> But again, by how much? What's the comparison between the delta with and
> without shielding, and the delta between the two delta-Vs involved? Can a
> millimeter of aluminum shield out radio waves, and if so, what more would be
> required for the baffle?
>

A millimetere or shielding is not sufficient for radio astronomy. Does anybody here know the right number?
Is SSI actively doing research in this field ? Since SSI proceeds from the original thinking of Gerard O'Neill, and since O'Neill clearly stated in THF that a telescope in free orbit would have many advantages over one on any celestial body, I would anticipate no.
Regards, Mike Combs

>
> Message: 3
> Date: Fri, 11 Apr 2003 10:15:21 -0500
>
> A millimetere or shielding is not sufficient for radio astronomy.
>
> Does anybody here know the right number?
>

> A millimetere or shielding is not sufficient for radio astronomy.

>
> Message: 1
> Date: Mon, 14 Apr 2003 05:16:58 -0000
>

> A millimetere or shielding is not sufficient for radio astronomy.
>
> Does anybody here know the right number?

Thought maybe I should post to offer an explanation as why I've been going on and on about this topic. A special interest of mine is planetary chauvinism. I see it as one of the primary barriers to O'Neill's concepts revolutionizing our vision of space settlement. I often advocate the O'Neill position in maillists such as this, on the New York Times Space Exploration Forum, and on Usenet. I've had some interesting reactions from some people. The bias I think we're all aware of is that it's "easier" to try and live on Mars than in HEO, because after all, Mars is a planet, and it at least meets us half-way, right? I've more than once been told that it will be easier to mine the moon than an asteroid because you obviously need gravity in order to mine. I had one person argue that heavy industry had to take place on the moon rather than in orbit, because the forces of a press continuously pounding away inside an orbiting industrial facility would cause said facility to shift in its orbit, which would have to be corrected. One wonders if he spent much time pulling himself around in a wagon as a child. On a related note, I had a PhD in physics explain to me that the moon was a much better site for a temporary storage facility to house asteroidal material bound for earth than one in HEO, because if one loaded several thousand tons of ore into an orbital station, it would change its center of gravity and cause it to shift into a different orbit. Even if one accepts this dubious assertion, for some reason the amount of fuel needed for orbital correction or station-keeping was seen as a show-stopper, while the amount of fuel needed to land thousands of tons of ore on the lunar surface and then lift if back up again was not any kind of problem. Another person involved in that particular conversation worsened matters by saying he saw in a documentary that NASA had to be careful on some Shuttle flights because astronauts bouncing around inside could change the orbit. Instead of correcting him that such movements might slightly disturb the attitude but not the orbit of the vehicle, the PhD gladly agreed that an astronaut inside a space ship could disturb its orbital path by thrusting off of the front of it. Neither offered to explain why NASA hasn't developed inter-orbital tugs which replace wasteful rocket engines with electric pile-drivers in the nosecone. One guy insisted that heavy construction couldn't take place in zero gravity and then launched into a lengthy thought experiment which essentially boiled down to the fact that one cannot push a string. I agreed that his scenario would not work, but said that only demonstrated he could think of a method which wouldn't work. I proposed a different arrangement (a ring shaped structure with the object being assembled clamped at the center) which I do not recall him responding to. Just as in the case of the PhD who thought the proper place for warehouses was on the moon, I think many people, when making a certain proposal, place them on some celestial body, because before you can build a warehouse/telescope/factory, you first have to find a plot of land to build it on, right? I think this is an unspoken and hence unquestioned assumption from the start. Only after being asked why orbital space would not be a better location does the idea pass through their mind for the first time, and they then begin to offer up difficulties with orbital space which have more to do with their imagination than anything. The arguments are ad-hoc, like those of the PhD who I honestly think preferred arguing about centers of gravity for literally weeks to simply saying, "Gee, I never thought of that 'til now". Regards,
Mike Combs

> I often advocate the O'Neill position in maillists such as this, on the New
> York Times Space Exploration Forum, and on Usenet. I've had some
> interesting reactions from some people.
I can think of two responses similar to the ones you describe. Some years ago on Fidonet, a space group was created in Denmark. I started talking about space colonies with another bloke. The moderator decided that this, being science fiction, didn't belong to a group dedicated to serious discussions of space-related topics. His reasoning was that such a small ecosystem can never be stable. You can't have a lawn as a stable, isolated ecosystem. I asked him why he thought that a space colony must necessarily be as simple as a lawn. It was his impression, and anyway, he had created the group and was the moderator. He was a biologist by training, and I assume that he was right in that a closed ecosystem as un-diverse as a lawn cannot be stable.
Two decades ago, in my teens, I talked to the older brother of a friend of mine about space colonies. He replied that it would never work, because people wouldn't be able to go hunting in a space colony. The hunting instinct is so central to man that if we cannot hunt we aren't happy. A few days later he conceded that he had made up this counterargument because of the novelty of the concept.

