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The Case for Beamed Sails

There is a natural path through solar sails, which are now flying, toward beam-driven propulsion, and it’s a path Jim Benford has been exploring for the last eighteen years. In my Centauri Dreams book I described how Jim and brother Gregory ran experiments demonstrating that carbon sails could be driven by microwave beams back in the year 2000. We learned that the theory worked — a sail could indeed be propelled by a beam of photons — and moreover, we learned that the configuration of the craft and propulsion system allowed it to be stable.

Now we’re talking about beam-riding, which the Benfords were able to demonstrate in later experiments. For it turns out that the pressure of the beam will keep a concave-shaped sail in tension, and as Jim pointed out in a recent email, the beam also produces a sideways restoring force. His work showed that a beam can also carry angular momentum and communicate it to the sail, allowing controllers to stabilize the structure against yaw and drift. This is as far as our microwave-beaming experiments have taken us so far, but as solar sails become less an experimental than an operational technology, we can move to space-based experimentation.

Image: A near-term sail experiment under microwave beam. Courtesy of James Benford, Microwave Sciences.

Robert Forward’s name always comes up in such discussions. An old friend of Benford’s, Forward developed enormous interstellar mission concepts using beamed propulsion, ideas that physicists like Geoffrey Landis and Robert Frisbee were able to tweak, just as Jim did, to produce smaller systems. Jim went on to take a cost-optimized approach to the issue, understanding that even the most ingenious of starship designs will be driven by economics. His new paper discusses the matter and notes that a design project using his methods called Project Forward will be undertaken by Icarus Interstellar, the group that manages Project Icarus.

Benford’s notions are solid and based on long experience. As he wrote recently:

I feel beam-driven propulsion is more firmly grounded, more thought through and quantified than nuclear propulsion methods at present. We should put more of our effort into beam-driven sails in this era of little funding. The on-going development of solar sails will tell us how to deploy and control sails, so we will keep close links with that community. This will lead to beam-driven experiments and simulations. Let’s get on with it!

Let’s talk for a moment about the experimental work on beam-driven sails, which was enabled by the invention of carbon microtruss material that is both strong and absurdly light. The material from which a sail is made is critical given that a certain fraction of the power the beam provides the sail will be absorbed and must be radiated away. Given that acceleration is strongly temperature-limited, materials with low melt temperatures like aluminum, beryllilum and niobium are ruled out for beam-driven missions, no matter how useful they may be for standard solar sailing, which uses solar photons rather than concentrated beaming to drive the spacecraft.

Carbon mesh materials work admirably for beamed-sail experiments because carbon has no liquid phase and sublimes instead of melting, as Benford explains in his new paper. These materials allow a sail to operate at temperatures up to 3000 C, allowing them to be ‘launched’ in a vacuum chamber here on Earth without burning. The Benfords were able to push ultralight sail materials at several g’s of acceleration, with the sails reaching temperatures in the range of 1725 C from microwave absorption while remaining intact. Bear in mind that various mission concepts call for lower power densities than the scientists used here. Operating on Earth, they needed a powerful push to get the forces needed for liftoff within a gravity well.

Robert Forward’s interstellar concepts were awesome in their scale, but Benford points out that there is a path to be followed before getting to the interstellar stage. From the paper:

It’s important to realize that for large-scale space power beaming to become a reality it must be broadly attractive. This means that it must provide for a real need, make business sense, attract investment, be environmentally benign, be economically attractive and have major energy or aerospace firms support and lobby for it. Therefore, we include missions that could lead to Starwisp missions, from an infrastructure base developed for smaller-scale missions.

Starwisp was another Robert Forward concept that came out of a time when the scientist moved from laser propulsion ideas to microwaves, whose longer wavelength allowed the sail to be little more than a grid — the wavelengths involved are comparable to the human hand, as Benford told me in an interview some years back, whereas lasers operate at minute wavelengths. A microwave sail, in other words, could be far lighter than the sail required for a laser push because the microwaves are stopped by a conducting surface with gaps smaller than a wavelength. From this, Forward came up with the ultralight ‘starwisp’ design.

Imagine a wire mesh about a kilometer in diameter that weighs no more than sixteen grams. You’ll want data return from the spacecraft so Forward included microchips at each mesh intersection. The craft would be so light and insubstantial that it would be invisible to the eye, but it could be accelerated at 115 g’s using a 10 billion watt microwave beam, taking it to a cruising speed of 20 percent of the speed of light within a few days. Forward’s Starwisp paper included his usual love of gigantic objects, including a beaming lens 50,000 kilometers in diameter.

