# Could a space based solar power system be used as a weapon?

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.

• 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. – Russell Borogove Oct 5 '15 at 3:40
• 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. – TildalWave Oct 5 '15 at 5:42
• 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. – spacer Oct 5 '15 at 11:06
• 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. – spacer Oct 5 '15 at 11:10
• 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. – Paul Omans Aug 15 '16 at 17:37