Space-based solar power (SBSP) has three advantages over terrestrial solar: flexibility, footprint and theoretical efficiency.

The classic case for flexibility is disaster zones; it's easier to set up receiving meshes than build new power plants. The problem is humanitarian crises don't pay. The real sell is military and mining. Beaming power to remote operations simplifies logistics.

Footprint is minor, but worth mentioning: the receivers for space-based solar are smaller and could be laid over e.g. farmland.

Last, but not least, the atmosphere absorbs a lot of solar energy. In this model, the space-based rig is a lens, converting the well-absorbed frequencies that generate power into microwaves to which dry air is more transparent. If you can get a focussed beam, a SBSP rig could also be placed far enough out that it's less subject to Earth's day-night cycles. These factors boost its theoretical efficiency.

I haven't run the numbers for several years. But they only become compelling if you have ISRU and in situ manufacturing of panels. I don't think SBSP is a beachhead market for asteroid mining. But it's feasible if we have it.

That’s only a problem if profit/exploitation is more important than humanity.

IMHO, space based technology needs to be public owned (not corporate) and not allowed to profit, with its goals and designs fit to benefit humanity before profit.

1. Geostationary orbits are in 24/7 sunlight except for a short period each day near the equinoxes

2. Geostationary polar orbits do not exist. Geostationary orbits are, by definition, 0° inclined. A polar geosynchronous orbit, like you mentioned, would not offer much besides being over the same point on the Earth at the same instant every day.

It would offer a lot: the ability to send down an absolutely massive amount of power without having the corridor shift around so that it can be reliably routed around by air traffic. Having the equivalent of a microwave scythe moving across a large area would be kind of counterproductive.

In fact, if we're already beaming power (assuming this works reliably enough) it might make sense to have a set of birds "surfing the terminator" in a polar orbits that can then transmit to geostationary birds which in turn transmit to ground stations.

That is the big potential with solar satellites, 24 hour a day power.

https://en.m.wikipedia.org/wiki/Space-based_solar_power#:~:t....

To achieve the energy transition, you need SO MANY solar panels and they will take up a lot of land. That land is most expensive near the very population centers which need the most power.

According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts."[1] (this isn't the most precise source I could find, but it's reputable)

[1] https://www.nytimes.com/2022/09/20/podcasts/transcript-ezra-...

> To achieve the energy transition, you need SO MANY solar panels and they will take up a lot of land.

This isn't true.

> According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts.

The estimates are that it's anywhere from 10000 to 30000 square miles which is compared to the 3.8 million square miles of the continental USA an absolute pittance. Doing this by tightbeaming the output of an equivalent solar installation in space is ridiculous.

The roof of every single new big box store should be covered in solar panels and we should start covering large parking lots in them too. Shaded parking that powers the area would be sweet.

There is even a potential convergence: https://en.wikipedia.org/wiki/Agrivoltaics

Getting rid of waste heat in space is harder than it is on earth!

If we used the area that is currently producing ethanol and "biodiesel" for solar panels we would be 80% done with the energy transition.

One standard design had a 10km contiguous(!) diameter receiver for a 1km diameter transmitter in GEO. You can lower the transmitter to about 6371 km[0] before having to worry about night, but that's only going to give you a factor of about 6 improvement. Different wavelengths can do better antenna gain, but start to need to care about atmospheric absorption. You could in principle also relay the power around the same sort of way we do for data, though that way you get a percentage loss from each relay (might still be worth it, might not, I've not tried to cost it).

You're also limited by the maximum safe power density; from what I've heard this is surprisingly close to what you'd get from the yearly average output of PV, which on the one hand means that although it can be an alternative to storage when the question is "what about night and winter?" it is fairly unhelpful when the question is "can we use less land"?

> That land is most expensive near the very population centers which need the most power.

Yes, but wire-based transmission is better than most give it credit for. And even if it was that bad, if you perfectly solved wireless transmission by any mechanism you like, it's a shorter route from the ground on the opposite side of the planet, to low orbit, around the planet, and back down again (~20e6m), than from geostationary orbit (~36e6m).

> According to Jesse Jenkins, to power America, "the solar farms are an area the size of Connecticut, Rhode Island and Massachusetts."[1] (this isn't the most precise source I could find, but it's reputable)

Sounds about right: https://www.wolframalpha.com/input?i=%28Connecticut%2C+Rhode... (20% efficient cells, 10% capacity factor).