Geoffrey Landis has shown that the wrong materials would cause a Starwisp to be fried by the powerful microwave beam thus generated, which is why people like Benford are looking at entirely new sail materials as they explore closer and more practicable missions. And practicality — a realistic path forward through solar sails to beamed propulsion — is what I want to discuss on Monday, when I’ll run through the mission concepts Jim Benford has looked at from the standpoint of cost-optimization. Because if we’re going to move beamed sailing out of the realm of science fiction, we’ll need missions that are near-term and offer a clear and economical way to deep space.

The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy.” I’ll post the link when this paper becomes available online.


Comments on this entry are closed.

  • Eniac December 15, 2011, 23:44

    Kalish, I was talking about 10,000 atoms per micrometer, not square micrometer. It is probably an overestimate, rather than an underestimate.

    I was working from an average ISM density of 1 particle per cubic centimeter. The density of condensed matter is roughly 10^22 particles per cubic centimeter. Divide the distance to Alpha Centauri by 10^22 and you get the “compressed thickness” of the ISM along that line. A light year is 10^16 m, thus each light year contains about 1 micrometer worth of material.

    The actual density of the ISM varies a lot and can be up to 10^6 particles per cubic cm, for up to a meter per light year of compressed thickness. I do not know the actual density is between here and Alpha Centauri. Does anyone?

    Assuming a micrometer, that would be about 10,000 layers of atoms. If condensed atoms were about 1 Angstrom apart from each other, we would see each square angstrom (each single file column of atoms in our sail) hit by an energetic particle 10,000 times during the trip. Once the kinetic energy exceeds the chemical energy holding atoms in place in a solid (at around 5-10 km/s) , each particle has the ability to knock many atoms from their places in the sail material, which is what leads to the erosion.

    James makes a few interesting points, but he neglects that our view of the universe is mostly through intergalactic medium, which is presumable much thinner than the interstellar type we would have to deal with here. Indeed, we can not even see lengthwise through our own galaxy.

    I would very much hear a more detailed explanation as to how a sail can affect an acceleration perpendicular to the surface normal direction, as James appears to be claiming. Conventional wisdom has it the force on a light sail is always perpendicular to the surface.

  • James M Essig December 16, 2011, 11:56

    Hi Eniac;

    The general idea for obliquely oriented beams involves the beamed energy incident on both sides of the sail. The sail could include a surface of hair like cilia or any other surface contour that would work so as to much more effectively grab ahold of the light .

    In addition, the sail could be fabricated from photovoltaic materials in order to provide power for electro-dynamic-hydrodynamic-plasma-drives or chargon rockets, or perhaps even photon rockets.

    For extreme gamma factors, the CMBR and starlight will be highly blue-shifted and will be relativistically abberated to what would approach a point source in front of the space craft at gamma = infinity. A sail parallel to the space craft velocity vector made of a suitable negative electromagnetic refraction index material will be pulled forward even by light incident on the sail at a very shallow angle from in front of the space craft.

    To enhance the negative refraction index sails capture of EM energy, the sails may have negative index hairs or cilia distributed along itslength.

    I can provide a much more comprehensive description of the concept by email for anyone wanting to know more about the concept as I envision it.

    My email address is jamesmessig@aol.com.

    Negative refraction index materials have actually now been measured to be pulled on by incident light. Duke University and other academic and government labs are researching the various aspects of negative refraction index materials.

    I have no problem with space craft being pulled forward by forward incident light. Afterall, the paradigm of light speed velocity limits may or may not have been shattered with any future validiation or not of the CERN superluminal neutrino results. The big bang may have been the most recent free lunch. There is no reason why the big bang could not have started with miniscule quantities of mass-energy.

    I will be away from my computer for most of the day tomorrow, Sunday, Monday, and Tuesday as I am going away to attend a funeral and burial service for a deceased Aunt. However, for those wanting to know more about the concepts, I can send you much greater details by email today, or after I return.



  • kalish December 17, 2011, 4:42

    Hi, sorry eniac I should have read carefully. James m essig, as i am a student in metamaterials i m interested by your negative index material pulled by light. It is very not known the radiation pressure can be seen in terms of forces as the lorentz one applied to material’s electrons, thus for an active anyyenna, thete is no problem to use the magnetic field of the light to be pulled, but for a passive material, i really don t understand how it could deal with energy conservation, do you have some references about?

