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Beamer Technology for Reaching the Solar Gravity Focus Line

Alex Tolley’s essay on using beaming technology to reach the solar gravity focus (SGF) caught the eye of Jim Benford, who has been exploring the prospects for beamed sails for many years. Along with brother Greg, Jim did laboratory work at the Jet Propulsion Laboratory some 20 years ago to demonstrate the method, and in the years since has written extensively on the uses of beaming within the Solar System as well as on interstellar trajectories. But what kind of beam are we talking about? Benford, a plasma physicist and CEO of Microwave Sciences, has done recent work on a gravitational focus mission in connection with Breakthrough Starshot. He points to the maturity of microwave technology and the cost savings involved in using microwaves for a mission far faster than anything that has yet flown.

by James Benford

An intermediate destination for beamed energy interstellar probes, such as Starshot, is the Sun’s Inner Gravitational Focus (SGF). Alex Tolley suggests using Beamer technology for this mission. Gregory Matloff and I studied this approach in 2018 in work on the Starshot Project and published it [1]. This is a summary and update of that work.

The on-going Starshot technology development program will build a modular Beamer system that will incrementally achieve steadily higher launch speeds. As the Starshot technology develops, velocity regimes beyond anything available now will be attained. This will include flyby probes of the outer solar system planets and moons, exploration of the Kuiper belt objects and interstellar precursors to investigate beyond the heliopause. All these missions have the advantage of not requiring any deceleration as the objective is reached. Thus consideration of earlier missions and destinations nearer than the Centauri system is in order.

Here we consider a specific application of the basic Starshot concept, to fly a mission at 100 km/sec. We take sailcraft parameters from Parkin’s Starshot System Model, a thin-film circular photon sail with a mass of 4 grams, a payload of 1.5 grams, a diameter of 5 meters and a thickness of about 0.1 micron (0.2 g/m2, in the range of graphene) [2]. In order not to choose the system parameters arbitrarily, we use the Beamer cost optimization method developed by Benford [3], which minimizes the total system cost.

Why Cost Matters

The approach in our paper is to stipulate the key parameters; mass and velocity, then minimize the cost of the system. All other parameters, such as the sail diameter and, most importantly, the frequency of the Beamer are varied in order to minimize costs. Why does cost matter? These are very expensive systems: note that Starshot is designed/optimized to have a system cost below $10B. We showed SGF Beamer Systems can be in order of magnitude lower.

Economies of Scale

The costs include the decrease in unit cost of hardware with increasing production, economies of scale [3]. The components we’re modeling here, sources of microwave, mm-wave and laser beams, antennas and optics, must be produced in large quantities for the large scales of directed energy-driven sails. High-volume automated manufacturing would drive costs down.

Cost-Optimized Systems

Microwave Beamer cost is 580 M$. (Parameters are wavelength 0.03 m, frequency 10 GHz) parameters for 100 km/sec, 3 gram, 5-meter diameter sail, perfect reflectivity, 0.3m wavelength.) Microwave costs have reached true economies of scale and are now available in quantity at about 0.01 $/W and about 100 $/m2. Consequently, there is no need to extrapolate future microwave cost because present costs are low enough to use.

Millimeter-Wave Beamer cost is 2 B$. Thus far, millimeter-wave (wavelength 3mm, 100 GHz) devices at ~ 1 MW are available at $6/W and 10,000 $/m2. No large market has developed for millimeter-wave devices, so economies of scale have not been firmly established. We assume the learning curve of millimeter-wave tubes will be approximately that of similar tube devices, such as klystron, for which the learning curve is well established. At present the largest application for a megawatt-level millimeter-wave sources is the ITER fusion project, which requires hundreds of devices. An emerging near-term application for millimeter–wave technologies is for 5G Wi-Fi. Although the power levels will be low because of the short-range requirement, mass manufacture of millimeter-wave transmitters and apertures may enable substantial cost reductions to be realized in the next few decades.

Laser Beamer cost is >5.3 B$. Parkin estimates contemporary costs as at least $150/W and 1M $/m2. There are several options for the technology of the laser Beamer: from small mm-scale wafers at ~ 1 W power to larger ~500 W lasers with long coherence length (a key constraint in operating an array). Cost elements include emitters, optics and amplifiers. Lasers are being used for LIDAR in autonomous vehicles and at powers of 10-100 W, cost 100-$1000 $/W. At the higher figure, the Beamer would cost 23 B$!

The large number of sails needed to provide a useful image of an exoplanet means that we must take into consideration the cost of sails. Each sail will cost far less than the Beamer. We estimated the cost of such sails at ~1M$ each [1].

Technology Readiness and Feasibility

  • State-of-the Art. Several practical factors favor microwave and millimeter waves over lasers, because they have practical advantages: Microwave equipment such as sources, anechoic rooms, antennas and diagnostics are commonly available than the emerging technology of high power lasers. That’s because microwave and millimeter wave sources, waveguide and supporting equipment, such as power supplies, are a developed industry. That means it is cheaper and faster to build systems. Lasers are developing fast, but at present are still expensive, and are produced in small numbers at slow rates.
  • Efficiency. Microwaves are more efficient than lasers, typically 50-90%. Millimeter wave generation technologies now make it possible to generate wavelengths as short as 0.1 cm with relatively high efficiency (>40%). Laser efficiencies are ~40% now and have been slowly rising.
  • Phased Arrays. Microwave phased arrays of transmitters and apertures are relatively easily done and are widely used, while phased arrays of laser beams, although possible in principle, subject to the coherence length constraint related above, are thus far little developed in practice. Work to date on laser phased arrays has been limited to small numbers of sources and modest power levels.