Those are 2.9e11, 6.3e10, 5.5e11 W, compared to recent US use of 4.6e11 W.

But they're also fairly small states, with 1%, 0.3%, and 2% of the US total population: https://www.wolframalpha.com/input?i=%28Connecticut%2C+Rhode...

[0] If I've done the maths right:

1. The radius of the inscribed circle of an equilateral triangle is r = (sqrt(3)/6)*a

2. The altitude of the triangle from any side is h = (sqrt(3)/2)*a = 3*r

3. So the closest you can get the satellites while being sure at you never have a period where the only visible satellites are themselves not in shadow is:

triangle_altitude - diameter_earth = 3*r - 2*r = r

I now think my assumption is wrong, that the closest you can get without worrying about night is minimal bounding square, not a minimal bounding triangle, so the orbital altitude should be:

(sqrt(2) * radius) - radius = (sqrt(2) - 1) * radius ~= 2639 km

so a factor of about 14 better than GEO.

https://en.wikipedia.org/wiki/Starlink#Military_capabilities

https://en.wikipedia.org/wiki/Starlink#Subscribers

If they try to do more than 400 users per satellite, the service throughout plummets.

Space X sells commercial falcon 9 launches for $ 67 million (to GTO opposed LEO) and this already includes a profit margin.

It still isn't obvious to me.

They announced 2 million users with fewer than 4k sats in orbit (presumably even fewer serving).

Let’s assume 2 mil by 4k for simple math, which yields 500 users per sat.

However, the vast majority of the sats are not where the bulk of their customer base is (US and Europe) at any given moment. That means that the 5% of sats over user dense areas at any given point in time are capable of carrying 100% of the current load.

Stated differently, they have sats capable of serving the US user density sitting over every other country in the latitude bands of those orbits just waiting for licensing. I.e. User count could 10x on the current constellation if the growth is outside of the existing markets.

This is complete nonsense.

That said I often wish there was an effective way to communicate whether one calls bullshit because of gut feeling or not convinced by the readily available evidence and/or priors, or whether one calls bullshit because they know more details and are just deflecting actual well known bullshit but for some reason preferred to omit the details (e.g. because they value terseness).

Terseness is beautiful, but not when it actually hinders communication

Of course, if a person is already poisoned by twitter discourse to the point where they substitute their own reality just to keep hating Musk, then all hope is lost...

What I'm saying OTOH is that I have no way to know whether you have some more insightful things to say about this since you just called bullshit without any further details. I hoped you had more details

rectenna arrays might have been a reasonable idea when solar panels were expensive. they're still reasonable if you're willing to use a microwave beam that's concentrated enough to be a devastating weapon of war. sunlight is 1000 watts per square meter, and that's already enough to heat up a tin roof or the interior of a car to temperatures that will burn or kill. if you boost that by a factor of 10 or 100, you could get a higher-density "power plant" on the ground that's cheaper than the equivalent solar farm

but you still have to build the solar farm, you just build it in space. right now that's much more expensive, and, as jacquesm points out, rejecting waste heat in space is more difficult. there are a few possible ways that building it in space could be better:

1. if robots are mining asteroids for minerals, they can build solar panels in space that don't have to survive earth weather, or for that matter launch or reentry. plausibly they could be under a micron thick, thus using thousands of times less material than earth solar panels. conceivably you could even do this with terrestrial factories if you have cheap enough and gentle enough launch, which you don't, but maybe with bfr you will

2. if barbarians are mobbing your castle with pitchforks and torches, banning borax and methyl ethyl ketone, coercing you into swearing loyalty oaths, or threatening to put your family in ovens because of your ethnicity, you might prefer to have your solar panels somewhere they can't reach

3. to make the transition from a kardashev type 1 civilization to a kardashev type 2 civilization, obviously covering the entire earth with solar panels is insufficient, so you have to put them somewhere else

4. you can put the solar panels much closer to the sun than the earth is, so they produce much more power per unit area

obviously none of these are relevant right now, but sooner or later they will be

Now, if you wanted to build a nuclear power plant in space and beam its power, then heat dissipation would be a big problem.

i think this depends in part on how high a temperature you can tolerate; at 20° the stefan–boltzmann emissivity is only 420 watts per square meter for an ideal black body, a third of the above-the-atmosphere solar irradiance. the blackbody equilibrium stefan-boltzmann temperature for 1300 watts per square meter is 116°, which is a bit toasty for efficient solar panels; even at half that (if you're emitting infrared from the back of the pv array as well as the front) you're at 54°. and if you want to move your power plant closer to the sun to harvest that sweet, sweet radiation, the temperature goes up further

so plausibly some heatpipes painted black sticking far out the backside would be helpful

So the benefit would be that substantially larger surface areas of renewables could be deployed with fewer maintenance concerns (i.e. thin antennas don't really care about dust, and present little cross section to being struck by debris).

