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Comments on Near-Term Interstellar Probes

If you have questions about beamed energy concepts, James Benford is your man. A plasma physicist who is CEO of Microwave Sciences, Benford has designed high-power microwave systems for the likes of NASA, JPL and Lockheed. Now Chairman of the Sail Subcommittee for Breakthrough Starshot, he is deep into the investigation of sail materials and design, as he explains below. After reading Greg Matloff’s Near-Term Interstellar Probes: Some Gentle Suggestions, Jim passed along his comments, which highlight the need for a dedicated laboratory facility to explore the Starshot possibilities. He offers as well his thoughts on where sails stand in the overall propulsion landscape, a position of growing significance.

By James Benford


My colleague and old friend Greg Matloff has given us a well-informed broad survey of propulsion options for interstellar flight. I’m going to contribute a few comments.

Even a century-long flight to Alpha Centauri requires a velocity of ~10,000 km/sec, which is about 500 times the fastest velocity that any human object has reached by rocket propulsion — Voyager 1 is at about 17 km/sec. On the way to getting to a velocity like 10,000 km/sec, we’re going to enable fast missions for exploration and development of the solar system and the near-interstellar region around us.

Beamed Energy Propulsion

The cubesat experiments described by Greg would probably have to be fairly massive in order to produce any substantial velocity. It’s far more expeditious and far less expensive to do such experiments in evacuated chambers in the laboratory. After all, the only flight experiments that been conducted to date, which I participated in 16 years ago, were conducted in just such apparatus. That’s the most straightforward way to demonstrate stability and high acceleration. No such laboratory exists at present.

I’m hoping that Breakthrough StarShot will soon develop such a Beam-Driven Sail Test Facility, because the pressing matter of stability and high acceleration will take some time to be researched. Of course such a laboratory has to be designed and built and that would take a year at least. The most suitable beam source will be either microwave or millimeter-wave, because they can be obtained off-the-shelf at high power and have quite reasonable costs of a few dollars per watt. (Lasers are now in the hundreds of dollars per watt range.)

Such a Beam-Driven Sail Test Facility would conduct experiments on the key Starshot issues, in order of priority:

    1) Beam-riding stability of various sail shapes.

    2) Sail material and its properties: to maintain a high reflectance with very little beam absorption or transmission.

    3) Ability to sustain high acceleration, which is necessary in order to achieve high velocities.

The Cosmos-1 experiments we (the Planetary Society, Microwave Sciences and JPL) had planned to do, and which unfortunately didn’t happen because of the launcher failure, should certainly be looked at again. We were ready to carry out an experiment to irradiate the sail with the Deep Space Network beam from Goldstone. This could have demonstrated beamed propulsion of a sail in space. The 450 kW microwave beam from the large 70-m dish can show direct microwave beam acceleration of the 30-m sail by photon pressure, and we can measure that acceleration by on-board accelerometer telemetry.

The key thing to do is to measure the acceleration on the spacecraft with an on-board accelerometer. The alternative, deducing acceleration from orbit changes, would depend upon multiple transits of the sail through the beam. It would be hard to winkle out of orbital data, especially if the acceleration is small. [see “MAX-Microwave Acceleration eXperiment with Cosmos-1,” James Benford, Gregory Benford and Tom Kuiper, Proc. Fourth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, also JBIS 59, pg. 68 (2006).]

Greg Matloff is certainly correct to point out that the sail material for Starshot must be able to reconfigure its shape. It must have very precise and accurate pointing and tracking, at both Earth and the star that it is approaching. The sail must be a very smart material with embedded and distributed artificial intelligence. How to provide the energy for reconfiguring the shape of the sail is one of the many challenges of Starshot.

Thermonuclear Fusion Rockets

Greg gives a good list of the many technical problems that face fusion rockets. Of course the greatest problem is to produce a fusion reaction at all! For interstellar flight they will be very large, very inefficient and very costly.

For comparison, here are some key parameters of a US supercarrier, two Icarus design concepts and the Starshot sailship:

  • Aircraft Carrier 0.3 km, 105,000 tons, 0.01 T$
  • Firefly 0.75 km, 23,550 tons, mass fraction 0.0064, rocket cost 40 T$, 4.7% c
  • Ghost 1.2 km, 154,800 tons, mass fraction 0.0008, cost 0.02-34 T$, 6% c
  • Large Sailship 10 km, 10 tons, mass fraction ~0.1, Beamer cost 40 T$, sailship capital cost ~ 1B$, 10% c [operating cost, electricity to drive the Beamer at today’s rate (0.1 $/kW-hr) is 0.5 T$.]
  • Starshot ~3 m, ~1 gram, capital cost ~10B$ [operating cost ~6 M$.]

And for a size comparison, see Figure 1. The ships that explored the oceans, such as the Santa Maria (19 m) and Kon-Tiki scale Polynesian rafts (45 m), as well as the Breakthrough Starshot sail (~3 m) could not be visible on this scale.

With the fusion rocket approach, the infrastructure necessary to build such huge vessels and supply nuclear fuel is a fixed cost. It is not easy to estimate, but will be quite huge.


Figure 1. Scales of Starships compared with largest Earth vessels. From top, largest Earth ocean ships, Firefly, Ghost, 10 km diameter Sailship, all to scale. The ~3m Breakthrough Starshot sail is actually not visible on this scale. (Figure from Michael Lamontagne).