  • Brett Bellmore December 17, 2011, 8:36

    “Fifth, the sail can simply be a deployed magsail or M2P2 type of sail or any other magnetic or plasma bottle sail. ”

    Indeed, while you might build a ‘starwisp” style probe, which survives continual exposure to relativistic interstellar gas by virtue of continuous self-repair, (Requiring mature molecular nanotechnology to build.) anything more macroscopic is going to require some form of shielding to survive exposure to the interstellar media at those speeds. Magnetic fields are the obvious candidate, as even those particles which aren’t initially charged will become so after encountering trapped plasma, and be subject to deflection by the field.

    As I conceive of it, your craft would be surrounded by a protective plasma sheath contained by a magnetic field. During the boost phase a continuous stream of small sails would be driven by microwave transmitters in the home system to kamikaze against the shield, transferring to it all the momentum they’d accumulated on their beam driven trip. (This way a relatively compact probe can be driven by a huge effective area of sail.) Once cruising speed was reached, the stream would cease, (Directed to propelling another mission.) and the ship would gradually slow as it’s magnetic shield deflected the interstellar gasses.

    Once crossing the “doldrums” of the destination star, the ship would be slowed even more by deflecting the solar wind. Finally, micro bombs could be detonated ahead of the ship, as a form of Orion propulsion using the ship’s radiation shield as a pusher plate.

    While sails have a great potential for directly driving very small, lightweight probes, I really doubt anything massive is going to be driven by a huge sail with guy lines tens of kilometers long, rather than using beam driven sails as a form of mass beam.

  • Eniac December 17, 2011, 22:34

    Brett, I agree that the idea of a beam of sails driving a bigger ship is worthwhile thinking about, although it does not really address the problem of sail erosion. Perhaps with the self-repairing nanotechnology, but not even the nanoest of technologies can bring back the material lost to impacting ISM particles, and taking a supply of spare materials is going to incur a huge mass penalty.

    About the ship slowing down, that is not going to happen. The shield and ship will have to be way heavier than the ISM it goes through, or else it will be eroded. That also mean there will not be any significant loss of momentum. If you want to stop at the destination, you have to apply the same energy for deceleration that was used for acceleration.

  • James M Essig December 21, 2011, 0:26

    Hi kalish;

    A good abstract for a great paper on negative super-pressure of light acting on a negative refractive index material is

    Henri Lezec
    (Center for Nanoscale Science and Technology, NIST)

    Forty years ago, V. Veselago derived the electromagnetic properties of a hypothetical material having simultaneously-negative values of electric permittivity and magnetic permeability [1]. Such a material, denominated “left-handed”, was predicted to exhibit a negative index of refraction, as well as a number of other counter-intuitive optical properties. For example, it was hypothesized that a perfect mirror illuminated with a plane wave would experience a negative radiation pressure (pull) when immersed in a left-handed medium, as opposed to the usual positive radiation pressure experienced when facing a dielectric medium such as air or glass. Since left-handed materials are not available in nature, considerable efforts are currently under way to implement them under the form of artificial “metamaterials” — composite media with tailored bulk optical characteristics resulting from constituent structures which are smaller in both size and density than the effective wavelength in the medium. Here we show how surface-plasmon modes propagating in a stacked array of metal-insulator-metal (MIM) waveguides can be harnessed to yield a volumetric left-handed metamaterial characterized by an in-plane-isotropic negative index of refraction over a broad frequency range spanning the blue and green. By sculpting this material with a focused-ion beam we realize prisms and micro-cantilevers which we use to demonstrate, for the first time, (a) in-plane isotropic negative-refraction at optical frequencies, and (b) negative radiation pressure. We predict and experimentally verify a negative “superpressure”, the magnitude of which exceeds the photon pressure experienced by a perfect mirror by more than a factor of two. 1) V. Veselago, \textit{ Sov. Phys. Usp. }10, p.509 (1968).

    Available at:


    Brett, who says the sail must be held by guy lines? A strong magnetic field based coupling or electrical charged based connection might work.

    Another option is to fabricate the sail guy lines out of graphene, carbon nanotubes, boron nitride nanotubes, graphene oxide paper, and the like. A cable constructed from such materials could stretch for about 20 to 50 kilometers yet still handle tens to hundreds of Earth G’s. The tensile strength of graphene is close to 18 million PSI for perfect forms.

    Materials such as solid quarkoniums and some how stabilized neutroniums, and perhaps even Higgsiniums would be better yet, but such materials may only exist in nature in extreme density states as of the present cosmic era.


    Who says the collection area of the sail cannot be very very large. A large electro-dynamic scoop could extent very far out from the sail.

