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Laser Beaming and Infrastructure

Looking at John Brophy’s Phase II NIAC award reminds us how useful the two-step process can be at clarifying and re-configuring deep space concepts. Brophy (Jet Propulsion Laboratory) had gone to work in Phase I with a study called “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” The work studied a lithium-fueled ion thruster with a specific impulse of 58,000 seconds. If that didn’t get your attention, consider that the Dawn spacecraft’s ISP is 3,000 seconds, and think about what we might be able to do with that higher figure.

I think about ideas like this in terms of infrastructure. The relation to interstellar flight is this: While we may well get robotic nano-probes off on interstellar missions (think Breakthrough Starshot) some time this century, the idea of human expansion into the cosmos awaits the growth of our civilization into the rest of the Solar System. Along the way, we will learn the huge lessons of closed-loop life support, means of planetary protection and propulsion technologies to shorten trip times. Building that infrastructure is an early phase of the interstellar project.

What Brophy has been working on with his Phase I study is the question of how to provide power to craft at the outer edge of our planetary system. When I looked at the 2017 NIAC awards, a stumbling block for Brophy seemed to be the need for a 10-kilometer laser array.

Brophy’s idea was to beam power to a lightweight photovoltaic array aboard the actual spacecraft — this is how he generates the power needed to drive those ion thrusters. Closer to home we would use solar power to do the trick today, while looking at nuclear power when far from the Sun. Brophy is talking about ion thruster operation in deep space without the need for a heavy onboard power source. This gets around the problem of increasingly inefficient solar panels as we go further from the Sun, as well as their alternative, a bulky nuclear reactor.

Image: John Brophy (JPL) initiated the NSTAR Project that provided the ion propulsion system for Deep Space 1, delivered the ion propulsion system for the Dawn mission, and co-led the study at Caltech’s Keck Institute that resulted in the Asteroid Redirect Mission. His Phase II NIAC award will be used to investigate beamed power delivery to a powerful ion engine. Credit: JPL.

If we can power up the lithium-ion engine, mission velocities of 100 to 200 km/s would be possible. This is exciting stuff: The Phase I study talked about a 12-year flight-time to 500 AU, which gets us into gravitational lensing territory, while flight time to distant Pluto/Charon could be reduced to 3.6 years. For that matter, imagine a Jupiter mission in roughly a year.

A 10-kilometer array is daunting indeed, but now we’re going into Phase II, which significantly refines the concept based on the results of the Phase I study. Phase I assumed a 100 MW output power at a laser frequency of 1064 nm, feeding a 70 MW electric propulsion spacecraft with a 175-meter photovoltaic array coupled to its lithium-fueled ion thrusters.

What the Phase I study revealed was a better set of parameters: Brophy will begin the Phase II study with a laser array of 2-kilometer diameter aperture with an output power of 400 MW at a laser frequency of 300 nm. Aboard the vehicle, a 110-meter diameter photovoltaic array is now considered. It will power a 10 MW electric propulsion system.

Results of the Phase I work brought the specific impulse down to 40,000 seconds, which is the figure of merit for Phase II, all in the service of a specific mission outcome: A journey to the solar gravity focus at 550 AU. The changes in configuration came from feasibility analysis — could we develop the required photovoltaic arrays with the required areal density (200 g/m2)? Could we achieve photovoltaic cells tuned to the laser frequency with efficiencies greater than 50%? And what about pointing the laser array with the needed accuracy, not to mention stability to supply the reference mission at the gravity focus?

In terms of beamed propulsion, Brophy’s laser array should remind us of Breakthrough Starshot’s own plans for such an array, though Starshot does not plan on building the array in space. Starshot’s array is to be on the ground, avoiding the huge issue of constructing and controlling a gigantic laser outside our planet, while creating a host of other questions, such as atmospheric attenuation of the laser signal. In both instances, however, we deal with a power source separate from the spacecraft, producing real benefits in terms of weight and efficiency, and creating a reusable driver for missions throughout the system.

Image: Graphic depiction of A Breakthrough Propulsion Architecture for Interstellar Precursor Missions. Credit: J. Brophy.

And what about the power developed in the onboard photovoltaic array? The output in the Phase I study was expected to be 12 kV, which goes a long way beyond the best solar arrays available today. Those on the International Space Station, for example, produce 160 volts. One part of Phase II, then, will be to show that the photovoltaic system can be operated at more than 6 kV in the plasma environment produced by the lithium-ion propulsion system.

Have a look at the precis to see what Brophy considers to be the outstanding technical issues, which include modeling the lithium plasma plume created by the engine and demonstrating a small aperture phased array that can become scalable to larger apertures. Depending on the outcome of these investigations, a roadmap should emerge showing how we might develop demonstrator missions along the way to finalizing a workable system architecture.

