Power beaming to accelerate a ‘lightsail’ has been pondered since the days when Robert Forward became intrigued with nascent laser technologies. The Breakthrough Starshot concept has been to use a laser array to drive a fleet of tiny payloads to a nearby star, most likely Proxima Centauri. It’s significant that a crucial early decision was to place the laser array that would drive such craft on the Earth’s surface rather than in space. You would think that a space-based installation would have powerful advantages, but two immediate issues drove the choice, the first being political.
The politics of laser beaming can be complicated. I’m reminded of the obligations involved in what is known as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (let’s just call it the Outer Space Treaty), spurred by a paper from Adam Hibberd that has just popped up on arXiv. The treaty, which comes out of the United Nations Office for Space Affairs, emerged decades ago and has 115 signatories globally.
Here’s the bit relevant for today’s discussion, as quoted by Hibberd (Institute for Interstellar Studies, London):
States Parties to the Treaty undertake not to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner. The moon and other celestial bodies shall be used by all States Parties to the Treaty exclusively for peaceful purposes. The establishment of military bases, installations and fortifications, the testing of any type of weapons and the conduct of military manoeuvres on celestial bodies shall be forbidden. The use of military personnel for scientific research or for any other peaceful purposes shall not be prohibited. The use of any equipment or facility necessary for peaceful exploration of the moon and other celestial bodies shall also not be prohibited.
So we’re ruling out weaponry in orbit or elsewhere in space. Would that prohibit building an enormous laser array designed for space exploration? Hibberd believes a space laser would be permitted if its intention were for space exploration or planetary defense, but you can see the problem: Power beaming at this magnitude can clearly be converted into a weapon in the wrong hands. And what a weapon. A 10 km X 10 km installation as considered in Philip Lubin’s DE-STAR 4 concept generates 70 GW beams. You can do a lot with that beyond pushing a craft to deep space or taking an Earth-threatening asteroid apart.
Build the array on Earth and the political entanglements do not vanish but perhaps become manageable as attention shifts to how to avoid accidentally hitting commercial airliners and the like, including the effects on wildlife and the environment.
Image: Pushing a lightsail with beamed energy is a feasible concept capable of being scaled for a wide variety of missions. But where do we put the beamer? Credit: Philip Lubin / UC-Santa Barbara.
The second factor in the early Starshot discussions was time. Although now slowed down as its team looks at near-term applications for the technologies thus far examined, Starshot was initially ramping up for a deployment by mid-century. That’s pretty ambitious, and we wouldn’t have a space option that could develop the beamer if that stretchiest-of-all-stretch goals actually became a prerequisite.
So if we ease the schedule and assume we have the rest of the century or more to play with, we can again examine laser facilities off-planet. Moreover, Starshot is just one beamer concept, and we can back away from its specifics to consider an overall laser infrastructure. Hibberd’s choice is the DE-STAR framework (Directed Energy Systems for Targeting of Asteroids and Exploration) developed by Philip Lubin at UC-Santa Barbara and first described in a 2012 on planetary defense. The concept has appeared in numerous papers since, especially 2016’s “A Roadmap to Interstellar Flight.”
If the development of these ideas intrigues you, let me recommend Jim Benford’s A Photon Beam Propulsion Timeline, published here in 2016, as well as Philip Lubin’s DE-STAR and Breakthrough Starshot: A Short History, also from these pages.
What Hibberd is about in his new paper is to work out how far away various categories of laser systems would have to be to ensure the safety of our planet. This leads to a sequence of calculations defining different safe distances depending on the size of the installation. The DE-STAR concept is modular, a square phased array of lasers where each upgrade indicates a power of base 10 expansion to the array in meters. In other words, while DE-STAR 0 is 1 meter to the side, DE-STAR 1 goes to 10 meters to the side, and so on. Here’s the chart Hibberd presents for the system (Table 1 in his paper).