The concept that Freeman Dyson originally proposed, using nuclear materials out of the nuclear arsenals to make explosives to drive starships, is a bit out of date now. As you can see in figure 2, the Cold War adversaries have been radically reducing the warheads they have, the rapid progress beginning during the Reagan–Gorbachev era. The Nunn-Lugar Cooperative Threat Reduction program deactivated more than 7,600 nuclear warheads. Highly enriched uranium contained in them was made into commercial reactor fuel which was purchased by the U.S. Few Americans realize that during the last several decades a fair amount of the electrical power the United States was generated by burning up Russian nuclear materials in fission power reactors!

There are now a few thousand nuclear explosives compared to the hundred thousand at the peak of the Cold War.


Figure 2: History of Nuclear Stockpiles.


Beam-driven propulsion is more firmly grounded and credible than nuclear fusion propulsion, as in Project Icarus. Fusion rockets remain far-term, distant on a timescale of decades, if not centuries. In today’s funding environment, that’s not likely to change: Due to Starshot, sails are becoming near-term.


Comments on this entry are closed.

  • Greg Matloff May 30, 2017, 10:14

    Hello Jim

    I am very happy that you agreed with my “gent;e suggestions” entry last week. I heartily concur that one of the accomplishments of Project Starshot should be an establishment of a Beam Stability Laboratory. It will be both fun and very instructive to observe how some of the proposed sail shapes perform when actually exposed to a collimated electromagnetic beam.

    Regards to all, GREG

  • Astronist May 30, 2017, 10:21

    “fusion rockets […] For interstellar flight they will be very large, very inefficient and very costly.” – I’m a bit surprised that you describe fusion rockets as inefficient. As you know, laser sail propulsion is energy inefficient unless the final speed is a large fraction of the speed of light, because one is stuck with an exhaust velocity equal to the speed of light. The Daedalus engine design is relatively energy efficient, assuming it can be built, because the exhaust velocity is of the same order of magnitude as the mission delta-V, and because the transfer of waste heat into the engine is small. Furthermore, the rocket can be used to decelerate at the destination, while the laser sail system depends on a deceleration laser having been earlier installed in the target planetary system.

    As to the comparative costs, that will depend very much upon how large a market grows up for use of the various propulsion systems within the Solar System over the coming centuries. I can see fusion, sails and high-power beams all having a role to play.


  • Alexander Tolley May 30, 2017, 12:19

    The stability experiments that have been done to date were with passive sails. Spin stabilization and various shapes have been tried. At the recent conference IIRC, computer simulation experiments were reported.

    What if the sail needs active control? I’m thinking here of the IKAROS sail’s use of surfaces that changed reflectivity to control orientation. It may require some active control of the sails shape or reflectivity to ensure the sail stays in the beam. Or can the beam intensity across its area be constructed to do the same thing?

    I can imaging beam manipulation being tested in a vacuum facility, but any sail technology that allows active control seems a way off to my limited knowledge.

    Having said that, sails and beams look like a very useful technology road for space exploration and exploitation. With parallel development, even if sails proved difficult to control with beams, those same beams could provide energy for space probes (e.g. powering ion engines and instruments) and facilities. Sails should be useful even if beaming has limited potential to be used with them, offering slow, but fuel free and reusability for a variety of missions difficult to manage with rocket propulsion approaches. If beams and sails are synergistic technology, even if star flight is not easily achieved, the roadmap could include a host of fast interplanetary missions. I would love to see mass produced sails of various sizes using beam facilities to host a wide range of sensors for exploration on a scale vastly exceeding anything we have done to date.

    • Michael May 31, 2017, 3:08

      I agree with you Alex, there is much this machine can do and will fill a huge gap in the space roadmap. It is this integration with the rest of the space game that will determine its success.

    • J. Jason Wentworth May 31, 2017, 5:25

      Such laser stations could also power laser-fed ion drive spacecraft (see: https://www.centauri-dreams.org/?p=37488 ), which have potential for solar system, ultraplanetary (interstellar precursor), and perhaps true interstellar travel.

      • Michael May 31, 2017, 16:06

        Once we get one of these beamers on the moon we are in a good position to send much heavier probes.

      • Michael June 1, 2017, 5:09

        If we had one on the moon it could be used as a spacecraft brake where the incoming craft is slowed to a stop with the excess laser light been used (bounced energy) to launch other payloads or communication satellites into space. They could also be used at the base of a space elevator to power the acceding and descending crafts, phased arrays offer a concentrated beam that would easily supply power along the length of the cable.

        It is the ‘other things’ that this machine can do that really excites me.

  • Brett Bellmore May 30, 2017, 13:05

    Fusion without bombs is currently technically infeasible. Fusion with bombs is “merely” politically infeasible at the moment. It’s anybody’s guess which form of infeasibility will be resolved first.

    Anyway, I agree that beam propulsion is the best bet given current knowledge.

  • Joe H. May 30, 2017, 13:45

    I might be mistaken but I think the 100 Year Starship project hopes to use Magnetic Inertial Confinement Fusion. In discussions of various fusion ideas, I haven’t seen this one mentioned in the last two Centauri Dreams posts. Can someone describe the technical challenges of MICF in a similar manner to the other types of fusion propulsion discussed?

    • Paul Gilster May 30, 2017, 13:57

      Joe, I haven’t heard any mention of MICF with regard to 100 Year Starship. But as to the viability of MICF concepts, I’ll let readers weigh in. I think Jim is right in saying that the key thing is to get fusion to happen in the first place, much less turn it into a working engine design.