The atmosphere attenuates our 1300W/m2 of solar energy down to 300W/m2 so for a 125M km2 planet you only get 40PW of available power, or 5PW after 80% loss in solar panels and distribution.

In terms of global energy demand we’re currently using 25PWh per year which is the equivalent of 3TW. Assuming oil runs out, politics annuls nuclear options, and we cover most of the planet in solar panels then peak terrestrial solar is three orders of magnitude away.

Covering over everything with panels would presumably have devastating effects on the environment just as much as fossil fuels but if we limit to 1% coverage then we’re only one order of magnitude away from terrestrial solar being unable to power the world. It’s neither sunny nor midday all the time either so solar is starting to look dangerously close to not cutting it, and yet harvesting solar energy could be our only option for power for the next ten thousand years.

I pulled all these numbers from Wikipedia and crunched them with GNU units — this is HN after all — but it doesn’t sound unreasonable to conclude that anything planet bound is going to be bounded by planet size. Moreover, if the goal is for power generation to have zero impact on our biome then we should probably not generate power within the biome itself.

What Malthus wrote about was basically the lived experiences of those around him for basically all of what he knew of to be recorded history: improved farming meant more people, not happier people.

Now, I'm not qualified to even judge if Jared Diamond's even fair let alone correct, but he wrote in Collapse (2005) that the Rwandan Genocide was brought about in part due to excessive population pressures, that it "illustrates a case where Malthus's worst-case scenario does seem to have been right."

On the other hand, it also seems plausible to me that widespread access to contraceptives may break the assumptions behind Malthus.

https://en.m.wikipedia.org/wiki/Sub-replacement_fertility

We had some impact as hunter-gatherers at 10M population, probably made some species extinct from hunting. If I assumed c. 1940 tech was the limit and wanted to minimise human ecological impact, this population would be the maximum sustainable given CO2 emissions.

Conversely, if we put submarine cities in deep water, powered by PV on the surface that are ready to withdraw when storms come[0] and feed ourselves on the most efficient foods we already know how to grow in sealed environments, we could probably support trillions of people… but at that level we might possibly need to find some asteroids rich in, IDK, phosphorus or something.

I have seen larger maximum population numbers, but they're for scenarios where you fairly strongly disregard the ecology — if you're willing to do weird stuff like putting up an L1 sun shade to block wavelengths of sunlight that serve only to heat us up because they get fully absorbed by the atmosphere, but somewhere in the 10-100 trillion range our direct heat emissions become dominant.

[0] ignore the expense, this is a thought experiment

There is but there is a 7/8ths probability that you're not going to like the answer.

https://en.wikipedia.org/wiki/Sustainable_population

I don't know if it is. The impacts could be positive, or at least counteract other negative impacts.

For instance, shading some ocean might counteract the heating of it.

There's other reasons not to put PV in the middle of the oceans, like the storms, and that if you're willing to make suitable transmission lines of that kind of distance, you can also perfectly well connect the Nevada and Namib deserts and put panels in each so the time zones are powering each other after sunset/before sunrise).

You're off by a factor of 1,000.

I think you've got a mistake and a bad assumption here, too. I think the atmosphere attenuates by not to 300W/m^2, but you might also mean that the earth being spherical rather than flat geometrically reduces the solar irradiation to an annual average of 340W/m^2? But then you'd be double-counting the effect of night, and ignoring weather. The currently installed PV systems produce about 10% of their nameplate capacity, so I tend to just use that in my approximations of all the different effects together, so a 20% efficient cell[0] would produce 1kW/m^2 * 10% (capacity factor) * 20% (efficiency) gets 20 W/m^2 in practice.

1% of land with those percentages gets us 29.79 TW[1] which is indeed only about 1 order of magnitude away from current use (and we definitely want to use more energy than we do as most people don't have the luxury of energy abundance found in developed nations), but that already accounts for it being "neither sunny nor midday all the time either".