    I still do not beleive that the interstellar medium would cause severe sail erosion even at high gamma factors. Where are the studies indicating the stated interstellar matter density. For such studies, there must be some papers arguing the reverse.

    Regarding nanotech self assembly mechanisms, just simply greatly increase the capture area of a electrodynamic scoop to collect enough interstellar materials and use most of the collected interstellar material as a EHPD, an MHPD, or a combination of the two and use the rest of the materials for sail repair.

    Regarding holding M2P2 plasma affixed to the ship under high gamma factor condition, simply increase the strength of the fastening fields.

  • Eniac December 27, 2011, 0:39


    I am not an astronomer myself, but I understand that both the existence and the density of the interstellar medium are well researched using multiple different methods and are in their essence undisputed in the field.

    There are even maps made of it in the local environment (http://en.wikipedia.org/wiki/Local_Bubble). Apparently we are in the middle of a “local fluff” of 0.1 particles per cubic centimeter. This is one order of magnitude better than I assumed, but, I am afraid, not enough to eliminate the problem.

  • James M Essig December 27, 2011, 13:35

    Hi Enaic;

    This would work out to be a layer of hydrogen or helium atoms about one atom thick for a column that is one light-year long. Not a show stopper for light sails or sails that are electrodynamically shielded or protected.

    If extreme materials are used with excellent reflectance, why not simply use a sail that has a thickness of one millimeter or more and which is monolithic and better yet use a sail with grid lines that are one millimeter or perhaps much greater in thickess. This way, a sail that has an area of only one square kilometer can intercept a beam having an equivalent black body temperature of several thousand Kelvins provided it is constructed of suitably refractive materials.

    What not simply use electrodynamic methods of grabbing ahold of the interstellar gas and diverting around the space craft and sail. The power to operate the electrodynamic mechanisms can be supplied by beams. The electrodynamic methods can include lasers for ionization, or rf radiation where the gamma factors are suitably large, magnetic fields, electric fields, plasma fields affixed to the space craft and the like.

    Then there is always the possibilities for sails comprised of truely exotic materials such as somehow stabilized neutroniums, quarkoniums, higgsiniums, monopoliums, and perhaps even raw space-time-mass-energy forms such as the “Yelm” of mid 20th Century big bang theory.

    Since one cubic meter of neutronium would have a mass of about 10 EXP 15 tons. A 1,000 kilomter long thread of the stuff that has a cross-sectional area of 1,000,000 neutrons would have a mass of only one kilogram. A 1 kilometer long thread having a crossectional area of 1 billion neutrons would have a mass of only 1 kilogram. Lines made of quarkoniums could have the same length and crossection but would be 10 to 1,000 times more massive. Higgsiniums would be all the more massive.

    Provided such extreme materials could be developed, they could also serve as electric current carrying magnetic sail components.

    Anyhow magnetic sails can be made of any ordinary conducting or superconducting period table materials.

    It is also conceivable that a hybrid sail can be used where a current carrying magsail would deflect plasma away from a monolithic and grid like light sail or rf sail.

  • Eniac January 3, 2012, 0:49

    This would work out to be a layer of hydrogen or helium atoms about one atom thick for a column that is one light-year long.

    More like a thousand, according to my calculations. See this, from earlier:

    I was working from an average ISM density of 1 particle per cubic centimeter. The density of condensed matter is roughly 10^22 particles per cubic centimeter. Divide the distance to Alpha Centauri by 10^22 and you get the “compressed thickness” of the ISM along that line. A light year is 10^16 m, thus each light year contains about 1 micrometer worth of material.

    1 micrometer would be around 10,000 atoms thick. At 0.1 particles we would get a tenth of that, or a thousand. Perhaps (hopefully?) I made a mistake?

  • Eniac January 3, 2012, 1:01

    Unfortunately, anything that is more than a few micrometers thick will not produce enough acceleration. Therein lies the problem: Too thick, and it won’t go. Too thin, and it will be disintegrated. At some speed yet to be determined by more in depth calculations. I am afraid that speed will turn out far too low for interstellar ambitions.

  • Eniac January 3, 2012, 23:05

    I don’t think there is a way to “electrodynamically shield and protect” a sail without using much more mass than that of the sail itself, which will render it ineffective for acceleration.

  • James M Essig January 4, 2012, 21:01

    Hi Eniac;

    No actually I was correct. (0.1)(10 EXP 2)(10 EXP 3)(10 EXP 13) = 10 EXP 17 = (300,000,000) EXP 2 or roughly a layer of hydrogen atoms one atom thick and 300,000,000 hydrogen atoms wide. Actually, the diameter of a hydrogen atom is 1/250,000,000 inch or about 1/100,000,000 of a centimeter and so perhaps the layer would be 9 hydrogen atoms thick.