  • Larry Kennedy May 30, 2017, 13:54

    How’s this? Build a ring in a chosen solar orbit with smart materiel 5 meters wide and 100 million kilometers in circumference. The axis would be aimed toward the target. The part facing the Sun would of course be highly reflective. A partial ring of lighter weight solar sails would hang around the outside. The sails would maintain the proper orbit and orientation. The smart materiel would supply the fine pointing and focus.
    I’ll let the optics specialists do the numbers but I have to think this could maintain acceleration on a large sail to extreme distances. It also seems to me this would be a far more effective signaling device than many that I have heard suggested.
    I’m positive this is not a new idea but I don’t know where it may have been detailed previously.

    • Michael C. Fidler May 30, 2017, 22:51

      The heck with using it to launch sails, just have the surface polished well and shape it into a giant optical reflecting telescope and point it at Proxima b and see more detail then the sails will when they get there! Putting it around the Moon would be even easier!!!

      See this article on the Russian “RATAN-600, The World’s Largest Radio Telescope”

      • Michael C. Fidler May 30, 2017, 23:01

        Or even easier, in a large lunar crater!!! ( Was that what the Russians were building in 2001 A Space Odyssey?)

    • Joe H. May 31, 2017, 11:05

      Smart material? Are you thinking of scrith from Ringworld? http://larryniven.wikia.com/wiki/Scrith

      Might was well be talking about unobtainium.

      • Larry Kennedy May 31, 2017, 12:16

        No I’m talking about things that are in the pipe or actually working today.
        I think you misunderstood my meaning. I’m not talking about a ring world type object with artificial 1 G. Each arbitrarily defined segment of this is orbiting whether attached or not, indeed no real requirement for it to be one piece at all. It just makes coordination somewhat easier.
        The only thing which makes this difficult is the scale, which makes it more directly achievable than many things discussed here.

  • Michael May 30, 2017, 13:54

    Reconfiguring the sail may be as simple as having carbon nano tubes to pull it into shapes. I put forward a simple idea on the breakthough website to join the currently shorter nanotube lengths together to form wires.

  • RogerPenna May 30, 2017, 22:44

    Are these Fusion Ships even comparable with the Laser Sail CHIPS?

    I might be wrong, but these fusion ships weighing thousands of tons are probably made to be crewed.

    So, why are they being compared to CHIP ships that can´t even stop at their destination?

    Oh, there is a 10 tons Laser Sail ship.

    Well, I really doubt the 10km Sail Ship weighing only 10 tons will be able to carry a crew on a journey spanning a decade or more.

    A 10km 10 tons sail ship might be feasible for travelling inside the Solar System. That’s how long the consumables will last inside a 10 tons ship.

    • Paul Gilster May 31, 2017, 8:19

      Roger, the Icarus designs you’re talking about are unmanned.

  • Nick May 30, 2017, 23:44

    Breakthrough Starshot is terrific, but what if wafer-sized spacecraft prove too difficult to work with, or if we eventually want to send something bigger? For the long acceleration times that larger craft would require, I’ve been under the impression that at least a couple things would be necessary (among others):

    1. Massive telescopes (used in reverse) that can project a laser beam deep into space with minimal beam divergence and spot size

    2. Sailcraft that can make their own adjustments to keep themselves centered on a beam for long periods of time, since it’s virtually impossible to make adjustments at the laser source when the craft is light-minutes away

    Is it reasonable to assume that those will be necessary, and if so, could they be developed on a long enough timescale?

    • Albert A Jackson May 31, 2017, 9:23

      Starshot is to work on problems like this. There have been concentrated group efforts , like the BIS Daedalus, still no study that has lasted decades. Hoping this will be a long term focused effort even is no metal is ever bent.

      • ljk June 1, 2017, 11:29

        Those chips or something like them should at the least make for excellent little minimally obtrusive robot explorers at a target star system, even if they end up having to be carried in a larger protective vessel.

        It would make sense to place multiple small sensors across an alien world not only for maximum coverage but also to reduce interfering with any potential native life forms during the effort to study them. I am not even talking about some Star Trekkian Prime Directive of Non-Interference, just the importance of examining a habitat in a way that will not disrupt the normal functions and behaviors of any creatures, smart or otherwise.

        This certainly raises the intellectual eyebrows in regards to all those UFO reports of giant shiny alien ships with big organic crews landing all over Earth and messing with the natives. Would not tiny automated machines do a much better job, assuming their purpose is scientific study? Or do I just not get the motives of advanced beings who can cross the interstellar voids?

  • Michael May 31, 2017, 6:01

    Although currently lasers are in the hundreds of dollars per watt if they are mass produced for this project they will drop significantly in price, 100 GW of laser power is significant. Which brings us to having to decide on which frequency to use and studies will need to be carried out to which is best. I would prefer we piggy back on the communication industry standards of around 1 and 1.55 micron which allows a natural converging of price reductions in quantity over time just like with microwave systems.

  • Adam May 31, 2017, 7:46

    Hi Jim
    Freeman’s interstellar Orion assumed pure deuterium devices, rather than present day Teller-Ulam devices. Since we still can’t do pure fusion detonations, it’s a non-starter.

    Purely electromagnetic beamers are promising but sail-based macron beams will be needed for pushing bigger vehicles at lower accelerations if humans are to get to the stars.

  • ljk May 31, 2017, 12:44

    Quoting from the main article:

    “The concept that Freeman Dyson originally proposed, using nuclear materials out of the nuclear arsenals to make explosives to drive starships, is a bit out of date now. As you can see in figure 2, the Cold War adversaries have been radically reducing the warheads they have, the rapid progress beginning during the Reagan–Gorbachev era.”

    My reply:

    There is a pretty good chance that the USA and Russia (and some other nations) will be building back up their nuclear stockpiles rather soon.