But also, 1% of Earth's land area is already built up[2], so I think this may be too tight a constraint (after all, PV can be used as a surface covering for buildings and even vehicles).

With sufficient political will (yes yes, I know that's wishful thinking), it's not unreasonable to build a thick enough set of conductors to make even a global power grid have negligible resistance — you'd need a 1m^2 cross section for an equatorial ring of aluminium[3] to have 1Ω resistance, and while this would be a big project today, it's not so large as to be absurd for a decade-long project.

[0] The record for efficiency is 47.6%, though this will only matter if we ever ran out of land: https://en.wikipedia.org/wiki/Solar-cell_efficiency

[1] https://www.wolframalpha.com/input?i=land+area+on+earth+*+1%...

[2] https://ourworldindata.org/land-use

[3] Don't put 20 TW through a single ring. If I did the maths right, the magnetic field at the surface will be in the order of 1 tesla depending on how you choose to distribute current and voltage.

Also, I have no idea how much energy such a thing would absorb from a CME, but I think the big CMEs can be in the range of 5e25 J, and if that's efficiently absorbed it would vaporise even a ring that big.

The downside is cost and scale. Launching enough mass to power the world would also approach pollution limits for injection of water into the stratosphere and mesosphere.

I have imagined it might be useable at small scale in specialized applications first, if the power is beamed by laser. This requires improvement in laser efficiency and cost, but it could be very nice to power long distance aircraft (at altitude above clouds) this way. The challenges are considerable, of course.

Large scale use of power in space would not need transmission of that power, so could be an easier step. Latency insensitive servers there? The heat needs to be dissipated.

Only uranium proudly stands apart.

https://world-nuclear.org/information-library/nuclear-fuel-c...

Well, kind of. That comes from fusion reactions in _other_ suns.

Though one might argue that they're also ultimately stellar-based power in the sense that in the case of geothermal energy both the residual gravitational thermal heat and heavy-element radioactive decay, and in the case of tidal power, the formation of our own proto-solar-system nebula were themselves based on earlier-generation stars.

But then so is nuclear fission based on heavy elements such as uranium, plutonium, or thorium.

Fusion on the other hand makes total sense if you can do it. It's just a really really really hard engineering problem.

Space based power belongs in the same category as solar roadways, energy harvesting speed bumps, solar windows, etc. Clearly if you want to scam investors an energy project is the way to go.

You could also say that space based solar makes total sense "if you can do it, it's just a really hard engineering problem"

For fusion, the equivalent envelope calculation is the cost of a large thermal electricity generation installation, where the heat source is, say, half the cost of fission or even a tenth the cost of fission. The floor of fusion's end cost is that thermodynamic conversion, if the promos of fusion is super cheap heat energy. And when you start calculating the cost of 2GW of turbines plus 2GW of cooling towers, one might conclude that there's only a decade or two left before solar and wind and storage makes thermal electricity generation obsolete. Thermal electricity generation equipment isn't getting any cheaper, but the competition is getting cheaper exponentially.

I would say that fusion is prettt close to a scam as a power source. There may be some cool physics and should be pursued for those reasons. But the idea of fusion as any sort of economical source of energy, when it has all the radioactivity problems of fission, massive scale challenges, and no reason to think that it will ever be anything except massive construction project, is just wishful sci fi thinking. I know it's not a popular opinion here, but "solar roads for sci fi fans" really is the shortest possible description of fusion power I can think of.

The thing is that while they are both really hard engineering problems, space solar (which includes energy transmission via lasers) is also a massive ongoing logistics and coordination problem.

Solar power without wireless transmission makes a lot of sense the closer you get to the sun and in the inner solar system it almost certainly makes more sense than fusion but once you get to the asteroid belt or further out, inverse square law would suggest that it quickly becomes infeasible.

Solar roads are expensive and worked out economocially nowhere, where they were build. Unless you mean roads that have roofs of solar panels - that is a sound concept, because you can have standard cheap panels and replace them at will, but is not what is generally understood with solar roads.

Also about fusion:

"when it has all the radioactivity problems of fission"

Not at all the same. What exactly are you referring to? Fission with heavy isotopes has radioactivity by design - but Fusion not really. There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity.

"massive scale challenges"

And where exactly are they? Once a small reactor runs, what is the problem to build a big one?