Keep scaling up and you achieve arrays of stupendous size, and in fact an early news release from UC-Santa Barbara described a DE-STAR 6 as a propulsion system for a 10-ton interstellar craft. It’s hard to imagine the 1,000 kilometer array this would involve, although I’m sure Robert Forward would have enjoyed the idea.
So taking Lubin’s DE-STAR as the conceptual model (and sticking with the more achievable lower end of the DE-STAR scale), how can we lower the risks of this kind of array being used as a weapon? And that translates into: Where can we put an array so that even its largest iterations are too far from Earth to cause concern?
Hibberd’s calculations involve determining the minimum level of flux generated by an individual 1 meter aperture laser element (this is DE-STAR 0) – “the unphased flux of any DE-STAR n laser system” – and using as the theoretical minimum safe distance from Earth a value on the order of 10 percent of the solar constant at Earth, meaning the average electromagnetic radiation per unit area received at the surface. The solar constant value is 1361 watts per square meter (W/m²); Hibberd pares it down to a maximum allowed flux of 100 W/m² and proceeds accordingly.
Now the problems of a space-based installation become strikingly apparent, for the calculations show that DE-STAR 1 (10 m X 10 m) would need to be positioned outside cis-lunar space to ensure these standards, and even further away (beyond the Earth-Moon Lagrange 2 point) for ultraviolet wavelengths (λ ≲ 350nm). That takes us out 450,000 kilometers from Earth. However, a position at the Sun-Earth L2 Lagrange location would be safe for a DE-STAR 1 array.
The numbers add up, and we have to take account of stability. The Sun/Earth Lagrange 4 and 5 points would allow a DE-STAR 2 laser installation to remain at a fixed location without on-board propulsion. DE-STAR 3 would have to be positioned beyond the asteroid belt, or even beyond Jupiter if we take ultraviolet wavelengths into account. The enormous DE-STAR 4 level array would need to be placed as far as 70 AU away.
All this assumes we are working with an array on direct line of sight with the Earth, but this does not have to be the case. Let me quote Hibberd on this, as it’s rather interesting:
Two such locations are the Earth/Moon Lagrange 2 point (on a line from the Earth to the Moon, extending beyond the Moon by ∼ 61, 000 km) and the Sun/Earth Lagrange 3 point (at 1 au from the Sun and diametrically opposite the Earth as it orbits the Sun). In both cases, the instability of these points will result in the DE-STAR wandering away and potentially becoming visible from Earth, so an on-board propulsion would be needed to prevent this. One solution would be to use the push-back from the lasers to provide a means of corrective propulsion. However it would appear a DE-STAR’s placement at either of these points is not an entirely satisfactory solution to the problem.
So we can operate with on-board propulsion to achieve no direct line-of-sight to Earth, but the orbital instabilities involved make this problematic. Achieving the goal of a maximum safe flux at Earth isn’t easy, and we’re forced to place even DE-STAR 2 arrays at least 1 AU from the Sun at the Sun/Earth Lagrange 4 or 5 positions to achieve stable orbits. DE-STAR 3 demands movement beyond the asteroid belt at a minimum. DE-STAR levels beyond this will require new strategies for safety.
Back to the original surmise. Even if we had the technology to build a DE-STAR array in space in the near future, safety constraints dictate that it be placed at large distances from the Earth, making it necessary to have first developed an infrastructure within the Solar System that could support a project like this. As opposed to one-off missions from Earth launching before such an infrastructure is in place, we’ll need to have the ability to move freely at distances that ensure safety, unless other means of planetary protection can be ensured. Hibberd doesn’t speculate as to what these might be, but somewhere down the line we’re going to need solutions for this conundrum.
The paper is Hibberd, “Minimum Safe Distances for DE-STAR Space Lasers,” available as a preprint. Philip Lubin’s “A Roadmap to Interstellar Flight” appeared in Journal of the British Interplanetary Society 69, 40-72 (2016). Full text.
I would have thought the obvious placement solution is on the lunar farside. It satisfies the requirement to prevent the lasers from being in the “line of sight” of Earth, but still effectively in space, Barring issues of directionality with targets that may be close to the lunar horizon, almost all targets can be safely reached during a lunar month.
