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Space based solar power systems that beam power back to terrestrial receivers have been discussed for decades. However, I've always had a nagging concern that such a system, once built, could be quickly repurposed as a weapon, and so no country would permit another country to launch one.

I know the power density on the ground wouldn't be high enough for a dramatic, fry-a-city-in-ten-seconds kind of weapon. In fact, Wikipedia cites expected levels of $23mw/cm^2$, which is only about twice the OSHA workplace limits. However, if you follow Wikipedia's reference to its source, on page 14 you'll find that this limit is designed to "avoid potential microwave interference with the D and F layers of the atmosphere."

For those who don't care about the ionosphere (which probably includes those hoping to create a weapon) it would likely be easy to design a transmission antenna that would usually limit itself to this power density, but could easily be focused to increase it by perhaps an order of magnitude. $250mw/cm^2$ would be about twice the solar constant, and would impart significant amounts of heat to anyone in the beam. Given that the beam could cover a large city, and add in infrastructure damage, and that's a pretty significant attack.

Is this a legitimate concern? Are designers of potential space solar power systems coming up with ways to eliminate the ability to repurpose them into weapons?

Edit: Most proposals put the transmitting satellite in geosynchronous orbit, so that it will always be above its dedicated receiving station. This puts the satellite out of the range of current anti-satellite weapons, and could even make it difficult to destroy in the future; it would be so large, and could be built from redundant sections, so that punching holes through it wouldn't significantly harm it.

I also understand the comparison to nuclear weapons. However, the use of nuclear weapons is such a bright line that the attacker would likely receive worldwide condemnation. Nor can you use a "little bit" of nuclear weapons, which also keeps that line bright. However, a divertable microwave beam might be easier to use briefly, and even deniably ("Ooops!"). Imagine a country briefly toasting one of its surrounding countries' capitals, and then immediately apologizing. The effect could be drastic, and could seriously destabilize the attacked country.

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    $\begingroup$ I would expect that any nation able to launch a big microwave beamer satellite would also have the ability to launch nuclear warheads, which would be able to deliver destructive energy to a target city at a substantially higher rate than 2 solar constants. $\endgroup$ – Russell Borogove Oct 5 '15 at 3:40
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    $\begingroup$ Are designers of potential space solar power systems coming up with ways to eliminate the ability to repurpose them into weapons? Not to my knowledge. But most countries' insurance policy against such weapons is that it would be trivially easy to destroy them either from the ground or on orbit, be it using same method (funny enough, beam sources like, say, phased array MW, don't really take well same level of input as they are capable of outputting) or kinetic projectiles. And then there's OST, not so much as a deterrent as legal grounds for subsequent destruction of it. $\endgroup$ – TildalWave Oct 5 '15 at 5:42
  • $\begingroup$ Most proposals for weapons in space (note the difference from militarization of space) put them in LEO. This makes the very vulnerable to ASATs. In fact, the MIRACL laser can damage satellites and that was done sometime around 1997 (?). To prevent this solar weapon's destruction you'd likely need to put them past GEO (some 35,000-40,000km). Yet that would greatly affect the efficiency of the suggested weapon. Is it possible? Yes, but likely to do a lot of damage over a period of time? Probably not. Repurposing the satellites would require autonomous robotic spacecraft that don't exist. $\endgroup$ – spacer Oct 5 '15 at 11:06
  • $\begingroup$ To be honest, I'd be more worried about someone purposefully crashing a satellite into a major city. That reminds me about the Rosat satellite stories. $\endgroup$ – spacer Oct 5 '15 at 11:10
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    $\begingroup$ As a side note, SimCity 2000 has a space based solar power system titled the "Microwave Power Plant" which carries the risk of its space based transceiver satellite misfiring, destroying large swaths of buildings in a fiery, death-ray-esque fashion. I'm quite glad to know my childhood fears have been resolved by the answers to this question. $\endgroup$ – Paul Omans Aug 15 '16 at 17:37
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There would be very little you could do to reconfigure a space based power bird to make it more lethal unless you had designed it as a weapon in the first place, and it's unlikely anyone would go down that route as there's much better technology to invest in if you want to destroy things. The best you could do would be to point the power output at a target.

It is possible that space based power systems might be used as disruptive weapons against civilian targets by a desperate nation. Space based power systems require ground based receivers which would be easy targets for conventional weapons, providing the enemy could reach them with their weapons. If the receivers were damaged or destroyed the space power satellites would have nowhere to send the power, and then some big brain might decide if they aren't supplying power to the war economy they could be used to fry a few enemy cities. The power beams could be re-positioned to point at soft enemy targets, providing that they are within reach of the satellite. This would not do that much damage, but it could be disruptive to electrical systems and impact the ability of an enemy to wage war.

You'd have to be in pretty dire straits to try this though as it would not be very effective and as soon as they are used this way the satellites would be huge, expensive targets. Once a nation uses space-based weapons all their other space infrastructure would likely be fair game as well. It would be very risky and foolish, but as history teaches the human race has an almost limitless capacity for destructive decisions.

In other words this is pretty unlikely and best saved for Thunderbirds Are Go! episodes.

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  • $\begingroup$ Do you think it could be used as an anti-satellite or anti-ICBM weapon? Turning it to track a satellite or ICBM might be a challenge. Would a tin foil hat be useful for a targeted pedestrian? :-D $\endgroup$ – LocalFluff Oct 5 '15 at 12:02
  • $\begingroup$ I think a tin-foil umbrella would provide better anti-power satellite coverage, a tin-foil hat would not protect the arms @LocalFluff. $\endgroup$ – GdD Oct 5 '15 at 12:14
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John Mankins is one of the top SSP researchers. Some power beaming architectures call for diffuse lasers and color-tuned solar cells, while others stick with microwaves. In an interview he reported some of the findings of a recent (2013 or 2014) European conference on Space Solar Power. One finding was that recent modeling suggests that beaming the power back with a relatively diffuse laser could still cause dangerous levels of heating if it was mis-targeted or intentionally targeted at cities. It seemed to be the death knell of the light-based power beaming architectures. Meanwhile, microwaves are not only diffuse but most designs build the frequency into the hardware. If it isn't built as a weapon then it can't be re-tuned remotely to increase the frequency and weaponize it. So in short, if you are using a microwave beamed power system then you probably can't repurpose it as a weapon unless you build that capability in before it launches, quite possibly requiring a lot of extra hardware and weight.

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