"and no reason to think that it will ever be anything except massive construction project"

Because fusion bombs work since 70 years. And fusion in labs are not hard either. We can do fusion - we just cannot do it in a controlled way yet. But recent records with controlling hot plasma indicate that there is real progress. So (allmost) no one is currently betting that we will soon have fusion power, but at some point we will, if we keep on working.

D-T fusion generates 17.6 MeV and 1 neutron, so 17.6 MeV per neutron. So for the same amount of energy produced it makes about 3.5x more neutrons. Worse yet, in D-T fusion, 14.1 MeV of the energy released is in the kinetic energy of the neutron. Compare this to the 8.8 MeV energy of all three neutrons in a fission reaction. Neutron activation and damage are significant engineering concerns for fusion reactors.

[0]https://en.wikipedia.org/wiki/Uranium-235 [1]https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi... https://en.wikipedia.org/wiki/Uranium-235

And then you need to use that neutron to breed new tritium from lithium-6, which is only about 2% of lithium. And since our only feasible source of tritium is a single tritium atom from that single neutron from a D-T fusion reaction that consumes a tritium, we had better be damn efficient at making sure the neutrons go only to breeding new tritium. Which is good, because then they cause less neutron radioactivity of all the rest of the reactor parts. But bad, because it's pretty unrealistic to capture all the neutrons via lithium blanket.

Which means we need ultimately need an outside tritium source, external from the fusion reactions, which means another neutron source, which means, fission reactors!

The whole idea of controlled terrestrial fusion being an inexpensive source of electricity is just absolutely baffling to me. As is the idea that it will somehow be a good extraterrestrial energy source, or power some sort of spaceship. Fission is far more realistic, likely to be compact, etc. Direct conversion of aneutronic fusion is complete sci-fi at the moment, we may as well be talking warp drives.

Ok well, I see the use of tritium only as a step towards normal hydrogen as fuel in fusion reactors. Tritium as fuel is just the "easy" target and yes, it is in limited supply and causes problems. But "in theory" we could have higher temperatures and not be dependant on tritium.

This is exactly my claim for fusion: an uneconomical idea that sounds really cool in theory.

> the basic idea of fusing hydrogen into helium works without radioactivity.

Where does the deuterium come from? There's going to be neutrons bombarding everything, by design, all sorts of radioactivity. It's literally at the core of the entire thing. This point is swept under the rug, but ensuring that the radioactivity is short lived will be a major design requirement of most components for the reactor.

> scale challenges

The scale challenge might be building a small reactor, instead of a large one. We don't know what sort of containment will work, and at what sort of physical scales. Which is why massive construction projects are a problem. We are terrible at massive scale construction, and much better at small scale manufacturing.

> fusion bombs

Not sure why fusion bombs would indicate we could ever do sustained and controlled fusion reactions. These are entirely different processes with almost no connection to each other. This is an error on the order of magnitude of thinking that fusion is not radioactive.

And this is why fusion is solar roads for sci fi fans. People love solar roads because they don't know the particulars about solar panels and roads and why they are a bad fit for each other. People love fusion power because they don't know the specifics about fusion or power and why they are a bad fit for each other.

Sea water, it's surprisingly plentiful. What I'm not convinced by is that the neutron emissions that come from D+D fusion are in any sense "safe" or claims that that the neutrons are in a form that "can't" ever be used for development of fission weapons…

Tritium comes from D+D reactions, which… make a lot of neutrons in the process.

This would be really cool and may make sense with cheap/efficient enough panels.

But of course, you have to solve the 11'8" problem wherever these things get installed.

The proposal and technology were obviously nonviable to anyone with the least technical knowledge from the beginning.

Wikipedia: <https://en.wikipedia.org/wiki/Solar_Roadways>

Phil "Thunderf00t" Mason on YouTube had a particularly brutal critique. Repetitively repetitious and occasionally repeating itself (as is his wont), but on point:

<https://yewtu.be/watch?v=H901KdXgHs4>

(Several follow-ups, including an on-site visit.)

In contrast solar roadways and space based solar don't make sense. Sure you could build them. But it would never make sense to do so because there's always a better way to spend your money. It's never* going to make sense to build a solar panel into a road than just on a field. And it's never going to make sense to build space based solar rather than just Earth based solar + batteries.