If manufacturing much of the mass of the laser array can be done locally, that would be operational and cost-effective compared to an array placed at L2, and the moon would be a stable platform obviating the need for propellant for station-keeping, let alone the mass of the support structure for an array in space that would be nigh impossible to stop from flexing and interfering with the phasing of the laser beams.
The main downside of placing an array on farside is that if there are a large number of lunar satellites, not to mention transport that requires orbital space over farside, the use of the arrays will have to allow for these to pass. [Any optical SETI on stars looking at our system might see not only a strong, narrow, monochromatic “signal”, but possibly pulsed irregularly as it is turned off to prevent damage to satellites and spacecraft passing overhead on farside. How would an ETI version of “The Mote in God’s Eye” or “Tower of Glass” be written with that as an inspiration?]
Agree. The inherent design of any such device must be so that it is physically impossible to use it as a weapon, so Luna’s far side, always facing away from Terra, is the obvious preference. Plus, there is the advantage of local resources, aluminum and etc.
What about the far side of the Moon?
When I read about building anything on the lunar farside – not dark side, Derek Miller – I am reminded of one of the major steps from Project Daedalus, the British Interplanetary Society (BIS) big white paper project conducted from 1973 to 1978 for developing an automated interstellar vessel that could reach Barnard’s Star in just 50 years.
Side Note: Why Barnard’s Star, a red dwarf sun almost 6 light years from Earth, as opposed to Alpha and/or Proxima Centauri, which are 2 light years closer to us? Because in the early 1970s, Barnard’s Star was though to have two gas giant exoplanets circling it, back when almost no stars were known to have such companions. Turns out it was a data error. On the plus side, Daedalus could reach Proxima Centauri – which does have at least one exoworld – in just 36 years.
Onward… Daedalus was planned as a fusion powered vessel. Part of the process would be the need for helium-3 as part of the fuel. As helium-3 is very rare on Earth, the project designers chose Jupiter as the place to mine for helium-3. The envisioned a robotic factory suspended by a balloon mining the resource for over 20 years, then transporting it back to Earth orbit where Daedalus would be under construction.
Yes, we now speak of mining helium-3 from the Moon, which would be much closer and relatively easier than Jupiter. However, the point I am trying to make it these plans involve a permanent space infrastructure that does not currently exist and probably won’t for decades – and little to none of it will be devoted to helping build any kind of starship.
It may have seemed neat and cool to add these elements to the design process, but did any of these designers really think about what would be involved in making their starship as conceived? We are struggling to get humans back to the Moon for brief expeditions at the moment – so imagine what it would take to build a radio telescope or a helium-3 mining facility on the lunar farside, or a megalaser facility for that matter.
Not trying to be a naysayer here, I am just frustrated that those who talk about building any kind of interstellar vessel talk about details of their projects as if they are going to happen naturally and easily along the way. They aren’t. Those who want to exploit space are not seriously looking at plans for interstellar vessels right now. So what do we do about that?
Perhaps it helps to have the perspective of one who watched the dreams stated during Apollo and plans after that drop away and stay down for decades. I am honestly amazed at even the relatively small progress I have seen in the last few years – and note how much of it happened due to private industry. Just remember, businesses are in the business of making money first and foremost. How will a probe to Proxima Centauri fit into their bottom line?
But recall that the DE-STAR laser array was originally conceived for planetary defense – to deflect PHAs that could impact the Earth. Hence the DoD was interested. That the laser array could power beamed sail probes was a later suggestion that IIRC was proposed at the “Starship Century” conference (or at least about that time).
The DoD has vastly more available funding than civilian space projects. If the military wants something, they have a fairly compliant US Congress to grant them funding, sometimes in excess of what they ask for.
