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Interviews: Ask Engineer and L5 Society Cofounder Keith Henson a Question
Keith Henson is an electrical engineer and writer on space engineering, space law, cryonics, and evolutionary psychology. He co-founded the L5 society in 1975, which sought to promote space colonization. In addition to being an outspoken critic and target of the Church of Scientology, Keith has recently been working on the design of an orbiting power satellite (video here). The proposed satellite would collect solar energy, send it to Earth via microwaves, and Henson has a plan on how to launch it cheaply. Keith has agreed to give us some of his time and answer any questions you might have. As usual, ask as many as you'd like, but please, one question per post.
I'm not Keith, but from what I've read about such systems, this isn't a concern: when they hit the Earth's surface, the microwave beam is very large so it wouldn't have a fatal effect on anyone or anything that happened to cross through it, though it might warm you up a bit. Also, the frequency is pretty important; microwave ovens work because they're tuned to the resonant frequency of water, which most of our food is largely composed of, so it excites the water molecules and makes them heat up. If the beam is tuned to something else, it might have very little effect on lifeforms crossing through it (which is probably the intention, since tuning it for water would make it attenuate too much on cloudy days).
... microwave ovens work because they're tuned to the resonant frequency of water ...
Bzzt. Microwave ovens use 2.4GHz because there's an ISM band there. There is no resonance at 2.4 GHz for water. If there was your food would explode in the oven.
Tiller's Rule: Never use a word in written form that you've only heard and never read. You will end up looking foolish.
I was born three years earlier than this project and that's the first time I ever hear about it.
The L5 Society is well known among space colonization kooks (SCKs). As a SCK, I first learned about the L5 Society in the 1980s. They do good work, advocating practical projects. Putting O'Neill Cylinders at the Lagrangian points is a much more sensible goal than trying to put a human colony on Mars.
Keith can give you the accurate and current story on it.
But as I recall it (from the proposals in the early days of the L5 society and some experience with microwave and synthetic aperture techniques):
The powersat has many transmitters. Each single transmitter, even with a very directional antenna, puts out a very wide beam. (It might cover the whole face of the planet (and beyond)). One transmitter would look more like a radar transmitter at least (26,199 - 3,959) = 22,240 miles away - about 9 times the distance from New York to Los Angeles.
The transmitter/antenna devices distributed across the platform are phase-synchronized by a pilot signal transmitted from the ground rectenna site. They compute the complex conjugate of the (equivelnt at their frequency) signal they receive and transmit that. This orchestrates them so they form a beam that retraces the path through space, correcting for flexing of the powersat structure, the turbulence of the atmosphere, clouds, aircraft - metal, wood, or feathered - rainstorms, ionospheric distortions, etc. and focuses on the pilot transmitting antenna(s), like a hologram.
A number of factors defocus the beam somewhat, so you get a spot that covers the rectenna efficiently rather than tightly focussed on the pilot transmitting antenna(s), or leaking all over the surrounding county. The main one is the diffraction limit at the frequency involved, given the size of the transmitting array and distance from it: The bigger the transmitting array, the more tightly focussed the spot on the rectenna site can be. You don't want it TOO tight, to keep the power density reasonable (like a few times sunlight's power density). But the battle is to get it tight enough so your rectenna farm isn't city-sized, not to keep it from being too tight.
If pilot lock is lost by a single transmitter, it no longer stays locked to the rest of them - its signal spreads out like that of any lone transmitter. It stops contributing to the power in the rectenna and "shines" on the whole face of the planet - becoming microwave background noise. If pilot lock fails completely, all the transmitters VERY quickly go out of sync with each other (and can be deliberately given individual drift rates to insure this happens quickly). They ALL shine, incoherently, all over the world. All the power spreads out over the whole face of the planet and beyond. That part of the sky gets loud in microwaves, so you don't want to park a commsat there. But it's not going to toast Tokyo, or cause malfunctions in old-style pacemakers in Cleveland.
Of course you can design the transmitters so any that doesn't have pilot lock just shut down, if the solar array can accept the loss of the load. You can also modulate the pilot with a cryptographic identifier, to keep people from stealing power - or warming Central Park slightly - by setting up a second pilot transmitter at another site and making the "hologram" deliver a second spot.