    Even if they do not, there is another nation with a both a big space program and nuclear arsenal that I think could be mighty interested in Orion. I wrote about them here:


    Quoting from the above linked article:

    “So could (and would) Orion be revived some day? While the United States and Europe (via the European Space Agency) could make the vessel a reality, Orion’s very means of propulsion remain among the major hindrances to such a plan for the foreseeable future. At present, the greatest chance for Orion lies with China. They not only have the means and the resources to undertake such a grand project (as well as vast, remote regions where they could safely test and launch the vessel), but Orion would be a logical extension of their current strivings for major science and technology goals. This author notes that he has no actual knowledge if China has ever conducted or will ever conduct such a project, only that of all the spacefaring nations on Earth, they make for the most realistic choice at present for reviving Orion.”

    You know what else is amazing about Orion, in addition to the fact that of all the relatively fast interstellar propulsion methods listed it is the only one that could be done NOW rather than decades or centuries into the future? That unlike most other methods of lofting payloads into the void, Orion’s very nature THRIVES on the concept of bigger is better. Read here:


    It is not that I don’t want to see the other methods of interstellar travel happen here, it’s just that as someone who was promised lunar and Mars colonies and manned missions to Jupiter with nuclear-powered vessels decades ago and is still waiting for any of this to happen, I am more than a little wary of endless academic studies of projects that sounds oh so good and then throw in little facts like, no one has been able to create controlled fusion reactions yet, or built and controlled an insanely-powerful laser that is supposed to keep a vessel stable and on course all the way to Alpha Centauri (and don’t tell me that wouldn’t be considered one heck of a deadly weapon in itself), or how antimatter would be great – if it didn’t cost trillions of dollars per gram or some other outrageous figure. And do not even get me started on warp drives and how NASA is secretly working on one, etc. If there isn’t a working machine available, it might as well not exist.

    Not that I have anything against the propulsion plan for Breakthrough Starshot in the academic sense, but I get the strong feeling that people support it for two main reasons: It conjures up romantic imagery of starships literally sailing across the galaxy and, far more key, it does not involve anything nuclear as in Orion.

    Is the Russian billionaire Yuri Milner going to keep pumping in funds for Starshot once the $100 million runs out? Who is going to fund and build that super laser? Where will it be kept and by whom? Tell me how this is going to get us to Alpha Centauri any sooner?

    • Alex Tolley May 31, 2017, 13:44

      Who is going to fund and build that super laser?

      I thought Lubin’s original pitch was pretty clear. His DSTAR laser arrays were to be used for planetary defense, most probably funded by the DoD. Given their use as weapons for killing satellites as well, not to mention possible use on ground targets, this is almost a no-brainer for the DoD that has experimented with laser weapons for decades. Even the Star Wars program proposed by Reagan had ideas of one-shot UV laser satellites powered by nuclear explosions. Don’t worry about funding, if they can be made as useful weap…err asteroid vaporizers, the DoD will fund them. Now whether they get any use as sail beamers is another matter.

      • ljk May 31, 2017, 15:04

        That is the problem and always has been: Science gets the table scraps, sometimes if they are lucky. SETI is a prime example of this until recently, and I am still worried once the main money goes, it will be back to begging for telescope time.

        I want to hope the DoD will handle this laser for space science the way they did with the Clementine lunar and (aborted) planetoid probe, but that was one small satellite: This is a super laser that will cost much, much more and do a lot, lot more.


        Should we place our hopes again on the private sector?

      • Antonio May 31, 2017, 16:27

        If the huge-weapon-of-the-judgement-day is not a real problem, why hasn’t Orion been developed yet?

        • Michael June 1, 2017, 5:27

          Orion although workable has some issues not just with political fallout but a radioactive one if used in our atmosphere and near space (magnetic field entrapment).

          • ljk June 1, 2017, 11:39

            Humanity set off nuclear bombs above ground in (relatively) remote regions of Earth intensively from 1945 to 1962 (excluding the end of WW2, of course), then sporadically after that. Not only has humanity as a whole survived all these deliberate detonations but our numbers are at 7.5 billion and growing without any obvious brakes.

            So that being said, if we had to launch a few Orions from Earth’s surface in remote regions (and there are still plenty of those places, especially the ones that were once used to test nuclear weapons), dare I say we might also survive these efforts? If it makes folks feel better, we can do just a few to get established in space an infrastructure to build more Orions far away from Earth and using the abundant resources found in interplanetary space.

            I will also bring up a reminder that humanity is still a major polluter of non-nuclear materials far beyond anything we did in those Cold War days, despite supposedly better awareness and some efforts to clean up our messes.

            • J. Jason Wentworth June 2, 2017, 4:57

              …And don’t forget the one or more early, un-shielded military satellite SNAP RTGs that burned up on re-entry, scattering plutonium far and wide (plus leaking radioactive coolant from Soviet Topaz reactor-powered ocean surveillance radar satellites, one or more of which have also re-entered)–yet no noticeable effects are evident.

              While I’m not suggesting that we should take a casual or cavalier attitude toward in-space nuclear power, humanity does need to outgrow its (overall) ignorant, childish, and self-limiting “Nuclear?! EEEEEEK!!!” reaction to nuclear power, whether for space or Earth use. Uranium, plutonium, and other such substances are energetic materials, and as such they should be handled and utilized with appropriate care and precautions. But not even the most ardent anti-nuclear activist thinks twice about pumping gasoline or diesel fuel into his or her car, even though both of those two fuels, with the correct proportions of oxygen, release several times as much energy as an equivalent mass of gunpowder, and much more energy than even such a mass of nitroglycerin!