Desorption-Assisted Sail Missions

A different method that the JPL group has apparently not noticed is to use the desorption of various materials from the sail, ‘paints’, as it passes perihelion near the sun. That multiplies the utility of the solar sail technique substantially.

Thermal desorption consists in atoms, embedded in a substrate, that are liberated by heating, thus providing an additional thrust. Desorption can attain high specific impulse if low mass molecules or atoms are blown out of a lattice of material at high temperature.

Desorption of materials from hot sails in flight was observed in 2000 in microwave beam-driven carbon sail experiments I was conducting [4]. We found out that photon pressure could account for 3–30% of the observed acceleration, while the remainder came from desorption of embedded molecules.

After we understood what we were observing, my brother Gregory suggested it be used as a means of propulsion for sails [5,6]. The extraordinary potential of this sort of propulsion mechanism: if properly used, desorption could enhance thrust by orders of magnitude, shorten mission times.

Roman Kezerashvili and his fellow researchers have conducted detailed studies using desorption for solar sail missions to obtain high velocities [7]. Kezerashvili recently published a review article about this [8].


Therefore if we are to send probes to the SGF in this era, my calculations show that the lowest cost Beamer will be a microwave system. This will enable a transportation system within the Solar System that could be realized far sooner than laser arrays.

A solar sail augmented by desorption propulsion may give better performance for solar sail missions to the Sun’s Gravitational Focus.

If exoplanet imaging from the SGF is to be done soon, microwave or millimeter-wave beam systems could be built with existing technology now. Developing the phased array laser Beamer and driving the cost down to where larger arrays can be afforded will take decades. Similarly, it will take decades to conduct the test demonstrations required to prove the solar sail approach in the inner solar system. Advocates of both approaches should acknowledge these necessary timescales.


1. James Benford & Gregory Matloff, “Intermediate Beamers for Starshot: Probes to the Sun’s Inner Gravity Focus”, JBIS 72, 51-55, 2019.

2. Kevin Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152 370, 2018.

3. J. Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy”, JBIS 66 85, 2013.

4. James Benford et al., Flight and Spin of Microwave-Driven Sails, Final Report, Contract Number NAS8-99135, 2000. See also short version: “Flight and Spin of Microwave-driven Sails: First Experiments”, James Benford, Proc. Pulsed Power Plasma Science 2001, IEEE 01CH37251, 548, 2001.

5. Gregory Benford & James Benford “Desorption Assisted SunDiver Missions”, AIP Conf. Proc. 608, 462–469, 2002.

6. Gregory Benford, & James Benford, “Acceleration of Sails by Thermal Desorption of Coatings”, Acta Astronautica 56, 593–599, 2005.

7. Elena Ancona, Roman Ya. Kezerashvili, & Gregory L. Matloff, “Exploring the Kuiper Belt with sun-diving solar sails”, Acta Astronautica 160, 601–605 2019.

8. Elena Ancona & Roman Ya. Kezerashvili, “Extrasolar Space Exploration by a Solar Sail Accelerated via Thermal Desorption Of Coating”, Advances in Space Research 63 2021–2034, 2019.


Comments on this entry are closed.

  • Michael January 2, 2021, 8:35

    The way I see it is lasers will win out in the end. Perhaps a small scale microwave array designed to examine asteroids and the moon and also as a aid for sundiver maneuvers, i.e slow them down to fall towards the sun and perhaps a boost on the way out.

    • Jens Fridthjof Knudsen January 3, 2021, 14:54

      What makes a laser superior?

      • Michael January 4, 2021, 1:03

        It’s power and resistance to spreading out.

  • Alex Tolley January 2, 2021, 15:14

    Great article with a welcome focus on cost rather than pure performance.

    Some questions:

    1. What is the sail material areal density before and after the desorption paint is expelled to produce thrust?
    (is there an optimum mass ratio, or is this mission-specific? – the non-JBIS papers need some study, and I cannot get behind the JBIS paywall)
    2. How does beam divergence compare between a phased microwave beam and an regular, unphased laser beam?
    3. What is the practical distance than a microwave beam can be used to power/propel a vehicle?
    4. If the microwave beam is used to provide power, what would you use as a low[est] mass rectenna to capture the beam for converting to electric power?

    • Robin Datta January 2, 2021, 20:21

      “I cannot get behind the JBIS paywall”.
      If you are on the faculty at UC Merced, the library privileges there might be of help.

  • Geoffrey Hillend January 2, 2021, 16:58

    I am not the expert on beamed propulsion, but the details of the photon pressure are not shown here, that is the comparison of mm wavelength microwave megawatt beam to the laser. The problem with radio waves is the inverse square law so radio waves like a flash light don’t have a coherent beam or wave so they spread out really quickly not to far away from the Earth but a laser does not. A lasers light waves are all at the same phase, direction and energy so the don’t spread out like a flashlight over long distance. I don’t see how microwaves can be as efficient a beam propulsion as lasers.