However, I agree with your main point. Corporate funding as investments for profit can be much more effective. It is focused and profit-driven. The downside, as we saw with the rapid hyping and subsequent demise of the asteroid mining craze, is that what might be scientifically interesting can be ended at a stroke, with no opportunity to persuade those with the pursestrings to change their minds. Government funding can “prime the pump” and place incentives for corporate involvement to run with the commercial operations. Science can piggyback on that. An oft-used example is working with oil? IDK, and so far all the ideas I have seen seem speculative. [I have Ruzic’s “The Case for Going to the Moon” (1965) that stated that the vacuum and extreme cold of the lunar poles should be ideal for some electronics industry components. Later, books by other authors proposed lunar He-3 (for non-existent fusion reactors) and platinum for fuel cells and the H2 economy. But technology often +sidesteps these resources. Solar PV and wind turbines are now the only viable non-fossil fuel energy generating approaches. Fusion reactors, even smaller scale commercial approaches, are still experimental and decades away from providing energy – if they even work. We don’t need larger amount of platinum for fuel cells, and H2 fuel cells are not the preferred way to store energy for electric vehicles (although they might be for airliners).
So what would a lunar base do? I can see plenty of scope for research about the Moon. But what commercial ventures make sense?
I am a great fan of the Eagle comic’s Dan Dare strip, started in 1950. I love the style of the various artists starting with Hanpson, and ending (at least for me) with Keith Watson. And the wonderful adventures. But was there even a hint at commerce in space, other than extractable resources? Even the initial need to grow crops on Venus to feed the Earth’s population turned out to be more like a giant aid package supplied by the Therons of Venus. (Such was the more post-WWII socialist-leaning politics of Britain). The same with the Jeff Hawke comic strip started at around the same time. Look at almost any Sci-Fi space movie and commercial activities in the background are usually absent, or used as a plot device and need to be curtailed. Even Heinlein’s stories, don’t have much in the way of commercial objectives. Even “The Man Who Sold the Moon” doesn’t IIRC, have a reason to go to the moon to exploit its resources or environment, other than as a billboard (the same idea as Clarke used in his short story “Watch This Space” as part of the “Venture to the Moon” stories).
AFAICS, New Space entrepreneurs are still looking for commercial prospects in space, most going the route of supplying “wagons, tents, and mining equipment” for higher-risk corporate ventures. More wishful thinking than hard-nosed ROI businesses.
Which means that piggy-backing on private space infrastructure may be very slim pickings, and the really interesting scientific questions won’t be answerable with corporate money. [Where are the corporate radio telescopes? All the asteroid probes are public missions, including the Lucy mission. Are any of the private Moon landers looking to determine He-3 resources and how best to extract it?). IOW, if we have to wait for commerce to build the infrastructure to support the development of a large starship, fuggedaboutit. I think building the smallest, feasible probes and using existing resources or ones that the government will build for other purposes, is the best way forward. If asteroid defense is done using other methods, and the military doesn’t want to build large-scale laser arrays, then the beamed sail approach will be a stillborn. We will need another technology to do it, or wait a long time for solar system settlement and economic growth to pay for a star probe. Like building cathedrals, we may just have to accept that starflight will be a centuries-long mission that cannot be planned and achieved during a human lifetime. We will need much longer mission horizons.
How about the “dark” side of the moon? Always pointing away from Earth! Radius is 1737km, which is a DE-STAR 6+ class.
There should be plenty of material to build the thing there, but solar power and tracking may be a problem.
This is a very interesting dilemma. Several thoughts occur: first, position maintenance for Lagrange point 3 could be provided by either solar sails attached to the laser arrays for corrections, which would be passive, or solar PV to electric propulsion motors (active). The latter might also be the power source to drive the laser arrays. Second, how about putting the laser arrays on the far side of the moon. This would allow any size array while blocking any direct-to-Earth beam unless a relay mirror system was added and this could be defended against. This location would face the same beam-steering challenges of the other locations, but at different time scales, at least in part. It would be close enough to allow quick visits to meet maintenance issues as they came up. Lunar dust might be a problem, however. If placed on Earth, perhaps the laser arrays could be either set in an annulus of large dimension or scattered almost at random over a large area. This would disperse the energy input to the atmosphere, perhaps lowering influence on turbulence and weather, but perhaps increasing the complexity of aircraft exposure avoidance. The beams would aim at a beam combiner at one of the considered distances mentioned in the post, chosen as appropriate for the power of the array. Techniques have been developed for maintaining beam integrity in the face of atmospheric turbulence both for laser weapons and for astronomical observations.