*standard HN pedantry disclaimer

This is inherent to the way fusion is even being developed. It's always assumed that the fusion part is going to be more expensive than the equipment to convert thermal energy to electricity, which is why it makes sense to make it as energy dense as possible, and therefore more cost efficient.

But from that very starting point, it's clear that there will always be better ways to generate electricity than fusion: solar and wind combined with batteries.

We have much more reason to be optimistic about the cost case of extraterrestrial solar than the cost case for fusion.

I am 100% for renewable energies and would not wait for fusion - but I am very hesistant to use the word always here. If there is a breakthrough in handling the plasma - and recent developements indicate that this might be possible - it could make fusion very cheap. So when you have a cheap, compact and reliable energy source - most people would favor it over something unstable that takes a lot of space.

That would be my gut feeling, but what simple calculation would you use to disproof it?

Fusion is currently not working at all - but we can transmit power (with big losses over that distance) via laser etc. But since space is expensive, I just see no way to make this work economically.

Edit: If you assume a ground based array has a 20% utilization factor, the space based panels + receivers can only be 5x the cost of the standard solar installation. Which requires ~free space launches, plus probably other technology we do not currently possess.

Both fundamentally are engineering problems now.

Also considering that there has been a lot of investment in space launch vehicles of late the cost to lift things into orbit are lowering.

Technology takes a while to develop and mature with applications for this beyond just beaming power to Earth.

Yes, but the cost of installing solar panels on the ground are astronomically cheaper than anything you can do in space and I do not see the gap closing in the near future, even with breakthroughs. Rather the contrary, we still have lots of cheap unused land (deserts). And solar panels are getting cheaper every day. Having a space based laser/power beam sounds cool - but complicated and expensive.

Additionally there was a lot more useful testing of various related aspects in this beyond just beaming power to Earth per the article.

Near future, I agree it's hard to imagine available resources for this outpacing available resources for fusion.

Though fusion does have some more difficult technical engineering issues than SBS.

By that I mean, as far I know, we have a decent understanding of what materials we would need and how to utilize said material for space based solar.

While with fusion we are close to self sustainings reactions, we still lack understanding on containing the reaction itself for it to be feasible. That is, understanding the magnetic fluctuations around the plasma reaction and having material to housing it for extended periods of time.

Of course with computing simulations becoming increasingly more accurate and powerful, those technical hurdles may not be as stringent as we see them today.

Given that, can it ever hope to work out cheaper than just putting solar panels on the ground?

New technology, especially when it comes out of a lab in an academic setting, is never commercially viable. That often takes years of refining based on lessons from the lab, and from "real" deployments that aren't profitable.

There are a few extant concept designs for space based solar power, but the most mature and most representative example is a design called SPS-ALPHA [1] which came out of a NIAC contract and has been refined by its inventor in the intervening decade. Essentially, the architecture assumes a large structure supporting an array of mirrors in geostationary orbit focused on a sandwich panel. One side of the sandwich panel contains solar cells, the other contains a retrodirective phased array which beams the collected power back to a site transmitting a pilot signal on earth.

This design is interesting and solves some issues that plagued the original designs developed in the 70s. The sandwich panel avoids the need to collect and transfer power between separate solar panels and an antenna, and the retrodirective array avoids the need for precise subwavelenth position knowledge of array elements across the antenna. Using mirrors also reduces the number of solar cells required, however, the achievable concentration ratio isn't very high (3x or so IIRC) due to thermal constraints requiring the sandwich panel to passively self-cool. The mirrors are gimballed, allowing the design to maintain peak power generation regardless of the position of the sun.

The problem is the sheer size of the structure, and it is dictated by physics constraints. Efficient microwave power beaming over a fixed distance imposes an inverse relationship between the size of the transmitting and receiving antenna: halving the area of the space based antenna doubles the area of the earth based antenna, and vice-versa. Both antennas can be shrunk simultaneously by increasing the frequency of transmission, however, atmospheric absorption caps efficient transmission at around 10 GHz. Most designs, including SPS-ALPHA, assume a 2.4 GHz transmission frequency (probably because of the ISM band and the feasibility of building high efficiency saturated amplifiers in this band). For SPS-ALPHA, this leads to an antenna about 1.7 km in diameter in space and 5 km in diameter on tbr ground.