    Now there can be made a MASER, which means microwave amplification through stimulated emission of radiation. A MASER has it’s waves which are all at the same phase and direction. It won’t spread out as fast as an ordinary microwave. I don’t know if these can be made as powerful as visible light lasers, and I think probably not. I am not saying a microwave beamed propulsion won’t work, but only that the radiation pressure can’t match a LASER or MASER.

    I am all for having a megawatt radio telescope. I would like to see what is the size of the microwave beamer. Nothing is mentioned about that in this article. I like the idea because of METI, and having a megawatt radar would be nice for defense, also for spacecraft observation, planetary surfaces, asteroid collision early warning, etc.

    • AlexTru January 3, 2021, 6:41

      Radio waves technology knows well (and new it long time before lasers invention) how to produce coherent (phased) beam, so coherency and narrow beam forming – is not a problem at all for mm frequency range.

    • charlie January 3, 2021, 18:39

      When you talk here about MASERS and other types of microwaves, do you or do you not encounter problems which result in, shall we call it, diffraction -even in a vacuum environment? I’m not at all certain about that but isn’t there some kind of problem even in the absence of matter where there’s a beam spread and that is an issue? Any type of microwave which would be useful for space propulsion I would imagine would have to be constructed in outer space to prevent any type of atmospheric losses would it not?

      On the plus side here is the fact that the technology I guess for microwaves is far more substantially mature compared to high-powered lasers-at least I would think so. The energy per photon I imagine is considerably less than what a visible light laser could manage so you get less bang for your buck.

  • Mike Serfas January 2, 2021, 20:24

    From the original Starshot proposal to the present, I can’t help but be reminded of plans to mine manganese nodules at sea. The resource does exist, and even now Wikipedia will give you optimistic projections of its future importance, yet the most striking effort to exploit the resource, the Hughes Glomar Explorer in 1968, was really just a CIA project to salvage a Soviet submarine.

    Massive arrays of lasers and microwaves have had obvious military application from the time of SDI and “Real Genius”. Today microwave attacks are the leading explanation for the “Havana syndrome”. There are reports in the British media that China has used a microwave weapon “Poly WB-1” in an attack on Indian troops at a disputed border. Most nations are also expressing some degree of justifiable paranoia about what uses foreign vendors will find for directional transmitters in 5G phones (though at present mostly due to their imaging capabilities).

    If astronomers propose any array of the type considered here, what could be done to convince the public that it is not primarily intended as a weapon? Is it feasible for an international observing agency to verify that microwave beams aren’t being aimed at humans from space, or that lasers aren’t being used to start wildfires or commit other arson? I think a strong international treaty to monitor and prohibit military uses of electromagnetic weapons in or reflected from space will be essential for any of these plans to be acceptable to very many people.

  • AlexTru January 3, 2021, 6:43

    The problem of whole Starshot idea is :
    very low, useless payload weight… Our technology cannot make (send) anything useful with this limitations, when project requires huge money amount, for useless or not scientific results.
    So mm, micro or light waves – it changes nothing, till it will be able to carry useful load – it has no any reason to be implemented.

    • AlexTru January 3, 2021, 14:46

      Want to make some explanation – more correctly, I meant that Starshot has no any reason to be implemented for requested price (billions $) :-) Too expensive and no useful.
      Especially when we are talking about very specific application – Gravity focused telescope.
      I suppose that Starshot adepts should , in the begging to prove concept – for example send 3 gram telescope to the Earth or Moon orbit and show it’s ability to do an useful job…

      • Alex Tolley January 3, 2021, 15:34

        If you read Philip Lubin’s road map on using lasers to propel sails. Slower, larger payloads can be propelled into the inner and outer solar system. So while I see Starshot as aspirational, there is certainly value to beaming sailcraft with probes to many destinations in the solar system. Building beamer stations, whether lasers or microwaves should be valuable infrastructure projects.

      • Patient Observer January 3, 2021, 16:15

        Yes, prove the technology first on modest solar system applications. For me, the concept of interstellar probes with masses measure in grams communication over 4 light years is utterly ridiculous with any reasonable extrapolation of present technologies.

        Another means to deliver a payload to the solar gravitational focal point is with nuclear electric rockets. If I did the math correctly, the rocket in the link below could achieve about 70 km/sec with a significant payload – a little slow but higher speeds may be achievable if optimized for higher specific impulse albeit at a slower acceleration,


        There would be copious electrical power available for instrumentation or station keeping once the destination was reached.

      • Brett P Bellmore January 3, 2021, 19:14

        My personal opinion is that Starshot has more potential as a mass beam system for larger probes. Imagine something more the size of Voyager, being pushed up to speed by impacts by a stream of tiny sails.

  • Andrei January 3, 2021, 16:57

    Thank you Mr Benford for your nice presentation, for once I am a bit aware of your work. So I am not entirely surprised that you make the conclusion that microwave propulsion is to be preferred.
    While I do agree that if we are to send a small probe with any beamed propulsion in near future, microwaves would indeed be the choice we need to make.

  • Geoffrey Hillend January 3, 2021, 17:55

    Quote by AlexTru: “Radio waves technology knows well (and new it long time before lasers invention) how to produce coherent (phased) beam, so coherency and narrow beam forming – is not a problem at all for mm frequency range.” This is not correct. The laser was invented in 1960, but the idea of light amplification by stimulated emission by radiation was invented by Albert Einstein in 1917. An ordinary radar like a military radar from the 1940s does not does not have a coherent beam like a laser which is why I mentioned a maser. Consequently, the beam is easily strong enough to get a reflection or “echo” of a target, but it is different story if used for propulsion. The cavity magnetron used for all military radars invented by British scientist won the war for the United States and allies.