    Still, many methods of shielding can be accomplished. For very refractory materials of extreme strength, I do not see how such relativistic sails cannot be produced. Simply produce a sail that is very reflective and transmits most of the non-reflected light or other EM energy.

    Some such extreme strength materials are as follows:

    1) carbon nano-tubes;
    2) boron-nitride nanotubes;
    3) buckyball-sheets;
    4) layered sheet arrangements of graphene;
    5) graphene-oxide paper;
    6) fabrics composed of a weave or knit on carbon atom chains;
    7) diamond fiber-based fabric;
    8) carbon nitride fiber-based fabric;
    9) combinations of two or more of the above, and the like material

    Metalization would help in these regards

    It is also conceivable that a hybrid sail can be used where a current carrying magsail would deflect plasma away from a monolithic and grid like light sail or rf sail.

    Now, regarding the subject of sail erosion by exposure to interstellar or intergalactic gas, we must realize that the kinetic energy of a gas atom traveling at a velocity of 86.7 percent of the speed of light with respect to the sail would be equal to the binding energy of roughly 10 billion atoms within a sail of micron thickness. Thus, the fact that 10 billion atoms could be dislodged should all of the energy of the gas atom be deposited within the sail. Incident gas atoms having even higher associated gamma factors with respect to the star ship sail could potentially knock loose even more atoms. Perhaps, there is no reason to worry about sail erosion in spite of this for the following reasons.

    First, extremely relativistic particles would likely deposit only a small portion of its energy within the sail thereby greatly lessening the number of atoms that would be knocked loose. This fact would apply to chargons as well as neutral incident particles.

    Second, for sails of near micron thickness, atoms that were knocked loose would likely simply be re-assimilated by the bulk sail materials. Perhaps the only chance for an atom to be knocked loose would include atoms located on the backward side of the sail. Atoms for which bonds where broken within the bulk sail material would tend to simply re-bond with adjacent atoms, certianly for nanotech self repair sails of self healing fabrics.

    Third, since the incident gas or plasma particle would deposit only a small portion of its energy within the sail, the kinetic energy per particle for particles that are knocked loose may be only slightly in excess of the binding energy of the dislodged atoms. Basically, the kinetic energy of the dislodged atoms could likely be re-absorbed and/or radiated away thereby promoting rebinding of the dislodged atoms.

    Fourth, for cases where the sail would completely absorb the kinetic energy of the incident gas or plasma particles such as an alpha particle, for the case of a one micron thick sail, the sail would obviously be able to complete stop the chargon without losing it. Thus, any atoms disbonded by the incident chargon would also likely be captured and prevented from leaving the sail material.

    Fifth, for grid like sails, the grid lines might be positively chargeable so that incident interstellar or intergalactic ions are pushed away from the grid lines and through the openings within the grid like sails. The effect would be similar to the Vander walls force that keeps neutral atoms from being squeezed together to tightly.

    I simply do not believe that relativistic sails cannot be fielded that permit high levels of acceleration of long time periods.

    Once again, the diametrical cross-sectional area of our observable universe is close to 10 EXP 47 square kilometers and the mass of the total mass energy of the observable universe is only about 10 EXP 50 metric tons of which only 4 percent is baryonic.

    Thus, an average column spanning the diameter of the entire visible universe would have an H2O STP matter thickness of only 25 micrometers for reactive matter.

    Quite simply, I have great trouble believing that the density of interstellar matter would pose a problem of a relativistic light or EM radiation sail.



  • Tom March 22, 2012, 18:43

    To live in such times. Reading the previous entries, I am relieved that ‘Photosails’ have such a latitude for development. What would be the best application for such systems? Aside from the potential of man-rated relativistic interstellar transport, what would be a good science mission with these parameters of capability, longevity and the scale of practical astronomy? Craft like these could post 1,000s of A.Us and immediately begin a very long lifetime as Giant, deep space Radio-observatories. The baselines alone could give network users unprecedented accuracy in interstellar cartography and lay down an invaluable resource lines for future astrogation. Between CMBR surveys, planet finding, Galactic time keeping/remote deep space probe networking and stellar explosion early warning…. maybe a little SETI?
    A lot of jobs and advanced data, ultimately you would turn this spiral arm of the Milky Way into real estate. We get the technology of the Ultimate ‘crystal ball’…. but you need ‘Celestial Palaces of Wonder’ to forge ‘the genies’.