              • ljk June 2, 2017, 8:49

                Did you know that some of the current environmental leaders were in the process of finding ways to have their followers become more acceptable of nuclear power, as it is much cleaner than various fossils fuels. Then the 2011 earthquake in Japan happened and the resulting disaster at the Fukushima nuclear power plant delayed those plans.

                Funny how the massive Deepwater Horizon oil spill in the Gulf of Mexico just one year earlier did not stop people from continuing to use their petroleum-powered vehicles. It’s always easier to protect Earth when it’s not an inconvenience.

                • Alex Tolley June 2, 2017, 11:11

                  There was a TED debate where Stewart Brand advocated for nuclear power. While there is potential for nukes, currently nuclear power is not cost-effective compared to other energy sources, and that is before such issues as who will insure the liability. Meanwhile, renewables are rapidly hurtling down the cost curve with the assurance that a technical failure won’t contaminate an area.

                  In the far larger picture, nuclear fission is a paltry energy supply compared to the sun’s output. We will ultimately source solar energy for the lifetime of the human species, even if that means that some of it will be converted to fissionable fuels.

                  • ljk June 2, 2017, 14:16

                    Yes but what do we do for missions and colonies that will eventually head into the outer planets region of the Sol system? Solar energy will not be enough.

                    • Alex Tolley June 2, 2017, 14:34

                      Solar energy will either be beamed directly to deep space, or converted to fuels and shipped, probably both. It is the same issue on Earth with solar energy. Create electricity in the desert regions and use transmission lines, or convert it to fuels and transport it for end-of-use applications. The sun is effectively inexhaustible and harvesting it directly and transporting that energy from locations in the inner solar system makes most sense to me.

                • J. Jason Wentworth June 2, 2017, 19:52

                  …And the Fukushima disaster was predicted (and was thus completely avoidable!) when the power plant was built decades ago. The engineers wanted to put the diesel back-up generators (which powered the pumps that circulated the reactor coolant during an emergency shutdown) on the hill above the plant, so that a tsunami wouldn’t flood the generator building, but:

                  The politicians, citing the limited open land in Japan, successfully agitated to have the generator building located closer to the reactor building, close to sea level.
                  After the earthquake struck, the diesel generators went into action just as they were supposed to, and were cooling down the reactors. But before long, the tsunami arrived–just as the engineers had warned–and flooded the generator building, which stopped the emergency reactor cool-down procedure…and the rest is history. Atomic power isn’t at fault for what happened–stupid and short-sighted politicians are to blame for it.

                • J. Jason Wentworth June 2, 2017, 20:27

                  Also, ljk (and Alex Tolley),

                  I strongly support the development of solar power in all of its forms (beamed, chemical and nuclear fuels made by utilizing it, photovoltaics, etc.), on Earth and in/from space, but for different reasons than the one that is usually spoken about (which the President dealt with yesterday–it isn’t something that I worry about):

                  Besides being a finite resource, it is a waste of petroleum’s vast potential to just *burn* the stuff–it is a valuable feedstock for countless industrial chemicals, lubricants, plastics, medicines, fertilizers, etc., and:

                  Being beholden to nations which are endowed with huge petroleum reserves–and which, in most cases, also embody odious philosophies and/or systems of political thought (which many of them are actively spreading, using their oil wealth)–is *not* a good position for us to be in. It will also probably not be a stable state of affairs over the long term. The world would be a much safer and more pleasant place if this “oligopoly” of oil-rich nations had its ‘collective monopoly’ of the world’s energy source snatched out from under it, via it being supplanted by another–effectively eternal–energy source.

            • Michael June 2, 2017, 9:56

              If a radioactive material is released into the radiation belts around the earth they can persists for a very long time and affect communications.

              Some of the worst bomb issues may be reduced with a getter or stabilising material say a low solubility glass as a reaction mass. The glass melts under the intense heat and absorbs either chemically or physical impact the radioactive decay products. Once they cool and solidify they drop into the water to fall to the bottom quickly preventing rapid uptake by living organisms.

  • torque_xtr May 31, 2017, 15:00

    It seems to me that particle/magsail propulsion has a number of advantages over laser, and they add up.

    First, particle beams are more energy efficient, and even at 0.2 c total energy spent on acceleration is much lower than with laser beams. More, they could be straightforwardly tuned for lower velocities and enable all the precursors, from shooting them at Kuiper Belt targets with months of transit time, through Gravlens and Planet Nine missions, to some substantial fraction of c. Nearest stars are fascinating targets, but particle/magsail propulsion could possibly revolutionize the exploration of Solar System as soon as 10 or 20 years from now. Shooting a 100 km/s particle beam at the probe requires drastically lower power levels than a laser beam for the same exerted force.

    Second, the same system, a superconducting loop and a mini-magnetosphere ,could be used for deceleration at the target, and it is fundamentally more robust than a lightsail. It does not need to be simultaneously atom-thin, mechanically strong at high temperatures and able to withstand tremendous irradiation, like a laser sail. Of course, it still has to be structurally strong and carry great currents, and requirements are inversely proportional to the beam collimating capabilities, and the steering is probably much more complicated. But the thing that matters is that particle/magsail propulsion is readily scaled all the way down, and things become much easier at the velocities that are already unreachable with chemical and solar/electric propulsion. The question is, how tightly the ion beam could be collimated? v_heat/v_beam = 100 m/s / 100000 km/s = on the subarcsecond scale – seems a natural limit to me (cooling the precursor gas before UV ionization and electrostatic acceleration is much easier in space), and with km-scaled magsail, this translates to millions of km for the acceleration leg.