    The maser was invented in 1953, before the laser but it was never used as a radar at least not commercially. https://en.wikipedia.org/wiki/Maser

    A Maser works the same as a Laser with a population inversion. Every atom has quantum jumps in it or energy levels. An interesting thing happens with a population inversion which is needed for all coherent photons, for all of the electromagnetic spectrum, gamma rays, x rays, ultra violet, visible light, infrared and radio waves is transmitted through the photon, the messenger particle of electromagnetic fields. In a population inversion, the election is kept at a higher energy level or jump let’s say the second level. (all atoms and molecules have quantum jumps jumps) When another photon hits that exited electron, it jumps up to the third level. When it falls back down to the second level, it emits two photons instead of one. “They both vibrate and move together, at the same energy, phase and direction. Ford, 101 Quantum Questions. The actual size of the election cloud is larger so that electron cloud of the atom is larger, atom appears larger in its excited state at the second level when a photon hits it. Then two photons come out, the extra photon being called a stimulated emission. The same thing happens with the mazer. The big deal is if you have a cavity of two mirrors, the photons are reflected back and forth and and if the two coherent photons hit another exited level two electron or enlarged atom, three coherent photons come out. If those three photons hit another excited level 2 atom, four photons come out, The result, is a cascade of coherent light where all the waves are at the same wavelength. There has to be a cavity of mirrors or some way for the photons to build up. If a photon hits a molecule or atom at the ground state, only one photon will come out, but not two or more as in stimulated emission.

    A radar is like a flashlight has it’s waves at different wavelengths like a flashlight or maybe without stimulated emission the photons don’t move together all at the same wavelength, frequency and direction so they spread out. I am not the expert on lasers. Maybe someone else can explain why a radar or microwave beam spreads out much faster than a maser but it does. There are natural masers in space in gas clouds of exited molecules with population inversions, but there is no cavity for a coherent beam to build up.

    • AlexTru January 4, 2021, 4:58

      Geoffrey, you give lot of laser/maser theory, but it has no connection to your statement – that radio waves (mm wavelengths etc.) cannot be so well coherent and beamed as lasers…
      Radio waves produced mostly by more traditional methods – electronic generators and amplifiers (vacuum tubes, semiconductors), and our civilization uses coherent signals for communication since 191x years till today (much earlier than Quantum Mechanic principles standing after lasers/masers was born) .
      So your arguments related to problems with coherency of radio (mm waves) signals are totally not correct.
      For radio , narrow beam forming is mostly problem of radiating antenna dimensions vs. wavelength ratio, that for mm waves can be implemented “good enough high” for narrow beam forming, but ,yes, it is always worst than for shorter wavelength – IR, visible light, etc…
      Summary – narrow beam shape is limited,
      for lasers, by non-coherency
      for radio waves , by antenna dimensions vs. wavelength ratio…

    • Patient Observer January 4, 2021, 14:36

      The cavity magnetron used for all military radars invented by British scientist won the war for the United States and allies.

      Heck, I thought it was the Norden bomb-sight. No, wait, it was the breaking of the German code. Oh, right, it was the development of Sonar.

      Or, it could have been the skill, bravery and sacrifice of millions of people fighting Nazism. I apologize for this off-topic comment but I was, coincidentally, looking at a photo of my father who fought in WW II. It was people like him who won the war. Enough said.

  • Wojciech J January 3, 2021, 21:05

    Off topic but interesting:
    Chinese perspective on SETI, with focus on cosmologist Zhang Tongjie who is leading China’s efforts in this aspect. Worthwhile to observe this, as we seldom read about non-Western endeavours, and Chinese significance is bound to rise in coming years.


  • Robin Datta January 3, 2021, 23:44

    I had the impression that longer wavelengths have better fidelity over longer distances: when in phase/cohered and collimated, they should stay true over much greater distances that shorter wavelengths. And they could be less disruptive than shorter wavelengths on small scales for the same energy.

    However they may be much more difficult to handle on equipment evolved from the handling of shorter wavelengths. They ideally might require Brobdingnagian equipment. For interstellar travel and communication, if wavelengths of several kilometers are in use by extraterrestrials, they might be overlooked by us.

  • Geoffrey Hillend January 4, 2021, 2:12

    I left out that in order to keep the electron at level two in the quantum jump, energy has to be continuously pumped into the atom which is why a ruby laser has a flashtube and a gas laser has a spark coil.

  • James Benford January 4, 2021, 6:24

    Michael: Alas, the core fact is that laser power has always been far more expensive than microwaves. I’ve been hearing for half a century that lasers will be much cheaper in the future. Fiber lasers have recently become cheaper; let’s hope that continues. Will lasers ‘win in the end’? Only if large-scale applications bring their costs down.

    Coherence issues: Wave constructive interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect sinusoidal wave. Classical devices (microwave, millimeter) can have very long coherence lengths because they are determined by the classical equations of electromagnetism. On the other hand, lasers have a particular limitation: they are quantum devices.