I think the “Outer Space Treaty” is relevant only to the extent the United Nations can enforce it. And I think it’s outdated anyway.
When corporations are ready to get serious about space business in space itself, they will either find ways to get around the UN Outer Space Treaty, or just ignore it altogether. The UN response will probably be a strongly worded protest.
https://www.projectstatecraft.org/post/fifty-years-of-the-outer-space-treaty-challenges-and-need-for-a-new-treaty
There is the possibility of using a large two sided reflective shield say at the geostationary or geosynchronous point. The ground based laser array fires through a hole in the centre onto the sail, the laser light is then forced to bounce between the space side reflective shield and the sail allowing multibounce to occur which reduces the capital costs of the laser system. The reflective side facing the earth has a dispersion reflectivity greatly reducing any back light towards earth. This can also be used on the moons darkside lagrange point.
Is there really any safe distance or placement options where a “direct line of site” cannot be manufactured with secondary, redirecting assets?
I feel like the military people regard the rest of us as simple-minded. Are we expected to believe that a device under the control of any individual, corporation, nation, or international NGO is going to expend thrust to actively maintain its inability to be relevant in the event of a global nuclear conflict? Does any military have a stronger imperative to be honest than to gather power and defend from attack? I think anyone actually working with such a device can see a clear moral path to ensure that it can be used that way… a path that leads onward until it is used in more aggressive attacks on civilian and military infrastructure.
I’d go so far as to guess that the “DE-STAR” name, which resorted to using an internal capital in its acronym, may be missing something like “…Antiballistic Targeting and Homing, …” that might appear in some more classified manifestation. We’ve seen such unsubtle naming with Palantir and Narya — now an election and a heartbeat away from having a man at the pinnacle of U.S. government. The dreams of Big Tech aren’t very secret – the CEO of X is testing brain microchips; the CEO of Oracle publicly dreams of making everyone’s daily life happen “under supervision” so they can be “their best selves”. Like elf-kings, we built an internet under the self-delusion there would be freedom of speech and decentralization, even as the cables carrying virtually all the traffic were laid down in the Dulles Toll Road median next to intelligence agency buildings. Who now thinks that unrestrained free enterprise and the marketplace of ideas have any role in what the internet is becoming?
In such a context, we ought to think beyond the plan we’re told will happen. If there is a plan to send a satellite array to the outer solar system, we should ask how soon it will come back, or what will happen if there is a failure in the interplanetary leg of the voyage to begin with. If an array is on the far side of the Moon, we should ask how readily it can be reflected to Earth. If someone wants us to buy a device to slay an ‘astronomically’ improbable natural bogeyman, we must look closely at whether we are helping a real danger to grow above our heads.
Even if we feel it is necessary to build launching lasers for an interstellar probe, is there any reason why we can’t converge many small lasers distributed among civilian buildings in cities all over Earth’s surface?
Mike, I think you meant to say “their best slaves.”
Certainly, just about any technology can be turned against us. Orwell assumed televisions would be watching us instead of us watching them in his 1948 novel, Nineteen Eighty-Four.
Sagan and Ostro warned in 1994 how methods for deflecting dangerous planetoids could be just as easily used to ai them at targets on Earth.
https://www.nature.com/articles/368501a0
Then there is this article:
https://www.jstor.org/stable/10.7249/mr1209af.18?seq=5
The question I have is: Would a megalaser be the most efficient means of propelling interstellar vessels even if one ignores its dangers as a weapon?
It is a current method of choice, judging by all the literature on it these days, but I think it has just too many technical and physics issues to be a viable choice for the near future.