Let’s put all this into perspective. The challenge of operating beyond 5 AU is the rapid dropoff in solar power available to a spacecraft, despite recent advances in photovoltaic array technology. A laser system like Brophy’s could increase the power density of photons available to a spacecraft bound for the outer Solar System by several orders of magnitude, giving us an ion propulsion system of considerable power. Moreover, this would be a spacecraft that does not need to create its own power but receives it, doing away with power processing hardware.

I come back to infrastructure when evaluating concepts like this. A workable laser array of this magnitude becomes a way to solve power issues far from the Sun that could drive missions throughout the system. Like the combined laser / neutral particle beam concept we’ve looked at over the last two days, it assumes a hybrid meshing of technologies that could go a long way toward enabling robotic and manned missions deep into the Solar System. That makes laser beaming to an ion engine an alternative well worth the continued scrutiny of NIAC Phase II.

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{ 46 comments… add one }
  • Thomas Goodey April 6, 2018, 14:08

    The article says “And what about the power developed in the onboard photovoltaic array? The output in the Phase I study was expected to be 12 kV, which goes a long way beyond the best solar arrays available today. Those on the International Space Station, for example, produce 160 volts.”

    That’s garbage. Power is measured in watts.

    • Patient Observer April 8, 2018, 18:16

      Yes, the voltage parameter is in itself irrelevant as a measure of power. The importance of high voltage from the photo-voltaic array may be the potential to be directly used in the ion thrusters thus avoiding the weight and reduced reliability associated with power electronics. Just a thought.

      • J. Jason Wentworth April 9, 2018, 3:11

        I agree–an ion drive’s power conditioner (as it was called in Robert M. Powers’ 1978 – 1979 book “Planetary Encounters,” in connection with Boeing’s SEPS [Solar Electric Propulsion Stage]) is a significant, though not hugely so, portion of the “mass budget” of an ion propulsion spacecraft. Reliable, long-life ones tended to be rather heavy in the old days, but improved electronics have reduced their mass requirements over the years, and:

        Along with thin-film, “roll-out deployment” solar cell arrays (thin-film solar cells are lighter–and also more radiation-resistant–than the older “wafer types”), the lighter power conditioners improve the Delta-V capability. Concentrators might not be hard to incorporate into thin-film solar arrays (“cast-on” or “3D printed-on” linear Fresnel lenses could be used, or they could be mounted on “flip-up,” edge-mounted stanchions if necessary for optimum light focusing). With NEP (Nuclear Electric Propulsion) ion drives, the reactor and power converter (for converting the heat [and possibly also the radiation, via alphavoltaic or betavoltaic units] to electricity; multiple methods can be used) are the most massive components of the system, but the U.S. SNAP-10A (of 1965) and the Soviet TOPAZ space reactors already had good power/mass ratios, and better designs are under development by NASA.

  • Alex Tolley April 6, 2018, 14:43

    An alternative to beaming is to use concentrators.
    Some calcs.
    The 110 diam solar array with a 200g/m^2 areal density masses 1900kg. The 10 MW output is of the same order as the solar irradiance energy at 1 AU, so the power beaming delivers this 1 AU of solar energy out to the edge of Sol.

    Now have the spacecraft use a concentrator instead:
    Assume a concentrator of a thin metallic foil of areal density 1g/m^2, on a par with a very lightweight solar sail.

    At Jupiter, the concentrator must collect 25x the sunlight, so the concentrator mass is 238kg. At Saturn, 100x, 950kg. At Neptune, 900x, 8550kg.

    Even if we increase the masses 10x to account for PV conversion ratios and extra mass for the concentrator and its supports, it seems to me that the masses are far lower than the 2km laser array.

    In practice, such foils, possibly inflated bubbles, are likely to mass more than 1 g/m^2, but it is feasible. (For 1um foils, AL = 2.7g/M^2, Li – 0.5g/m^2, Na = 1g/m^2).

    Spieth and Zubrin have written about ultrathin perforated foils just a few nm thick, even doped carbon nanotube sails. Any of these technologies would make concentrators extremely lightweight allowing solar PVs to be used to at least Neptunes orbit. O’Neill suggested that for his colonies, a conical Al reflector massing the same as the colony would allow a colony to experience normal 1 AU sunlight out to 2.7 light days, about 460 AU. Lightweight foil or refracting concentrators to power solar arrays has considerable possibilities to provide energy for electric propulsion, IMO.