    When the laser power is sufficiently high, nonlinear effects in the laser make the coherence length very short. The practical limit in today’s fiber lasers, the current favorite in high-power laser technology, is about 1 kW per laser. Above that power coherence length shortens, making laser arrays difficult to build because to get high power, many lasers are required. Microwave & millimeter sources are commercially available at ~ 1 MW, coherent lasers at ~ 1kW.

    Phased arrays of laser beams, although possible in principle, are little developed in practice. Work to date on laser phased arrays has been limited to small numbers of sources and modest power levels. In contract, microwave phased arrays of transmitters and apertures are relatively easily done and are
    widely used.

    Alex Tolley: Any beam divergence will have a divergence given by the wavelength divided by the aperture diameter. The laser’s very short wavelength is an advantage. Their disadvantage is that laser apertures are very expensive;
    so far the largest apertures are about a meter. / Optimized microwave beamers tend to have higher powers because it’s cheap. Accelerations are high, acceleration lengths short, above 100 km. / A rectenna for capturing the beam for
    converting to electrical power would be very impractical. Rectennas weigh a lot, cannot take high power densities on them and are increasingly inefficient at increased frequencies.

    Geoffrey Hillend: You’re comparing incoherent microwaves to coherent lasers. That’s not what I’m talking about. Microwave devices of many varieties (magnetrons, Cerenkov generators, backward wave oscillators, etc.) use other processes to generate coherent beams. Coherent microwaves have been
    demonstrated to propagate over large distances with high efficiencies. In the 70’s efficiency of 80% was demoed over a mile distance. A Maser is a different type of device using stimulated emission of radiation to generate microwaves, same process as a laser.

    Mike Serfas: Beamers on Earth could attack satellites, which would be an act of war. But the Beamer would itself be quite vulnerable to attack by conventional military means.

    AlexTru: I’m making the point that early use of the developing Starshot program might use microwave or millimeter sources because the tech is already here. And early missions can be done with larger masses because the velocities would be much lower.

    • Patient Observer January 4, 2021, 22:18

      Thank you for great followup discussions to your post. I do have a simple question (but likely requiring a lengthy answer if answerable at all). Given the high power and apparently high degree of directionality of microwave beams, why not simply use them to transmit energy to a spacecraft able to receive and convert the beam power to electricity to power an ion engine with a very high specific impulse?

      If Wikipedia is to be believed, the conversion efficiency from microwave energy to electricity can be as high as 95%. If a megawatt of power can be received by said spacecraft (and having to radiate away only 50 kW of waste heat) then the balance of power can be applied to well-developed Hall-effect thrusters. True, the beam would need to be on target across millions of miles until the desired velocity has been achieved. But, the energy levels and control required seem relatively small compared to the current discussion. Or, I could be wrong:)

      There is certainly nothing new in the above proposal – just wondering if this would be a promising line of discussion.

    • AlexTru January 5, 2021, 3:50

      Only want to pinpoint my hesitations:
      Today we know well how to send payload that has 0 rest mass i.e. photon’s beam with speed of light – even do not need high power to do such things, but this “payload” – is not useful.
      With our present technology – 3 gram payload same way useless as photon’s beam payload.
      So to prove EM beam (no matter microwaves or light) concept we need to know what should be scientifically useful instrument payload mass , when we know mass we can calculate EM beam power requirements.
      I suppose that to find what is minimal scientifically useful payload mass we need prove of concept – i.e. build minimal mass astronomical tools that can be sent to the desired destination using more conventional methods and probably it can prove that data supplied from this payload can bring to science new data.
      By the way, present situation with multiple university funded nanosatellites that constantly launched to the Earth orbit, proves that it can be very not expensive project. In same time despite those satellites called “nano” – they have mass much higher than 3 gram and they all narrowly specialized devices, but give not so much useful scientific feedback, far from the dream to have high resolution telescope in Solar System Gravity focus.

      • Ron S. January 5, 2021, 11:15

        What you say is true and, in general, I agree. However…

        When we say that we can’t do A because we first need B, and we can’t do B because we first need C, and we can’t do C because…etc., it would be very difficult to take on any difficult project! Only ever doing project steps in series and not in parallel can be paralyzing.

        Of course if any of those steps is technically or theoretically impossible the concern must be addressed or the entire project is doomed. But uncertainty or even the need to wait for enlightenment on one, two or more of those steps is a risk that may become managable. Abandoning a difficult project in the face of risk can be wise or unwise, and it can itself be difficult to assess because of what we don’t (yet) know.

        A judgment call is required. “Series-ism” as a guiding principle can too easily lead to self defeat. There is no simple answer, so I won’t be too harsh on what Breakthrough Starshot and similar projects are attempting despite some near impossible challenges. If nothing else, we will learn something.