    Finally, another thought cought my attention. The earth’s atmosphere may seem transparent for the visible light, but, starting from megawatts per square meter, the absorption would considerably heat the air in the beam. This air will rise vigorously and create additional turbulence, very likely making adaptive optics ultimately useless and fundamentally limiting collimation from Earth much more strongly for the mega- and gigascale beams. Maybe into 10s of arcseconds at MW/m2-level already. So we need to mount lasers in space, too, but if so, why not to start with particle beams? (and relieve politics)…

    • Volucris June 1, 2017, 10:44

      Good points, unfortunately it can’t be tested from earth, it requires accelerator infrastructure in space, which we don’t have.

      • torque_xtr June 1, 2017, 17:03

        I think small prototypes of particle beam accelerators can be tested in large vacuum chambers quite similarly to electric propulsion. Testing magsails on the ground could be much more complicated than laser sails (because of Earths magnetic fiend, and because their efficiency goes up with their size), but maybe some basic questions on beam/sail interaction could be answered. (maybe with some help from simulations based on the first experiments)
        There is one thing, I’ve read somewhere that steering and beamriding with magsails could be really troublesome, but I still cannot find those papers…

    • Michael June 2, 2017, 6:22

      ‘The question is, how tightly the ion beam could be collimated? v_heat/v_beam = 100 m/s / 100000 km/s = on the subarcsecond scale – seems a natural limit to me (cooling the precursor gas before UV ionization and electrostatic acceleration is much easier in space), and with km-scaled magsail, this translates to millions of km for the acceleration leg.’

      We could use an ionised C540 atomic structure filled with other elements, then we don’t have to worry about the temperature divergence issue to much.


      • torque_xtr June 3, 2017, 10:02

        Large clusters need to be detached from the surface of solid phase with random lateral velocity as low as possible to achieve the required collimation. Large molecules freeze easier, and I think nothing can remain in the stationary gas phase with Maxwell velocities substantially below 100 m/s. (He at 2 K).

        But I thought of another way – to extract working gas atoms from low pressure reservoir with concentric extraction cones, like the ones used in ICP-mass-spectrometry. Only the molecules/atoms aimed straight in the direction of axis exit, and the rest are rejected (and possibly returned into the reservoir). On Earth, this will suffer from imperfect vacuum and deflection by gravity, but in space, it will work much better.

        Then, of course, is the issue of acceleration field homogeneity…

  • Robert Clark May 31, 2017, 16:04

    NASA just announced a solar probe to travel quite close to the Sun, about 3.7 million miles from the solar surface:

    Nasa’s hotly anticipated solar mission renamed to honour astrophysicist Eugene Parker.
    Renamed the Parker Solar Probe to honour solar astrophysicist who predicted high speed solar wind, the spacecraft will attempt to get close to sun’s surface.
    Wednesday 31 May 2017 07.08 EDT

    Spacecraft able to get this close to the Sun could potentially allow beamed interstellar propulsion. For a spacecraft of any size, you would need huge amounts of beamed power. Where to get it? If you make the beam be solar-powered then can just use space-borne mirrors to focus the Suns rays. But the mirror(s) would have to be impractically large if they were in Earth orbit.
    But what if we placed them close to the Sun? At the distance quoted of 3.7 million miles away from the Sun a mirror 1 km on a side could collect a terawatt worth of power.

    Bob Clark

    • J. Jason Wentworth June 2, 2017, 2:59

      Robert, I think your idea deserves examination. Chapter 9 of Arthur C. Clarke’s book “Profiles of the Future” (at least in the revised 1977 Popular Library edition–there have been earlier ones) is titled, “You Can’t Get There from Here,” and it covers two things that are relevant to your idea:

      He discussed the rise in temperature as one approaches the Sun, and the materials which can withstand those temperatures; his “reference stations” are also of interest. For example:

      For a spaceship whose hull was at a comfortable temperature of 65 degrees F. in the vicinity of Earth (using appropriate radiating and absorbing coatings to achieve such a temperature), as it passed Venus–67 million miles from the Sun–while moving inward, its hull temperature would reach 160 degrees; at Mercury’s orbit (36 million miles from the Sun) it would be 400 degrees. The ship would have to get within 10 million miles of the Sun before the temperature passed 1,000 degrees. At 5 million miles from the center of the Sun, it would be nearing 2,000 degrees. At 1 million miles out (only about half a million miles above the Sun’s surface), the ship’s hull would reach 4,500 degrees. There are materials which remain solid at temperatures above 6,000 degrees F.:

      Graphite begins to evaporate at about 6,800 degrees, and hafnium carbide resists evaporating up to 7,500 degrees. (Clarke also pointed out that properly-shielded instrumented probes can even *land* on the Sun and return data from its ~9,000 degree surface before they disintegrate–they would need to be protected by layers of refractory material that would slowly boil away. Even more hair-raising is his suggestion that a manned spaceship that remained in the shadow of a Sun-grazing asteroid [one perhaps “nudged” into such a trajectory]–or scientists who set up an observation station below the surface of one–could safely pass just above the solar surface, flashing by it at a million and a quarter miles per hour!)

