiency would likely dictate a more custom crafted solution. The same goes for manipulators turning cranks to drive rotary tools….

  • Ron S January 17, 2010, 12:31

    Eniac, I think you misunderstand me. I am not trying to argue for the elimination of manipulators, but rather for reducing their complexity and dependence upon them. This is simply for the reason that anything in a machine that “moves” is more prone to failure than something that does not.

  • Eniac January 17, 2010, 17:03

    Ron, I think I do understand, and I am arguing the opposite. All self-replicators have to move, because growth and movement are connected by the exclusivity of space. Thus, the dependence on moving parts is already there, and absolute.

    Using manipulators for more tasks does not increase that dependence, and using them for less does not decrease it. Not using manipulators for a particular purpose, on the other hand, requires alternative equipment. That in turn may require more blueprints and more production facilities. In the end, ironically, it may require more manipulators, as well.

    Phrased a little differently, reuse of the same equipment for many different purposes (universality) is a powerful way to keep complexity of an SRM at bay, or at least in the software. Manipulators, like human hands, are some of the most universal tools. Anyone who as ever played with a erector kit knows that human hands, some simple structural parts, nut, bolts, and a spanner and screwdriver can produce a very large range of complex machinery. This kind of universality is a core principle of SRM, and is also observed in biology (think ribosome).

    Being failure prone is not quite as much of a problem in an SRM, because self-maintenance can compensate for a certain amount of failure, although there are, of course, limits set by parts production capacity.

  • Ron S January 17, 2010, 20:17

    I think we’ve taken this as far as we can reasonably go since, while we do understand each other, we are now arguing the specifics of technologies that do not exist, even if tantalizingly almost within reach. I am not so wedded to my position that I want to defend it at all costs.

    Given the current situation, arguing the cost-benefit analysis of the alternatives is somewhat interesting but not very fruitful.

  • Eniac January 17, 2010, 23:42

    Ron, you are right, of course. I did not mean to quibble about specific cost/benefit analyses. I meant to emphasize the principle of universality, which is more important for an SRM than it is in conventional engineering.

  • drpayton January 17, 2010, 23:47

    http://www.tethers.com/HiVOLT.html. High-Voltage Orbiting Long Tether (HiVOLT):A System for Remediation of the Van Allen Radiation Belts Summary:The space radiation environment presents a significant impediment to both human and robotic exploration and development of space. The Earth’s magnetic field traps high energy charged particles generated by cosmic rays, solar storms, and other processes, forming the “Van Allen” belts. The high fluxes of energetic particles in the radiation belts will rapidly damage electronic and biological systems in these regions unless extraordinary and expensive measures are taken to harden or shield against these particles. Even with hardening measures, the lifetime and reliability of space systems is often limited by the steady degradation caused by very energetic particles. Under funding from NASA’s Institute for Advanced Concepts, TUI is currently investigating a novel concept for remediating the radiation belts to improve the safety and reliability of manned and unmanned missions in Earth orbit. The High Voltage Orbiting Long Tether (HiVOLT) System, illustrated in Figure 1 below, will utilize long, lightweight, conducting structures deployed in the radiation belts and charged to very high voltages to scatter the energetic radiation particles, causing them to leave the radiation belts. Preliminary analyses indicate that a HiVOLT System can reduce the MeV particle flux in the inner electron belt to 1% of its natural levels within about two months. Figure 1. The HiVOLT System Concept for Radiation Belt Remediation.