    • J. Jason Wentworth April 7, 2018, 6:33

      Thank you for mentioning solar concentrators, including O’Neill’s extremely long-range ones for providing distant space colonies with 1 AU sunlight intensity, and:

      If–this is a big “if”–the PROCSIMA “composite” laser/neutral particles non-spreading laser beam scheme works, that could further reduce the required size of the spacecraft’s laser photovoltaic array and/or the laser projector’s aperture. Another possibility might be refueling such an ion-drive spacecraft with the PROCSIMA composite beam (the neutral particles could possibly be lithium atoms, which the vehicle could collect either for immediate use, or to store over time, to enable subsequent “acceleration runs”). Also:

      The 58,000 seconds (or even “just” 40,000 seconds) specific impulse of lithium-fueled ion drives is astonishing! Lithium isn’t common, but it doesn’t seem to be so rare–as is becoming the case with xenon–that we need to “worry about running out of wonder fuel.” There is also another high-performance ion thruster fuel, which is very common on Earth and in space:

      In his now-classic article titled “Interstellar Flight” in the February 1952 issue of JBIS (“Journal of the British Interplanetary Society”), Dr. Leslie Robert Shepherd calculated that an ion-drive starship using carbon as its propellant (with the carbon ions perhaps being generated by an electric arc [as in an arc-jet electrothermal thruster]?) would have an exhaust velocity of 6,300 km/s.). If so, at a 0.02 c (1/50 of c) cruising speed (with a brief acceleration and deceleration period at departure and arrival, respectively) the transit time to the Alpha Centauri system would be just over 215 years (John W. Macvey used these velocity and voyage duration figures in his 1970 book “How We Will Reach the Stars” [this is a later edition of his 1965 book “Journey to Alpha Centauri,” which contains more technical data]; he also referenced Shepherd’s JBIS article in “How We Will Reach the Stars”).

      • Brett Bellmore April 7, 2018, 14:13

        As I understand it, the chief limit on ion propulsion is the cost of ionizing the propellant, which must be expended before even one erg is spent accelerating it to produce thrust. Also, the difficulty of handling extremely high currents in order to get significant thrust.

        Ionization energies at least *tend* to drop with larger atoms, and the current necessary to get a particular thrust declines with atomic mass; Thus Cesium, with its high atomic weight and low ionization energy. Or Xenon, high ionization, but at least easy to handle and massive.

        So, why Lithium? Sure, the ionization energy is fairly low, but the atomic weight is very low, leading to extremely high currents in order to get your thrust, and a high ionization energy per kg of propellant.

        It seems to me the real promise with ion engines is molecular ions, which can have absurdly low ionization energies, and almost arbitrarily high mass to charge ratios. I’m talking electrospray thrusters.

        • Michael April 8, 2018, 3:02

          Lasers and particle beams strung about solar system will allow us to move matter at low cost. If matter can be entrapped in the laser beam it would make a very efficient transport system. The matter can be decelerated and the energy recouped as well as the energy from the laser beam. Perhaps in the far, far future it will allow us to move the earth outwards by the use of laser entrapped matter from mercury thereby extending the earths habitable lifespan.

        • J. Jason Wentworth April 9, 2018, 3:51

          Also, Brett, lithium is pretty reactive in certain situations (you don’t want any moisture around [cesium also “doesn’t like” water, exploding even on contact with ice!]). Using charged dust particles as ion thruster fuel, as was proposed in the 1950s (the NanoFET Thruster, which is covered here on “Centauri Dreams,” and also *here* http://www.google.com/search?source=hp&ei=jhHLWsDhFezCjwTj1ofYAg&q=nanofet+thruster&oq=NanoFET+thruster&gs_l=psy-ab.1.0.0i22i30k1.1946.543896.0.546225.24.17.0.7.7.0.379.2899.0j14j1j2.17.0….0…1c.1.64.psy-ab..0.24.2911…0j46j0i131k1j0i30k1j0i8i30k1j0i46k1j0i10k1.0.B3TeHqEEALE , is similar), would provide more massive exhaust particles, and:

          A carbon-fueled ion thruster (as Dr. Leslie Shepherd and John Macvey both advocated for propelling non-relativistic starships) might be able to function as an “afterburning” arc-jet engine. In the basic engine–much like an ordinary arc-jet rocket–the carbon could be vaporized by the arc (another propellant such as hydrogen or ammonia could also be injected, as is usually done in arc-jet engines). This would create a hot, ionized, high-velocity exhaust, which could be further accelerated electrostatically, as in an ordinary gridded ion thruster or a Hall Effect thruster. Such an arrangement would produce a fairly “massive exhaust” (due to carbon’s atomic weight, plus that of any additionally-injected fuel), and the engine could be operated at different exhaust mass and exhaust velocity settings, depending on the circumstances. Also, concerning your comment about using molecular ions:

          Busek Space Propulsion Systems (see: http://www.busek.com ), an electric thruster manufacturer (they make Hall Effect and gridded ion thrusters, electrospray thrusters, electrothermal thrusters, etc.) has done this. They have successfully run Hall Effect thrusters on *ordinary air* (which is, of course, a mixture of nitrogen [N2] and oxygen [02] molecules, with a bit less than 1% argon included), as well as on xenon, argon, iodine, krypton, bismuth, zinc, and magnesium (and their gridded ion thrusters run on xenon, krypton, argon, and iodine).