        • AlexTru January 6, 2021, 3:45

          “Series-ism” as a guiding principle

          Sorry I do not propose any series-ism or parallelism at all. Even do not touch this point.
          I am only analysing actual reality.
          For example development of small sized image sensors is done right now, by different commercial companies in semiconductors industry. And small sized image sensors required not only for Starshot project, there are many potential customers for this sort of hardware.
          But to be more connected to Gravity Focused telescope, I am sure at first someone should ask astronomers what is their requirement for such instrument, astronomers know well what they need. And analysing existing astronomic project, I am sure that required payload mass will be many orders higher than 3 gram…
          3 gram payload exactly equal to no payload at all , go out of home with laser pointer or generic flashlight point it to the sky – and you can be sure, that you sent “army” of photon (wavelength) sized payloads to the Universe with speed of light, so task to achieve speed of light is done very easily, now you should to solve the task how to get usable feedback (information) from this payload.
          (When I open my minds – I can immediately “invent” to use quantum entanglement, in conditions that there is not as local, as well non local hidden variables there :-)

          • Ron S. January 6, 2021, 13:53

            Sorry to imply I was accusing you of anything. What I was trying to do was point out that although, as you say and I agree, the 3 gram payload limit is exceedingly small for practical use this fact should not detract from working on other parts of the project, provided there is funding and people willing to do the work. The same can be said for any part of the project, including currently “crazy” ideas like square kilometer sails and hyper-powerful lasers/masers/etc.

            Everything about interstellar travel is crazy, so far. Yet we still work on it, and with the likelihood of future advances we can turn crazy into tractable. Or so we hope. There is a sometimes subtle difference between crazy and impossible. We just need to be careful not to confuse the two and, perhaps, choose to work on the parts of the work that are merely crazy.

    • Mike Serfas January 5, 2021, 10:00

      Considering the tight collimation of the beam, I don’t see why the Beamer array could not be placed in a hardened bunker far below the surface. I would be surprised if that is not where it is constructed, perhaps under guise of protecting neighbors from interference or accident. If more than one geosynchronous relay satellite were desired for backup in event of space based attack, multiple apertures could be drilled, but it seems more likely a “constellation of mirrors” travelling relatively together in geosynchronous orbit would be used.

      The concept of building a ground-based laser array and relay satellites to return a laser attack to the ground was explored in the “Air Force 2025 Final Report” (written 1996, mirrored at https://www.bibliotecapleyades.net/sociopolitica/sociopol_weatherwar08.htm ; see Volume 3 Page 42). To be sure, that was not official and the unclassified version was withdrawn after public commentary…

  • Alexander Tolley January 4, 2021, 12:43

    I found this 2006 reference of a comparison of laser vs microwave power beams for Earth-to-orbit launchers.

    A Comparison of Laser and Microwave Approaches to CW Beamed Energy Launch
    Jordin T. Kare and Kevin L. G. Parkin

    AFAICS, the bottom line is a 3:1 advantage for microwave beams with a possible lower risk of reaching TRL 8/9.

  • James Benford January 4, 2021, 13:30

    Alex: Kevin, in conversation with me, said that the ratio should have been higher than 3! He said that he was generous to Jordan in the negotiations.


    • Kevin Parkin January 11, 2021, 19:27

      You have a good memory! I was a grad student back when Jordin and I collaborated on that paper. It was about dueling models and assumptions and approaches.

      Perhaps I was too deferential at that time, but I do not think it matters. Microwave and laser thermal rockets are both possible and would save more than $100M per week at this point. Intentionally and otherwise, we are working toward both.

  • Geoffrey Hillend January 4, 2021, 17:21

    James Benford, I meant radar microwaves like 3 to 30 GHZ. All radars and microwave communications use incoherent EMR. Only a maser use coherent microwaves, and I mean the word coherent light and EMR as defined on Google.

    The waves of a powerful radar will spread out because some of them are out of phase and will destructively interfere and they are not of all the same wavelength and frequency as in a wave packet. Radar is the only type of beamed microwaves. It uses a parabolic dish to focus the microwave beam and it needs a magnetron to give it the power. It is cheap, yes, but it can only get high power in short pulses and a small object in space let’s say in orbit can be detected, but no matter what the power, beamed microwave propulsion will not be strong enough to break the gravity and leave orbit since a lot of the microwave energy is spread out and moves around the object or target and only a small amount is backscattered or reflected off the target back to the receiver on Earth, but most or all of the EMR of a maser and laser will hit the target in space. It might be able push something in deep space, but I still don’t think microwave beamed propulsion would be as efficient or as strong a maser or laser. I still like the idea of bouncing signals off spacecraft and planets and other bodies the solar system though.

    I am not an expert on laser propulsion. Which would work better, group of pulse lasers or a continuous beam? Pulse lasers can be really powerful like the new Thales laser which is very powerful and can get so hot it can be used to study atomic physics. Another kind of laser design chirped pulse amplification is cheap and powerful.

    I think some masers are used as radars nowadays.

    • Adam January 5, 2021, 5:43

      Geoffrey, you’re mansplaining to a world expert in very high power continuous wave microwave systems, so I’ll be very interested to see Jim’s reply.

      • Geoffrey Hillend January 5, 2021, 16:52

        Mansplaining? really. I always thought quantum field theory was complicated. Every part of the electromagnetic spectrum is incoherent from what I’ve read. We can also manipulate any part of it to have a population inversion and amplification through stimulated emission of radiation. Scientists can make a xaser and gaser or x-ray and gamma ray amplification through stimulated emission through radiation. They have also made a free electron laser which does need to use a cavity with mirrors to make the beam and uses wigglers and undulators. https://en.wikipedia.org/wiki/Free-electron_laser

        I will admit that I am not the expert in beamed propulsion and microwave communication and I will say that I recently from what I have learned through reading an article here on Centauri Dreams I think that there might be a lot of fear that powerful laser used for beamed propulsion might be used as a weapon to vaporize spy satellites which we don’t like spying on the United States. One could design a laser which both pushes a spacecraft safely from orbit and also includes a secret, hidden, super powerful pulse laser which is designed to damage targets in space. An infrared lidar with telescope could locate them or maybe a low frequency radar from a radar telescope to defeat stealth technology.