Here's the key part: these dimensions are independent of the amount of power transferred. For efficient transmission, you need to build a spaceborne megastructure regardless of if you're transmitting 1 MW or 1000 MW. You can't really do a small demonstration plant, you pretty much have to go for the whole meal deal. And that structure is huge: the largest thing we've ever built in space by several orders of magnitude, in a very hostile radiation environment (GEO). The costs would be incredible.

Could you build it? Technically, yes. But there is no universe in which it makes economic sense.

[1] https://www.nasa.gov/general/sps-alpha-the-first-practical-s...

Here’s the interesting part: I don’t see the supporters of SBSP disagreeing about antenna sizes. Yet, they somehow still see economic viability in the future. How?

As far as I can tell, in order to get close to commercial viability they’re doing the math with the launch services market of 2040, but comparing it against the cost of ground-based solar in 2023. As long as you’re making the solar panels on earth, it’s always going to make more sense to install them there.

At 9.84 GHz their ~meter scale aperture from 400km LEO orbit to the biggest NASA dish in existence (that could track LEO) would have a path loss of 76 dB. That means something like (being generous) 0.0000000001% of the power would make it. Plenty for carrying information but useless for beaming power.

Space solar power requires coherent emitting apertures on the order of a handful of kilometers to be feasible (from LEO). We can't build kilometer scale anything in LEO. End of story. Once that changes we can think about space solar power.

Kilometer space structures need not be very massive, as the loads they are subject to are quite small. More often achieving a certain stiffness, such that the structure doesn't wobble around too much when being repointed, drives design more than resisting structural loads. The main loading case they consider in [0] is solar radiation pressure.

Making the entire structure coherent gets interesting as it's expected to flex a bit. However, they have demonstrated methods to correct for this flex[1].

One problem they have is that their satellite has to consist of a number of smaller spacecraft connected to each other in a grid because they can't launch the whole structure at the same time. One person involved with the project had concerns about how they would align each spacecraft to within subwavelength accuracy.

[0]https://arxiv.org/ftp/arxiv/papers/2206/2206.08373.pdf

[1]https://www.nature.com/articles/s41528-022-00218-z

I think you are reading something in the headline that isn't there. They ended their first space based mission which was to test various things. It wasn't to beam power back to Earth. The details in the article are actually interesting.

>"Solar power beamed from space at commercial rates, lighting the globe, is still a future prospect. But this critical mission demonstrated that it should be an achievable future," says Caltech President Thomas F. Rosenbaum, the Sonja and William Davidow Presidential Chair and professor of physics.

This mission did nothing to show that was achievable. The hard part isn't the vacuum. It's the distance which requires, because of physics, a large coherent emitting aperture. And the larger your aperture the harder it is to point. If they really wanted to make progress towards this they'd be building a km-scale coherent phased array for power transmission here on earth.

The EISCAT scattering radars in the artic circle are much closer to proving feasibility for space based solar power than this is.

Complaining about a scientist saying "it's a future prosect... that should be an achievable future" is just silly.

Look, I get it. I see a PR and I get angry at the implications. But it's just not worth overinterpreting this PR as justifying future research into beamed power delivery.

My guess would be this has space-based military applications they're not discussing but that's purely speculation based on what we believe the military space community is already exploring.

Whether it makes any sense to continue this line of research is an open question; they seem to be very far from having something that would produce useful power, with a long line of technical challenges facing them.

Could a similar mechanism be used as well for space to earth energy transfer, or is that unrealistic at the orbital speeds of LEO?

Where people post inspiring and interesting things we can learn from, we can aspire to and maybe even improve some things from time to time.

There's no need for all the negativity every time someone tries something new. It serves no purpose.

The only reason Caltech is doing this is that a really rich person with delusions gave them 100 million dollars to research space based solar power. They know as well as anyone with a bit of RF experience that what they're doing isn't really increasing the TRL of space based solar power transmission. It's just humoring the donor for the money.

The best that can be said is that it's the first flex pcb RF system in orbit. That's kind of cool but as a person looking to be inspired and improve things it seems like a significant waste of money. It's not like high power RF transmitters haven't been tested in vacuum in LEO before. There have been many, many radar satellites.

They should've been spending they $100 million attempting to make coherent kilometer scale transmitting apertures on Earth and actually demo something that'd prove the feasibility of space based solar power transmission.

I'm not sure you understand how the world works. You don't get to decide how institutions spend money that is donated to them, and I really don't know why you think you do.