          • Brett Bellmore April 9, 2018, 12:55

            Dust has some obvious advantages, but I was really thinking about electrospray thrusters using ionic fluids.

            Ionic fluids are basically very low (Sometimes cryogenic!) melting point, high boiling point salts of complex organic compounds. With the right microscopic geometry and the application of some voltage, they can be induced to fire off charged molecules or droplet streams, with fairly high mass to charge ratios.

            The Brane craft proposal distributes such thrusters across a thin film solar panel, obviating the need to distribute power long distances. As proposed it would “only” have an ISP of about 4,000, but I think there’s no real obstacle to pushing the exhaust velocity higher.

            • J. Jason Wentworth April 10, 2018, 3:07

              Indeed (and thank you for reminding me about the Brane craft), contact-ionization (utilizing charged needles raising cones of the propellant fluid, which is ejected) electrospray thrusters are in use today. They tend to have lower exhaust velocities and thrusts, though (they are usually employed as low-thrust, precision-maneuver thrusters for satellites), BUT:

              This is not a “hard-and-fast rule,” because high-performance, gridded ion thrusters using cesium as propellant have operated using the contact-ionization principle (SERT-1, the first in-space ion engine test, involved a cesium contact-ionization thruster [which unfortunately did not “ignite”] and a regular mercury-fueled thruster, which did run). As the Brane craft research suggests, the performance potentials of contact-ionization electrospray–and gridded ion–thrusters has yet to be explored.

  • Ioannis Kokkinidis April 6, 2018, 14:48

    400 MW array in orbit. Isn’t that James Bond villain territory? Somehow I doubt that several countries will be pleased with that

    • Andrew Palfreyman April 6, 2018, 19:04

      100 GW on the back side of the Moon would be acceptable though. One just has to promise not to craftily install mirrors in space around it to bring the beam back home.

    • Patient Observer April 6, 2018, 19:24

      The fiendish plan will be revealed only after it is too late to stop! Kneel before your new masters! ha ha ha!

      -feel free to delete, just been a tough week:)

    • Kuho April 7, 2018, 11:39

      Propulsion is only a subset of what this technology can offer..

      Need power for our base on Ganymede? No probs, we beam some over

      Need power for our rover on Triton? No probs, we beam some over

      Civilization-ending asteroid inbound to Earth? No probs, we alter its course by burning off material

      Want this ice comet redirected to Mars? No probs, we use our Deimos laser to beam some power for a steam rocket

      Want to terraform Venus? No probs, we superheat its atmosphere to above escape velocity (ok we will need a huge array for this)

      Fears about having a laser in orbit is unfounded. A PV array this size will be a sitting duck for kinetic impactors. The designers probably picked 300 nm lasers bearing in mind UV will get greatly attenuated by the ozone layer.

      Really wish this proposal can be given more positive support by the press.

      • Alex Tolley April 7, 2018, 13:21

        Now ask yourself how many of those benefits will be available in the near future, versus its almost immediate military use to subdue populations on the ground?

        If the uS builds one, why would this not be seen as an arms race for other nations to build theirs, just like other weapons? If another nation builds one for military purposes first, why would the US also not build military versions? Would a military version also be a plowshare, or would civilian use designs need to be rather different?

        I’m all for building beamers if they make economic sense, but I am very wary when the military gets involved in funding due to the likely compromises to ensure its military use.

        Some space technology is very hard to press into military service, like solar sails, or concentrators to deliver solar energy to PV arrays for propulsion.

        • Kuho April 7, 2018, 17:06

          Alex, you make claims that it can be used to “subdue” populations, please elaborate?

          Like, what frequency do you think they will use? As I said, the 300 nm that is being studied is UV and is greatly blocked by the ozone layer.

          And what power density will the beam have by the time it reaches the surface? A tin foil hat can fully reflect the 2kW/m2 odd that the NIAC team is looking at. Regardless, bodies have a thermal time constant and won’t immediately cook, plenty of time to run indoors surely?

          And how does directing lasers at humans not constitute a contravention of the Protocol on Blinding Laser Weapons?