We would love to see an interstellar vessel at least built and launched in our lifetimes. Yes, the fact that it could get a fleet of probes to Proxima Centauri in just 20 years is attractive to beings that live about 80 years on average.
But I don’t see the megalaser method happening any time soon. Nothing has been built. Nothing has been funded. No one can decide who and what is going to pay for and operate it — but you know it is either going to be a powerful wealthy government or an even more powerful corporate conglomerate.
So how can we have hopes for the laser starship any more than a fusion or fission powered one? At least with an Orion version, we COULD have had it ready to go decades ago, but everyone still seems squeamish about a nuclear bomb-powered vessel.
However, as I said in an essay I wrote on Orion published in this blog in 2016, there is at least one country that has the capabilities and lack of such concerns who could make such a vessel happen. My stance has only gotten stronger since then:
https://www.centauri-dreams.org/2016/09/16/project-orion-a-nuclear-bomb-and-rocket-all-in-one/
So, if you are in a real hurry to get to the stars, go with Orion. Otherwise, why not focus on a starship design that is self-contained and doesn’t require on the good faith and good nature of humanity back home? Not to mention REALLY good aim.
‘Certainly, just about any technology can be turned against us. Orwell assumed ——-televisions would be watching us instead of us watching them in his 1948 novel, Nineteen Eighty-Four.’
If they controlled the output from the TV say via propaganda and got a ‘citizen’ to dob a naysayer of the narrative in, is that not in a way the same. A sort of western type of control as opposed to soviet type.
Our Final Hour (2003) by Sir Martin Rees itemizes the many technologies we have that could be used as weapons and even destroy us – in this century.
We should understand that as our technologies become ever more powerful and cheaper, they will be used in ways whether deliberately, or inadvertently, against our existence.
Forget Orwell. Mass surveillance by corporations (as well as governments) has become ubiquitous (“better living through surveillance”?). Who would have thought back in the mid-1990s that the WWW would be used as a medium to destabilize democracies? We don’t need huge, fast-moving objects or high-energy beams as weapons. We have far more subtle, and much cheaper, ways to attack other people or nations. Just look at the chaos caused by the Russians’ use of the Novichok nerve agent in Salisbury, or Ricin in the 1990s (One only needs to send an envelope with some white powder to shut down a facility). A contemporary US politician just needed to make up a story of “hateful others” to trigger chaos in a town.
Large, easily targeted facilities like vast, space-based laser arrays, are probably the least of our worries. It is the unsuspected use of technologies or their unexpected side effects that will prove to be our demise. Whoever thought that CO2 emissions from burning the fuels that powered our Industrial Revolution and improved the well-being of billions of people could be an existential threat? And look how hard it is to reverse course.
We have a long ways to go yet from some of the temperatures in the past.
https://tse1.mm.bing.net/th/id/OIP.PAjiekxZbUq6AYnS63x7EAHaDG?pid=ImgDet&w=197&h=75.23050458715596&c=7&dpr=1.1
Not as much as one might think. The Eocene thermal maximum is depicted at about 80F (300K), the Permian extinction about 90F (305K). In less than 2 centuries we are on course to raise the temperature 2.5F and we could push 8F in a pessimistic scenario within another century or two. Humanity would be reduced to a much smaller population huddled at the extreme northern ends of their major continents. It is conceivable we could build large arcologies to house and protect the human population and selected biota, but many of the important, and charismatic species, would face extinction, including our food crops. While we probably won’t cause the temperatures to rise to the land biosphere extinction temperature of 338K (149F), we really don’t want to be testing the limits given what we are seeing already.
Our planet 0.5-1.9 bn tears in the future with the modeled temperatures would be rather grim and likely severely depleted in biodiversity, at least with the species we currently live with. However, we cannot predict what species might evolve to adapt to the higher surface temperatures and whether the Gaia hypothesis would have a more ameliorating effect on the temperatures than the authors’ model. [An extreme “Daisyworld” would have vegetation with highly reflective foliage and flowers to increase the albedo. As Dave Moore posits, plants might redirect their metabolism to reduce carbon fixation for cellulose and produce more nitrogen-rich proteins instead, or like diatoms, use silica instead for cell walls. Or perhaps they create reflective clouds by emissions of SO2.]