Meanwhile you're not going to have roast birds falling out of the sky (like you do with the point-focus solar power systems). Microwave ovens cook because they use a frequency that is strongly absorbed by water. Milimeter-wave power systems use a frequency that is chosen to NOT be strongly absorbed by water. This lets it go THROUGH clouds, and birds, rather than being absorbed and heating them. They're not PERFECTLY transparent to it. But at the frequencies and power levels involved at the best focus you can get it's more like having an incandescent lamp in the room than like being in a microwave oven.
Meanwhile the rectenna is spidery enough that most of the sunlight passes through it, and efficient enough that most of the power does not. You can put it up on poles and graze cattle under it, without cooking the cows or the grass.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
It was a major letdown in my young life. Instead of talking about things like orbital mechanics and how to make the economics work, the meeting consisted of kooks and was centered around making fun of people who didn't know the space shuttle couldn't land on the moon and a short speech from a woman who was writing a sci-fi book about sex in space. The best term that I've heard about those people is space nutters. I went because I wanted to learn and do something, but only learned that space is a cargo cult to many.
Why not L1 or L2?
L1 and L2 are not stable. A slight perturbation can push you out of orbit, and energy must be continuously expended to stay in position. L4 and L5 are stable. After a slight perturbation, a space station would settle back into the original orbit.
They both have a lower delta-v budget than L5.
Sure, but a bit extra delta-v is no big deal once you are "in space". The hard part is getting out of the atmosphere and into orbit. You need expensive chemical rockets for that. But once in orbit, you can take your time, and use cheap ion thrusters to move to your final position.
I've been working on a textbook about Space Systems Engineering: http://en.wikibooks.org/wiki/S...
In section 4.9 I do the numbers for orbital mining: https://en.wikibooks.org/wiki/...
The first product of asteroid mining is likely to be rocket fuel. Some asteroids (the carbonaceous type) contain up to 20% water and carbon compounds. This can be processed to Oxygen + Hydrocarbons, which is a common high-thrust rocket fuel. The lifetime mass return ratio of an asteroid tug is ~350:1, and if 20% is usable fuel, then you gain 70:1 just on that one product. Extracting water and carbon compounds only requires kitchen oven level heat, which is easy to do by concentrating sunlight.
There are lots of other products we can potentially extract from asteroids, but that's the easiest and most useful, since most anything you do in space needs some fuel to get where you want to go.
Asteroids did have geological processes, just different ones. The "metallic" type come from protoplanets which melted internally from radioactive decay early in their history. The iron and iron-loving elements sank to the core because they are the densest. Later collisions broke up the protoplanets, exposing their cores. The metallics are a high percentage of iron, nickel, cobalt, and a few other elements. The "stony-irons" come from regions that didn't fully separate the core and rocky layers. They range from low to high percentage iron, with the remainder being rock.
The other process that happened is thermal. Depending how far from the Sun a given asteroid first formed, and later orbit history, certain compounds condensed or not, and then could be baked. Probably the most significant difference is due to the "frost line", the distance at which water ice can remain solid in a vacuum. It happens to be right in the middle of the Asteroid Belt, where Ceres is. Objects beyond that distance tend to have a lot of water. Anything closer tends to have little water, though it can contain "hydrated minerals", where the water is chemically bound.
We actually know quite a bit about the composition of asteroids. Nature delivers samples to Earth in the form of meteorites. We can compare the spectra of meteorites to those from asteroids we get through telescopes, and infer what they are made of. We have flown past or orbited several asteroids, most notably the Dawn mission to Vesta and now Ceres, two of the largest asteroids. Spacecraft carry a larger variety of instruments and can do a better job of telling what the asteroids are made of.
As far as materials processing, we can design machines based on meteorite samples, or simulated samples, since meteorites are rare and valuable. If the Asteroid Redirect Mission that NASA wants to do happens, we would have a sizable boulder to experiment with. After taking science samples, they could try various processing methods on an actual piece of asteroid rock, in zero-g. I don't think we can design serious production units without a a few rounds of trying it on a small scale. For that, we would need at least a small asteroid tug that fetches back chunks from different asteroid types, so we have enough raw materials to experiment on. Most known asteroids are too big to move whole. A 30 meter one is anywhere from 18,000 to 90,000 tons. So for early space mining, we are talking about scraping loose stuff off their surfaces, or grabbing boulders.
my email is the same as my user name here, but lowercase, and add (at)gmail. Feel free to contact me if you want more information. I can point you at sources I have, or send you stuff directly.