          I just think that in an age where genetically tailored bioweapons and fully autonomous drones are fast becoming reality we humans have much more to worry about than a huge array that’s visible to the whole planet in an orbit that can be predicted weeks or months in advance

          • Alex Tolley April 7, 2018, 20:53

            I understand what you are saying. The NIAC study suggests a low-intensity beam, just a lot of them to generate the total power.

            Since we can make far higher intensity beams, the economics suggests to me that we use these instead if they have higher specific power. We already have those on military aircraft and ships being tested. If the military gets involved, I would see high power lasers being used instead.

            So perhaps the issue is the UV laser and the tuning of PV material. AFAICS, the tuning increases the efficiency to 50%. High-performance arrays already reach 20+% and 2 layer experimental arrays potentially double that. For a craft, it is the power to weight ratio that is relevant. Those UV tuned arrays may be the best way to get the performance, I don’t know.

            For any sort of beamed system, I would have thought that developing a basic system with “off the shelf” components would be a better way to get started that proposing a new technology that may not be necessary. Economics, not highest performance is the way to go. NewSpace companies are taking this approach, and SpaceX for one is eating the OldSpace lunch.

            If you accept the premise that high power lasers in space, pointed at the ground are weapons the military would like, then they would be useful ways to “subdue” populations as I have already laid out. Even wide area, low power lasers can be used this way if there is a focusing mechanism – mirrors or lenses. Curved mirrors to burn the Earth’s surface have been proposed in the past. While space-based assets are easy targets, so far no militaries have targeted these assets in a war as far as I am aware. In proxy wars, opposing great powers could target these assets, but don’t seem to. Spy sats are not blinded and comsats are not destroyed, even though these would hamper current military operations. We cannot assume that a space-based laser would definitely be targeted either. Indeed its large size might be a deterrent as the many pieces that would result could damage other satellites, as we saw when the Chinese destroyed one of their own satellites with a laser some years ago.

            • Andrew Palfreyman April 9, 2018, 13:08

              A gentle reminder that the chief impetus for the StarShot beamer designs comes from Professor Lubin, who got his start doing military-funded research for his DESTARS project.

            • Patient Observer April 9, 2018, 15:13

              Even modest lasers, in comparison to these 400 MW monsters, could be used to quickly knock out early warning, GPS and communication satellites thus rendering a country blind to a potential attack. Said country would know that something bad is about to happen but would be clueless on the appropriate level of response – launch everything they have and end civilization or wait to see what comes over the horizon.

  • J. Jason Wentworth April 7, 2018, 7:51

    You all are half-joking about it, but this concept could potentially become another “Project Orion affair.” Even things far less spectacular than this nuclear bomb-propelled spaceship can breed opposition, as I have personally witnessed:

    RTG (Radioisotope Thermoelectric Generator [essentially a type of “atomic battery”]) units whip up protests, like those before Cassini’s launch. Even Dr. Michio Kaku, who–of all people–should know better, opposes RTG-powered outer solar system probes; he actually advocated that the Cassini mission should have been ended after its Venus flyby, saying “they’ve already learned enough from it” (long before it ever reached Saturn, its destination), and:

    The RTG powering a remote automated weather station in my area (outside Fairbanks, Alaska) was replaced with a diesel generator system–after the locals discovered the RTG was there, even though it had operated for years with *no* problems whatsoever–because all the ignorant locals knew was, “nuclear is bad!” They had no idea that surgically-implanted, plutonium-powered atomic batteries power many surgically-implanted heart pacemakers, keeping their patients alive and healthy (how dangerous could such devices be, if they’re safe enough to be implanted *inside* human bodies?), but:

    I can see today’s journalists (and remember, members of their profession–with precious few exceptions, sadly–thrive on sensationalism, controversy, and emotion, rather than facts and reason) seizing on such powerful space-based laser projectors as (to give just three possibilities):

    [1] Starting a new arms race;

    [2] Accidentally blinding people and animals, and/or incinerating people, animals, forests, crops, buildings, aircraft, etc., and:

    [3] Falling into the hands of terrorists. Now:

    Any of these dire things could, of course, happen, but would they be *likely* to happen–likely enough that we shouldn’t pursue such technologies at all? I don’t think so (jetliners have been misused–as cruise missiles–and cargo jets could be as well, but no one has stopped using these aircraft out of fear of future misuses of them; instead, security precautions to prevent more such incidents have been instituted). The same could be done, from the start, with laser-powered ion-drive spacecraft, but:

    Because of the novelty and lack of immediate economic necessity of such spacecraft (as opposed to jet transports), sensationalistic journalistic coverage of them could, without prior public education, create public opposition to them which could prevent their development. High-energy-powered spacecraft need proactive education and advocacy of the enthusiasm-generating kind that Wernher von Braun provided for satellites, manned space flight, and lunar and Martian exploration in the 1950s.

