Maybe Avoid the "Nuke" Idiom

This week I saw the news that Caltech had completed a proof-of-concept mission demonstrating power transfer from space using microwaves. I was instantly transported to my childhood playing SimCity 2000, which offered a microwave solar power plant for more advanced cities. I hadn't realized such a plan was actually feasible, but it's been considered since the 90s. The main obstacle has been cost, since the project requires many solar panels, all delivered to space. With the price of solar panels going down though, it's come back into the realm of possibility. However, I was curious about the possible risks of such a system, since SimCity (a credible source if ever there was one) suggested the possibility of accidental fires set by the system.

Before getting into that though, let's discuss how these systems work. To get the maximum amount of power from a solar cell, we want it to be constantly illuminated, but for a panel on Earth this isn't possible, since it spends roughly half its time in night. To get around this, we could put the panel is space, where it can always face the Sun, but now we have a new problem: How do we get the power it produces back to Earth? The simplest solution is to send back electromagnetic waves, but why choose microwaves? To answer that, we need to look at the absorption spectrum of the Earth's atmosphere (click to enlarge):

Wikipedia

We're interested in the regions with low absorption, since we want our beam to go through the atmosphere to a receiver on the surface. Microwaves have a wavelength around 12 cm, which falls neatly in the dip on the right side of the plot.

Since we want to keep the beam fixed on a single receiving station, we want the satellite to be in geostationary orbit, which requires a distance of 36,000 km. This page gives the size of the receiving antenna as about 3 km in diameter, which corresponds to an angle of about 5 millidegrees from the spacecraft. That page also gives the total power transfer as about 1 GW. Given how tiny that angle is, it's easy to imagine the beam being knocked off center, so how much damage could it do?

With the numbers above, the spot would have a power density of 141 W/m^2. For thermal radiation, this is well below the level that can burn you. Of course, these are microwaves, commonly used for cooking, so how does it compare to what you have in your home? A typical microwave oven has an area of around 20" x 24", and puts out around 1000 W, which comes to 3200 W/m^2, almost 23 times what our beam is sending!

So you'd be able to take a nap on the receiving dish without getting cooked, but your WiFi uses the same 2.4 GHz frequency that this system does, so would you be able to read this blog? I found a paper discussing the power density from WiFi base stations as a function of distance:

Figure 9

Note the scale is in milliwatts per square meter, meaning this is several orders of magnitude below the beam's power. Even this weak microwave signal can knock out your wifi!

It seems my childhood fear (or fascination) of fiery beams from the heavens was unfounded, but if they do build one of these, it has the potential to knock out your internet nearby...