      A statite (a solar sail that is designed to hover over one point on the surface of a star–or above a pole of a planet that orbits sufficiently close to a star–instead of orbiting the body) that is designed to hover as close to the Sun’s surface as you suggest might not need to be as reflective as a sail usually is, due to the much higher sunlight pressure at such a low altitude above the solar surface. (As an alternative, such beaming stations could orbit very close to the Sun’s surface, keeping their sail propulsion beams on target by using movable mirrors. A “belt” of such stations, spaced equidistantly around the Sun, could maintain beam illumination of the outward bound sail probes. There could even be two or more belts, at different orbital altitudes and orbital inclinations, so that multiple stations could hurl more photonic force at the sails.) Also:

      At such extreme proximity to our local stellar fusion reactor, multiple types of energy collection to power the propulsive beam projectors (for the interstellar sail probes) might be practical–visible light solar cells, thermovoltaic cells (infrared photovoltaic cells, utilizing the intense infrared light), ultraviolet solar cells, X-ray solar cells (if such devices are possible), thermocouples (which would utilize the difference in temperature between the sunward-facing and the shadowed “space-facing” surfaces of the facility), etc.

  • James Benford June 1, 2017, 20:07


    Energetic efficiency of sails vs. fusion rockets: This has been in the literature for several decades. It was first introduced by Meyer [1] and then by McInnis in his solar sailing book [2], and will soon appear in my paper critiquing Project Icarus [3]. (References at the end of this comment).

    The argument is basically this: using the rocket equation, integrating the equation of motion of a rocket gives the final energy as a function of the final (payload) mass, exhaust velocity and the final velocity of the rocket. The equation of motion for sail is a similar expression, which depends again on just the velocity change and the sail mass. Then a figure of merit is the ratio of the rocket energy to sail energy, which is a function of only the exhaust velocity and the final speed.

    For example, for chemical propulsion (H2/O2) the sail’s efficiency exceeds the rocket’s efficiency at 62 km/s. They are more efficient than fission rockets above 155 km/sec, for missions within the solar system. For interstellar precursors >1,000 km/sec, beam-driven sails are more efficient than ion rockets, such as VASIMR. For nuclear fission it’s 155 km/s. For thermonuclear fusion at 10 keV it’s 16% of the speed of light, at 100 keV, 25% of lightspeed. Sails are clearly more efficient than chemical rockets etc.

    And, as always, a deficiency of rockets is that the huge complex machine is thrown away for each mission. Whereas the expensive part of beam propulsion, the Beamer, stays behind and can be used again immediately. These economic efficiencies are a strong argument.

    Alexander Tolley

    Active control was studied by the University of New Mexico group of 15 years ago. For high accelerations this doesn’t work out well because the light transit timescale is longer than the instability growth time.

    Certainly beamed energy can provide for moving energy around the solar system in a very economical way. Of course that means having rectennas to collect the beams. The basic advantage is that in a gravitational well you can move energy in the form of photons with no loss to gravity, whereas moving mass around has a big penalty in changing its potential energy [4].


    Putting a Beamer on the moon, while expensive, could of course be used to decelerate incoming payloads. However, using millimeter or microwaves gets rid of the atmospheric attenuation issue that lasers are having to confront. So Earth-based Beamers will precede space-based Beamers.


    Certainly larger mass sail ships would require larger Beamers. For sails to shift to adjust themselves to stay on the beam actively would require propulsion. This is not out of the question. However it would increase the mass. These matters may become necessary in the future when sailships have become commonplace.


    Lasers have a particular limitation: they are quantum devices, whereas the microwave and millimeter sources are classical devices. As a quantum device, lasers have a coherence length, which is the propagation distance over which a wave train maintains a specified degree of coherence.

    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 arrays difficult to build; laser arrays will involve very many lasers. For Breakthrough Starshot, the required 10’s of GW means millions of individual lasers which have to be built into a coherent array.

    On the other hand, classical devices can have very long coherence lengths because they are determined by the classical equations of electromagnetism. Classical microwave and millimeter wave devices can operate at MW levels and therefore each GW of radiated power requires ~1000 sources per GW. Starshot would require perhaps multiple thousands of sources rather than multiple tens of millions for lasers. Therefore a fundamental difference between the quantum and classical devices is their complexity.


    The Beamer in Starshot operates only for minutes, not for the entire flight time. And yes, Starshot is very much a private-sector initiative at present. Funding for the construction of a Starshot Beamer would involve considerable contributions from billionaires and might involve governments. But that’s well over the horizon for now as the R&D, which could result in a demonstration Beamer, must be done over the next decade in order to show the potential of the full system.

    Robert Clark

    The Parker Solar Probe will carry a shield to protect the probe itself from the intense fluence of the sun, which will be several hundred times the fluence here on Earth, ~MW/m2. Building a large solar receptor at that point would mean it would operate at extremely high temperatures. Moreover the solar photon pressure and solar wind pressure would make it move outward. To prevent that, one would have to provide propulsion to keep it in orbit. It’s altogether too large an enterprise to consider.

    [1] Meyer T.R., et al, “Rapid Delivery of Small Payloads to Mars”, in The Case for Mars II, C.P. McKay (ed), Vol. 62, Science and Technology Series, Am. Astronomical Society, pp.419-431, 1985.

    [2] McInnes, C. R., Solar Sailing: Technology, Dynamics, and Mission Applications”, Praxis Publishing Ltd., Chichester, pp.275, UK, 1999.

    [3] Benford, J., “Fusion Rockets vs. Sailships: A Contrarian View”, in press.

    [4] Benford, J., “Space Applications of High Power Microwaves”, IEEE Trans. on Plasma Sci., 36, pg. 569, (2008).