    • Alex Tolley April 7, 2018, 13:11

      I well recall the fuss about the Casini probe as it was going about one of its gravitational swings about Venus and Earth. The scare was about a mistake with the probe disintegrating in Earth’s atmosphere and spreading plutonium over the planet. It was definitely fearmongering. It was similar to the demand to refrain from turning on the LHC in case a mini-black hole was created that would doom the Earth. GMOs are another that are effectively banned some continents.

      I think the space-based lasers are not quite the same issue. The military has been experimenting with lasers to strike targets for many years. Lubin’s lasers were funded by the military. Their use to deflect asteroids looked a bit like a cover to me. The military potential against ground targets was clear, and a descendant of other orbital technologies like “brilliant pebbles”. The military routinely uses drones. How much better to use space-based beam weapons on ground targets. No fuss, no muss, no parts to leave at the target, and no way am operational failure puts the technology in the hands of the enemy.

      While many technologies are dual use, some are clearly of more use for military purposes. Beamed energy is clearly a useful space technology, but its use is not going to economically benefit the taxpayer. If we had a beamer today, hawks would muse about using such a facility for military purposes. This is not farfetched when we already have a US president musing about “why do we have nukes if we don’t use them?”

      If the military develops such a weapon, I think we should be careful not be in any way complicit in its development, lest that sully space activities and lead to more questions about funding. I would be interested in knowing what fraction of the population would still oppose another Cassini-type launch despite its obvious scientific success, because of the same contamination fears.

      • J. Jason Wentworth April 9, 2018, 5:21

        New Horizons and Ulysses, both of which used/use RTGs, didn’t seem to generate any protests (as far as I can remember). The prospect of Cassini’s RTGs re-entering–had some spacecraft malfunction resulted in an impact rather than an Earth flyby–didn’t concern me, but I was also aware of RTGs’ demonstrated re-entry safety (involving Apollo 13’s LEM re-entry and a Nimbus satellite’s Vandenberg AFB launch failure [plus an earlier–in 1964, I think–*un-shielded* plutonium SNAP RTG aboard a Transit satellite that burned up on re-entry, but harmed no one]). Most people, including journalists, were unaware of these incidents, and:

        While I wish it were otherwise, we have no choice about developing laser weapons, or at least effective defenses against them (with one possible exception), because China and Russia and other countries will. Hypersonic cruise missiles are another example of such new technologies; even India, which developed the Mach 3 BrahMos missile in partnership with Russia, is now working with them on the hypersonic (Mach 7) BrahMos II. The possible exception is this:

        We could adopt a policy of, “Any laser weapon attack on the U.S. or our allies will be responded to with a nuclear attack on the laser attacker’s nation” (but those nuclear weapons, if not laser-shielded in some way, won’t be a credible threat, so even in a defensive way, we have to get involved with laser weaponry, whether we want to or not). Also:

        The President’s musings regarding “Why do we have nukes if we don’t use them?” isn’t about using them, but–as with the enormous projected cost of the next Air Force One plane, which his inquiry (“Does it ^have^ to cost *that* much?”) showed to be ‘well-expense-padded’ by the contractors–reflect his frustration that we have to spend so much money on nuclear weapons (at least the current-generation ones), when other possible nuclear deterrence options (which could be equally effective, but cheaper) haven’t been examined or pursued. Being a businessman, he looks at costs and asks, “We’re spending a lot on this–what does this big expenditure do for us, since it costs so much?” In addition:

        Without military-funded rocketry, the Space Age would probably still be a dream for the future (even SpaceX’s accomplishments were enabled by NASA research, whose roots are in the Cold War-caused race to the Moon, in which science was just a “hitch-hiker”). Even if we never field space-based laser weapons, I doubt if interplanetary or interstellar applications of such high-powered, space-based laser projectors will ever come to fruition without military-funded R & D work on them, just as Project Orion (which lasted for several years) would never have occurred without the Cold War-induced development of compact and lightweight thermonuclear weapons. I don’t like this state of affairs, but it simply is what it is, and grandiose plans for space travel and settlement much negotiate the socio-politico-economic-military-industrial terrain as it exists.

      • Andrew Palfreyman April 9, 2018, 13:12

        That fearmongering can be laid in large part at the door of Michio Kaku. Unsurprisingly, these days he never talks about his erstwhile plans to scuttle Cassini.

  • Michael April 7, 2018, 8:26

    I am surprised it is only 100 to 200 km/s, surely a powered drop to skim the sun and then a boost outwards would be better.