If such a device is built, governments in several countries would request that extreme measures would be taken to prevent any hostile takeover of the laser system. Either trough direct physcial take-over of the control facility, remote hacking or even by sending false instructions from another site. (Assuming it’s remote controlled without any personnel on site.)
And they might indeed quote the Outer Space Treaty – now that is supposed to be applicable to nations and governments.
But in certain situations a country can be responsible for what their citizens and corporations do. And there’s legal precedents for that situation.
So I have a hunch that if and when an apparatus is built that also would be a potential superweapon – I’m certain the lawyers will get quite busy.
But yes, as mentioned above, if it’s located on the far side of the moon the direct risk would be removed.
Not that this is any immediate concern, as said in the main post, Starshot envisioned they would be doing this in the 2050’s.
A World-Changing Gamma Ray Laser Is on the Horizon. It Could One Day Unlock Interstellar Travel.
Published: Aug 01, 2024
Popular Mechanics, but behind a paywall.
Wireless communication technology based on gamma ray…
https://www.sciencedirect.com/science/article/abs/pii/S0168900222003692
So where is SETI?
Just gather enough smoke detectors and you could be talking to aliens right now!
;-}
There is another way to use gamma rays to communicate and that is to have a stream of electrons going across the gamma ray beam. every time a gamma ray interacts with an electron it is knocked off course into a receiver.
Several comments have been made about not having an antenna completely filled with many small apertures, but instead have a larger diameter array filled sparsely with widely spaced antennas, leaving empty spaces in the antenna plane. The advantage is supposed to be the larger diameter could then be focused on a smaller area, increasing the power density on target. The reason to not do this is known as the “thinned array curse” (sometimes the “sparse array” curse.)
The thinned array curse is very simple. If a transmitting antenna is filled by an area fraction F (where F < 1), then the power that is lost by emission into side lobes of the beam, and hence is not directed into the main beam, is proportional to 1 – F. So thinning the array causes inefficiency.
This can be derived from the brightness conservation principle, which is directly derived from the third law of thermodynamics: an antenna that is not fully filled cannot make a spot on the ground that is brighter than a filled array of the same diameter, and hence any array phasing that makes a smaller spot on the ground must also direct less power into the main beam.
I had heard of the ‘thinned array curse,’ but never understood it until now. Thanks, Jim!
Although the thin array has some big issues the side lobes can still be used to propel other payloads that require less velocity, say satellite infrastructures. Over time the array can be filled in with more units.
Here is a little mathematics around the thin array, a large part of the losses can be circumvented by clever placement of the array components.
https://www.antenna-theory.com/arrays/geometry/thinnedarrays.php
Thank you for your clarification on this detail James. (And if I guess right on who this is I assume it was from a highly qualified person – if so I add: Thank you sir for some good reads.)
While the details and situation. The proposal of having smaller receivers to create a virtual larger disk for receiving a signal from one interstellar probe using quantum means described in ‘Are Interstellar Quantum Communications Possible?’ would not work either.
It appear this person who mentioned that have been thinking of interferometry which is all good for creating images of higher resolution for a distant object. But what would be needed is an actual reflector of that enormous size since it’s to any light gathering telescope where every photon counts.
Jim is president of Microwave Sciences, and the author of the key textbook in the field, High-Power Microwaves, just entering its 4th edition. And you’re right, Andrei. His brother is science fiction author Greg Benford.
Being based on terra firma is not much of an obstacle to weaponization if we’re contemplating tech that is 20 years down the road. Recall the missile defense ground based laser studies back in the 1980s, where orbiting relay mirrors were used to transport the beam wherever it was needed.
As many have pointed out, the farside of the Moon seems a safe place. The beam director for a relay back to Earth would be so big that it would be quite obvious.