        I do know any type of beamed microwaves must use a parabolic radar dish and it makes a cone of microwaves with the point or smallest diameter at the dish and it spreads out wider as it moves away from the dish. One could also use a radio telescope to make a large radar. I will admit that I don’t know a lot about laser beamed propulsion or James Benford’s microwave beaming, and I would like to see the details of the beamer; it’s size and power.

        A visible light pulse laser will loose some energy when it passes through the air and the energy is lost through heat and sound., but pulse lasers have more peak power but only for a short time than a continuous laser.

        • Brett P Bellmore January 5, 2021, 20:59

          There’s nothing magical or even particularly “quantum” about coherency. EM radiation is “coherent” when the individual photons, (Even radio has photons, they’re just macroscopically large.) have the same wavelength and phase.

          For light this requires some quantum gimmickry, because the individual oscillators originating the photons are atoms or electrons, controlling them is tough. They emit the photons when and how they “want” to. You have to set things up so they “want” to emit it in the same phase.

          For lower frequency EM radiation, coherency is easy, because the oscillators are circuits you can control in real time to produce the same frequency and phase. In fact, the lower the frequency of EM radiation, the harder it becomes to avoid coherency, because if a source of EM radiation is smaller than the wavelength of the radiation, it can’t avoid being coherent, at least in terms of phase. (Though the coherency length, the frequency stability, isn’t guaranteed.)

          So, yes, microwaves can be coherent, and quite easily.

          • Geoffrey Hillend January 6, 2021, 16:09

            It’s the electrons in the atoms which have to be forced up into a higher energy level. For example, a population inversion results when the electron falls back down from the third level to the second level, it emits two photons, which in the same direction, energy and phase or wavelength. With the excited atoms, there is no lasing effect of coherent light.

    • Brett Bellmore January 5, 2021, 15:50

      “All radars and microwave communications use incoherent EMR.”

      This isn’t remotely true, phased array radars are coherent, it’s the basis of their operation. Coherency is relatively easy to achieve with radio or microwaves, because you can phase lock multiple oscillators by ordinary electronics.

      • Geoffrey Hillend January 6, 2021, 16:04

        All phased array radar is many small radars or transmitters put together into one which allow the beam to be turned without turning the radar dish. The F22 raptor, a stealth airplane has a phased array radar in its nose cone, so it does not have to turn the plane to turn the beam. https://en.wikipedia.org/wiki/Phased_array A phased array radar does not use a coherent beam like a maser.

        • Geoffrey Hillend January 6, 2021, 22:11

          Pardon me. The sentence should read without any exited atoms to the third level, there can’t be any population inversion, lasing effect or coherent light or EMR

        • Ron S. January 6, 2021, 23:52

          Do you read the material you cite?

          “Mathematically a phased array is an example of N-slit diffraction, in which the radiation field at the receiving point is the result of the coherent addition of N point sources in a line.”

    • AlexTru January 6, 2021, 3:27

      Geoffrey, I suspect that your understanding of coherency somehow different from mainstream.
      3GHz-30Ghz (10cm – 1cm) wavelength easily maid coherent even with obsolete electronic devices, for engineer even there is no any need to know/use quantum mechanic for this task. It is well described by macro-models, where quantum effects can be (and are) neglected.
      By the way, too narrow EMW radiaton beam can be real headache – not easy till impossible (for astronomic distances) to align two ends… so suppose in real beam propulsion system, bean will be intentionally divergent and far from ideal shape.

      • Mike Serfas January 9, 2021, 13:31

        Would it make more sense to use additional beams for aiming? With three weak, blurry guide beams of different colors aimed in a triangle in a broad region around the tight main thrust beam, the probe could get precise information about how far it had strayed from its proper position, and use a diagonal mirror to get a tiny bit of sideways thrust from the guide beams until it is struck by the main beam again.

  • James Benford January 4, 2021, 21:02

    I’ll just reply by saying that AlexTru’s remarks are right.

  • Michael January 5, 2021, 15:10

    These microwave beams would very useful in moving material between the earth and the moon, if we shot a meshed structure to the moon with material we could use a reflective microwave system to regain the energy landing it as it reflected between the two reflective surfaces, that would be a huge saving in energy. I would be very much in favour on the microwaves been used to develop near field infrastructure as we move out into the system.

  • Michael January 5, 2021, 16:59

    Interesting article on deabsorption.


    It makes quite a difference.

  • Geoffrey Hillend January 6, 2021, 17:17

    I will admit that we could look at beamed microwaves as propulsion in a simple way and miss something. We can compare a radar with a lidar. Radars use beamed EMR. The radar makes radio waves in a cone with the point at the emitter or radar dish. A lidar is simply a laser used as a radar, the word lidar meaning light detecting and ranging. A lidar makes a cylindrical beam which does not spread over distance. It will spread out after a long distance say the Moon, but not as much as a radar.