  • Geoffrey Hillend April 7, 2018, 16:48

    The idea of making a laser in space is possible, but expensive especially if it is large. I have some questions based on my doubt of the efficiency of a photovoltaic cells tuned to the laser frequency. The problem with this idea is that the laser is not explained. Photovalaic cells which are simply solar cells can be tuned to a laser frequency, but how do we get the lasing effect and how strong it is per square inch? How can you get a 100 MW power spread out over only 110 meters? One hundred megawatts is a lot of power, but spread out over a large area it might not provide much thrust.

    The lasing effect in LASER or light amplitude by stimulated emission of radiation was Einstein’s idea. It requires a population inversion which is possible with solar cells, but the thrust by itself is from only two photons which vibrate and move together at the same frequency, phase and energy will not be much of a push compared to Breakthrough Starshot and certainly not like a particle beam weapon. When the ground state of an atoms absorbs light or a photon at a resonant or the same frequency, if it has enough energy, it raises the electron to a higher level and it falls back down to the ground state, rest state, the lowest state to emit light at exactly the same energy or frequency. If we keep pumping light into the atom at the same energy, it will keep the electron stuck at higher energy level or quantum jump. Now add even more light while the photon is kept at a higher energy, level 2, It will raise the s energy level to level three, but when it falls back down it will release two photons instead of one. The two photons will be at the exact same energy, phase and direction. This is the lasing effect and called a population inversion since we get two photons instead of one..

    All leasers need mirrors for the cascade effect which involves two mirrors. One of those mirrors requires a partially transparent mirror or like a ninety eight percent albedo. The light bounces back and forth between the two mirrors, the photons fill up, and cascades so two photons become four, eight, sixteen, and so on. When the amplitude is high enough, the threshold is reached and the light can pass through the partially transparent mirror and you have a powerful beam of cascading coherent light. There are H20 and OH masers in space in the HII regions but they can push very much. Without the mirrors, you can’t get a powerful laser. Only two photons will be coming out at a time which might add up over a large surface of 110 meters but I don’t consider that to be much of a weapon or even thrust compared to a laser with mirrors. 100 megawatts yes but spread out over a wide area. Correct me if am wrong. I am not an expert on lasers, but I would like to see the laser explain better.

  • Geoffrey Hillend April 7, 2018, 16:50

    Excuse me for the error, Masers in space can’t push very much

    • J. Jason Wentworth April 9, 2018, 4:10

      Masers *can* push suitably-made sails effectively, but not over a very long (compared to a laser) “acceleration runway,” because microwave beams diverge more rapidly than laser beams. Robert Forward’s “Starwisp” microwave-pushed integral sail/starprobe (I call it the “Flying Tam Tam cracker” because its hexagonal shape suggests one) was designed to be quickly accelerated to 0.2 c, but later analysis indicated that its particular (very fine, extremely lightweight metal mesh) design would sizzle and fry instead of accelerating away.

  • Michael C. Fidler April 9, 2018, 11:16
    • Andrew Palfreyman April 10, 2018, 14:50

      Clearly the cooling problem is well in hand here, but how do you plan to get the particle beam up through the atmosphere?

      • Michael C. Fidler April 11, 2018, 10:17

        Take a look at the second page on the upper pdf file, Dome C is 10,000 feet high on Antarctica’s ice and Dome A is over 13,000 ft. The air is dry, clear and with little turbulence. The laser will clear the way thru the thin atmosphere and AO (Adaptive Optics) will keep it from blooming. No jet stream and a perfect shot at Alpha Centauri and Proxima Centauri 24 hours a day!

        • Michael April 11, 2018, 13:10

          But what is the air mass index and also if we need a shot for a gravilens probe it might just be out of reach as it is in the other direction.

          • Michael Fidler April 11, 2018, 17:41

            Yes, good point that is why originally I was thinking of Ross Island, closer to the 60 degrees south of Alpha Centauri, but it would be only high in the sky for part of the day. The solar gravilens is something I considered also, the north artic could be used for a northern beamer but more politicly touchy. The Antarctica is more conducive to such a project as in the International Geophysical Year in 1957-58, more of an international project. The separate continent and beaming were it can not be easily used for military purposes would be a BIG plus for any such project.