    • Michael June 2, 2017, 3:25

      I agree that microwave transmitting will be less complex but fibre lasers have the advantage of been able to be mass produced. A laser fibre with a transverse laser configuration will allow us to mass produce these long laser fibres. Controlling the beams into a coherent one will be a challenge but I don’t see it as an insurmountable one. We have built small very coherent arrays but it remains as you have indicated to be demonstrated on a much larger scale.

      That is why an experimental lab would be a very good idea and we need to keep in close contact with the communication industry who have a vested interest.

    • Robert Clark June 5, 2017, 21:11

      Thanks for the response. If a spacecraft does not have enough orbital speed then it will descend down towards the gravitating body. Then we could make the orbital speed of the satellite be such that the outward light radiation pressure and solar wind pressure would keep the satellite in place.

      Bob Clark

  • Andrew Palfreyman June 3, 2017, 4:16

    I would like to point out Zac Manchester’s sterling work with optimal beam shapes for Breakthrough StarShot

    • Michael June 4, 2017, 8:17

      Phased arrays allow us to have many different laser profiles, I think we will need a passive and active system here. Having the reflective medium as a retroflective one has the advantage of the beam been reflected always back to the source which means less destabilising forces.

  • ljk June 4, 2017, 23:27

    NASA is Planning to Test Pulsars as Cosmic Navigation Beacons


    To quote:

    If successful, NICER could revolutionize our understanding of how neutron stars (and how matter behaves in a super-dense state) behaves. This knowledge could also help us to understand other cosmological mysteries such as black holes.

    On top of that, X-ray communications and navigation could revolutionize space exploration and travel as we know it. In addition to providing greater returns from robotic missions located closer to home, it could also enable more lucrative missions to locations in the outer Solar System and even beyond.

  • Robert Clark June 5, 2017, 22:56

    I think it should also be investigated using self-assembly in space to get large spacecraft, not just nanoprobes, for beamed interstellar propulsion.

    The problem is small craft have a good area to volume ratio, and with volume as a proxy for mass, a good area to mass ratio. This is important for getting a high acceleration due to light pressure.

    However, one way to maintain the high area to volume(mass) ratio for a large craft would to keep the large craft very thin, just as thin as the small craft. The large mass is obtained by making the areal dimension large.

    Bob Clark

  • Robert Clark June 6, 2017, 8:52

    The Breakthrough Starshot program has been envisioned as a flyby. However, it may be possible to slow it down so it stays within the stellar system. One method is by breaking against the solar wind emitted by the destination star.

    A preliminary calculation suggests that for a 10 meter wide sail carrying a gram sized payload, for the solar wind similar to that of the Sun, a craft moving at relativistic speeds should be able to brake against the wind to slow enough to speeds to remain inside the stellar system.

    Bob Clark

    • Paul Gilster June 6, 2017, 10:58

      Bob, also check “Proxima Mission: Fine-Tuning the Gravitational Assist,” re Heller and Hippke’s ideas on deceleration:


      and “Mission Concepts: Bound Orbits around Other Stars”


    • Larry Kennedy June 6, 2017, 16:04

      1/10th gram per meter and near perfect reflectivity is an enormous ask.

      • Larry Kennedy June 6, 2017, 16:07

        Correction. That would be more like 1/100th gram per sq. meter.

  • Robert Clark June 11, 2017, 13:22

    For the solar sail material I want to suggest some adaptations of the transparent carbon nanotube sheets discussed here:

    Saturday, April 19, 2014
    Economical Space Solar Power Now Possible.

    It refers to this research:

    Researchers produce strong, transparent carbon nanotube sheets.
    Aug 18, 2005
    “Strength normalized to weight is important for many applications,
    especially in space and aerospace, and this property of the nanotube
    sheets already exceeds that of the strongest steel sheets and the Mylar
    and Kapton sheets used for ultralight air vehicles and proposed for
    solar sails for space applications, according to the researchers. The
    nanotube sheets can be made so thin that a square kilometer of solar
    sail would weigh only 30 kilograms. While sheets normally have much
    lower strength than fibers or yarns, the strength of the nanotube
    sheets in the nanotube alignment direction already approaches the
    highest reported values for polymer-free nanotube yarns.”

    A 1 km square sail weighing only 30 kg, corresponds to 0.03 gm/sq.m. So for the 4m x 4m sail discussed by the Starshot project, it would be a weight of 0.48 gm, which is likely sufficient for a 1 gm scale micro spacecraft.

    However, these nanosheets are transparent. We could coat them with aluminum but the 100 nm thick aluminum coating normally put on mirrors would make our sail 4.4 gm. This would slow down our acceleration.

    It would be cool if we could use or adapt the transparent nanotube sheets themselves. One possibility is using the fact that nanotube’s electronic properties are highly tunable according to the arrangement of the carbon atoms. See for instance:




    Atomic structure and electronic properties of single-walled carbon nanotubes.
    Teri Wang Odom1, Jin-Lin Huang1, Philip Kim2 & Charles M. Lieber1,2
    Nature 391, 62-64 (1 January 1998) | doi:10.1038/34145; Received 10 October 1997; Accepted 26 November 1997

    Then we might be able create lightweight sheets that are instead reflective as a mirror.

    Another possibility is that their electronic properties can also be changed by doping. So we may thereby be able to make the sheets reflective.

    A different possibility would use them in their transparent form. We would shape the sheets so they act as a lens. Then the collecting area of the sheet would focus the collected light down to a smaller mirror area, thus having a smaller weight which would still have the effect of inducing an acceleration on the spacecraft.

    Bob Clark

  • Andrew Palfreyman June 11, 2017, 20:08

    @Bob Clark: That last suggestion of a lens+mirror is very creative.