    We’ve looked at laser technology, but not radio transmitters. A cell phone makes radio waves or micro waves by electric current. The battery accelerates electrons a piece of metal, the antenna. Fayer 2010. When electrons are accelerated, they emit electromagnetic radiation, the radio waves. A cell phone signal can travel twenty miles as long as there is nothing blocking the signal like a hill, mountain etc in the line of sight, so a cell phone needs repeater towers in different locations to boost the signal and get around the mountains, curvature of the Earth etc. A cell phone only uses less than one watt of power. My year 1996 TRC 222 radio shack walkie talkie has four watts of power and uses eight one volt batteries. A switch on the side allows me to change between one watt of power to four watts. My brother Mike who has an interest in electrons and transmitters showed me this experiment. He has the same type of walkie talkie, but with the plastic tip broken off so the tip of the metal antenna is exposed. He told me to put the metal tip on my lip while pressing the transmit button. I noticed that I could feel some warmth on my lip at four watts, but nothing at one watts. The warmth was caused by the four watt radio wave causing the electrons in the water in the skin of my lip to be accelerated. He also built a 30 watt transmitter from scratch that is he soldered the parts together. He told me to hold the antenna while it emitted radio waves at 30 watts of power. The antenna was quite warm so radio transmitters have an antenna temperature which makes them not very power efficient. Just think of the antenna temperature at one million watts and one gegawatt.

    I was thinking about the electromagnets in the LCH, the large hadron collider which use super conducting magnets and a nutty idea came to me. An ordinary electromagnet gets hot before the magnetic field can get too strong and those electromagnets in the LCH would melt long before their magnetic fields got strong enough to bend the proton beams, but they don’t melt because they use super conducting magnets which causes the electrons to flow through the metal without any resistance to make the metal hot. The resistance is caused by phonon scattering at normal temperature, but at very cold temperature, phonons don’t travel any distance. Randall 2013. There are no phonons to cause scattering of the electrons. In an ordinary electromagnet, the phonons absorb the energy from the electrons and heat the metal. Phonons are photons in metal. Fayer 2010. The result much less energy efficiency than a superconducting electromagnetic.

    We might be able to make a superconducting transmitter without any antenna temperature? If so, then it could operate at very high power for a long time and not get hot. If the physics of this is possible, then it might not be to mobile, but it might not have to have steel as thick as the LCH electromagnets, but it would be expensive. Whether one is building interstellar communications or beamed propulsion it still will be expensive because this technology has not ben designed or build yet and necessity requires innovation.

  • Geoffrey Hillend January 7, 2021, 16:41

    I am surprised not anyone challenged my idea of a superconducting antenna. I talked to my brother about it and he said it was a crackpot idea and an antenna does not have a temperature so the LCH superconducting electromagnet does not apply to antenna’s. The warmth I felt was in my skin from the radio waves exciting the atoms. A apologize for not researching it on the internet first.

  • Eric Strobel January 11, 2021, 14:49

    Two questions:

    1) Any thoughts on engineering a microwave sail with desorption material on both sides and an intermediate layer that reflects at one frequency, but allows a different frequency through (to the desorption material on the other side)? The thought being to get the high speed outgoing, but allow a way to reduce speed somewhat at the SGF.

    2) Since we’re talking RF… has anyone in the SETI community given thought to what Earth would look like in the RF when viewed by gravity lens?


  • Kevin Parkin January 11, 2021, 20:05

    To answer your first question, jettisoning any part of the sail’s mass in the name of propulsion means that you are governed by the rocket equation.

    To reach 40 au/yr (the peak speed of the Parker Solar Probe) , and given the exhaust velocity of desorption, let’s say (generously) 10 km/s, then the rocket equation predicts that the sail’s initial mass must be 170 million times the final mass. If your final sail had a mass of 1 gram, your initial sail would have a mass of 170 metric tons.

    On the other hand, sails that are accelerated by microwaves alone will likely be able to accelerate at approaching Earth gravity anyway (and I mean this in a rough order of magnitude sense). The exact number depends on thermal, electrical, and mass properties of the sailcraft.

  • James Essig January 13, 2021, 2:37

    I really enjoyed reading this thread. Although I do free-lance theoretical work and writing mostly on relativistic rockets, it is good to consider multimode propulsion systems.

    I cover a very wide variety of propulsion methods, and sailing methods, in my spacecraft concepts presented in my ebooks, “The Monolithic Christmas Tree Light-Sail.” and “The Monolithic Christmas Tree Light-Sail. Expanded Edition.”

    “The Monolithic Christmas Tree Light-Sail. Volume 2.” is in the publication process.

    These books hit on the kind of light-sail techniques that would be useful for high end Keplerian velocities to ultra-relativistic velocities. So they can have merit for for beamer methods to the solar focus line.

    An interesting idea would involve a powerful light beam that would be intake into a PV, thermo-electric, or other conversion mechanism. The electrical energy generated could be used in an electrodynamic-hydrodynamic-plasma-drive. The way I see it, there are 4 main types of such drives. These are; 1) magneto-hydrodynamic-plasma-drives; electro-hydrodynamic-plasma-drives; electro-magneto-hydrodynamic-plasma-drives, and electromagnetic-hydrodynamic-plasma-drives.

    Another method that Paul Gilster has mentioned previously is beamed energy for powering ion rockets. Here, the energy to produce and accelerate a plasma to near light-speed in the spacecraft reference frame would not be sourced from onboard matter and antimatter fuels, but instead derived from powerful beams of light, or perhaps from phased array microwave and rf source beams.

    Regardless, it is nice to see that folks are working on this kind of stuff.