  • Geoffrey Hillend April 9, 2018, 16:04

    Quote by J. Jason Wentworth “because microwave beams diverge more rapidly than laser beams.” This is not correct. There is a difference between mircowaves and a maser. The microwaves in your microwave oven spread out, but the microwaves in a maser if powerful enough could cook your food or boil your pot of water a mile away, but not ordinary microwaves. A maser uses the same exact principle as a laser. The light from an ordinary light bulb is spread out over many different wavelength so the light dissipates or spreads out, but a laser has the light all at the exact same wavelength and a maser has all the microwaves photos vibrating and moving together in one coherent beam with no spreading out. Ordinary microwaves spread out. Also there are also lasers in the HII regions in outer space. https://link.springer.com/chapter/10.1007/978-1-4612-2378-8_57
    You’ve completely missed my point though. Imagine your solar cells in outer space. These can be made to produce a lasing effect but it will harmless like lasers in the hydrogen gas surrounding stars in space.
    One has to have mirrors to cause the cascading effect, the building up of many photons between the mirrors which cause the cascade or emission of many combined electrons, not just two at a time like in the HII regions of stars or a large solar cell array. There are lasers that don’t need mirrors like a free electron laser, but the laser in this article, Laser Beaming and Infrastructure is not explained. How do you go from lasing solar cells which don’t emit much over a large or small area to a concentrated beam which is made of a lot of photons. It seams to me one can’t leave the details of the laser out since without the powerful laser, you spacecraft goes nowhere, and some lasing solar cells don’t put out much. The lasing effect does not produce a very strong beam since there are only the emission of two photons at a time. The light from them would have to be concentrated, so the amplitude becomes very high as the result of a combination of many photons. You could float in front of an array of lasing solar cells and not notice anything. It might only move the array a very small amount.

    • J. Jason Wentworth April 10, 2018, 3:35

      I was referring to using a maser (which was developed a few years before the laser, utilizing the same principle). But even totally incoherent light can push a sail, as Echo 1 demonstrated back in the 1960s. An incoherent but powerful beam of microwaves could push a metal sail, but not as efficiently as a coherent maser beam.

  • Geoffrey Hillend April 9, 2018, 16:19

    A maser has all the photons at the same energy, wavelength or phase and direction. Ordinary microwaves are not the result of a population inversion, the pumping up of an electron with EMR photons to a higher energy state, holding it there with continuous EMR, and afterwards adding additional energy to the electron to raise it to an even higher level and when it falls down to the ground state, it re emits two photons at the same time instead of one.

  • Geoffrey Hillend April 9, 2018, 16:58

    I think I have misunderstood the design of this spacecraft. The laser does not use the photovoltaic cells. They just receive the laser light. My only criticism is that it’s harder to make a laser to be used in space since weight becomes a problem and one has to engineer a smaller, lighter laser that is still powerful.

  • Ivan Vuletich April 9, 2018, 21:27

    How would the lithium ion plume interact with the incoming laser?

    • Michael April 10, 2018, 15:21

      This depends on the frequency of the laser and absorption spectrum of ionized lithium, I don’t think it will be a problem.

      • J. Jason Wentworth April 11, 2018, 2:21

        I’m inclined to agree with you, but Ivan brings up something that might–but might not–be a problem:

        The exhaust beam of an ion thruster has to be neutralized, in order to prevent the spacecraft from developing a net charge, which would draw back the exhaust beam ions, nullifying the thrust. Next to the exhaust beam grid (or the ring-shaped [or cylindrical] “exhaust cavity,” in a Hall Effect thruster) is a hot cathode filament or an electron gun, which emits the electrons that were knocked off from the propellant atoms during the ionization process. in order to prevent the spacecraft from developing a net charge, and:

        It is possible that the incoming laser power beam, approaching the spacecraft from the rear as it accelerates away, *might* heat and re-ionize the exhaust beam to some extent. While I think it should be analyzed, I suspect that this shouldn’t cause a problem, for two reasons:

        [1] If the exhaust beam re-ionization occurs, it may be too far behind the spacecraft for the re-ionized exhaust to interact with the vehicle, and:

        [2] If the “composite” laser-neutral particles power beam can be held “tight,” as the PROCSIMA team’s work is attempting to do, it should be able to be focused on the spacecraft’s laser-tuned photovoltaic array, and “miss” the ion exhaust beam.

        • Brett Bellmore April 11, 2018, 13:21

          I really kind of doubt you’d want a particle beam intense enough to help keep a laser beam focused hitting your photovoltaic array. So you’d have to find some way to separate them.

  • Michael April 11, 2018, 13:53

    “[1] If the exhaust beam re-ionization occurs, it may be too far behind the spacecraft for the re-ionized exhaust to interact with the vehicle, and:”

    The laser light is not of the correct frequency to re-ionise the lithium.

    “[2] If the “composite” laser-neutral particles power beam can be held “tight,” as the PROCSIMA team’s work is attempting to do, it should be able to be focused on the spacecraft’s laser-tuned photovoltaic array, and “miss” the ion exhaust beam.”

    With a tight exit beam which is ideal for thrust perhaps we could have the solar cells further away from the engine, it is unlikely that the exhaust will be pointed at the laser beam all the time.

    My feelings it that the lithium will be to diffuse to cause an issue.

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