His reasoning was that such a small ecosystem can never be stable. You can't have a lawn as a stable, isolated ecosystem. I asked him why he thought that a space colony must necessarily be as simple as a lawn. It was his impression, and anyway, he had created the group and was the moderator. He was a biologist by training, and I assume that he was right in that a closed ecosystem as un-diverse as a lawn cannot be stable. I would say those sealed flasks with the shrimp, algae, and bacteria you can buy now give lie to that assertion. Of course they're not always stable forever (I was just reading Carl Sagan's account of his purchase of one in "Billions and Billions"; his died after a few months). But sometimes they are stable for quite some time. Also, one does not have the opportunity to make the kind of internal adjustments to a sealed desktop curio that one would have inside a space settlement. One of the criticisms leveled at the Biosphere II project was that they tried to reproduce many and varied habitats in their enclosure (basically tried to reproduce the Earth in microcosm), when a simpler, more utilitarian ecology might have functioned better.
Regards, Mike Combs

>
> I would say those sealed flasks with the shrimp, algae, and bacteria
> you can buy now give lie to that assertion. Of course they're not
> always stable forever (I was just reading Carl Sagan's account of his
> purchase of one in "Billions and Billions"; his died after a few
> months). But sometimes they are stable for quite some time. Also,
> one does not have the opportunity to make the kind of internal
> adjustments to a sealed desktop curio that one would have inside a
> space settlement.

> > He was a biologist by training, and I assume that he was right in that a
> > closed ecosystem as un-diverse as a lawn cannot be stable.

> Date: Tue, 15 Apr 2003 07:54:26 -0500
>
> I wonder if you could direct me further. I've only looked at
> http://www.setileague.org/iaaseti/lunar02.pdf
> http://www.setileague.org/iaaseti/lunar02.pdf> so far, but I can't see
> that this answers the question of the necessary shield baffle thickness.

Charles F. Radley wrote: So far nobody has done any such comparison so far as I can see. Note that HEO is not sutiable, we need to be at least 100,000 km or so beyond GEO, which puts us into the regime of the Lagrange orbital locations, that could loosely fall within the defintion of HEO. L5 would be a good choice I think- it's far closer to LEO and the moon.
I went back last night to have another look. Unfortunately, the only specific size O'Neill gives for the baffle is the diameter: 2x the diameter of the dish. He does not give a number for the thickness, and merely describes it as a "thin" aluminum skin stretched over a geodesic frame. What geometry ? It sounds like he is assuming a disk. Radio waves are pesky things. Most people think about them as "line of sight", but that is not the case at all. Sidelobes and harmonics are everywhere. Radio signals diffract around corners and edges, The equation for the angle at the edge is 1.22*wavelength/diameter (angle is in radians i.e. 2 pi to a circle, diameter is size of disk); so you might have a problem below a wavelength of about 10m (about 30 Mhz) for a 100 meter diameter disk.

You've all been talking about orbiting radio telescopes. I've attached a marketing plan that I came up with a couple of years ago to do just that. We formed a company around the idea, but have closed it since because we could not raise the necessary capital. You'll notice by the date that it was formed a month before 9/11 which certainly didn't help matters any. The version I've attached has had all the names and addresses removed so there may be some points where it looks like something is missing or has xxx in it. This is just because I didn't want to put the names and addresses of those involved without asking.
If anyone would like to take it and run with it, please feel free. It would be nice if someone were to get it off the ground (no pun intended) that we could be at least peripherally involved, but again, not absolutely necessary.
DBJ
Ian Woollard wrote: Charles F. Radley wrote: So far nobody has done any such comparison so far as I can see. Note that HEO is not sutiable, we need to be at least 100,000 km or so beyond GEO, which puts us into the regime of the Lagrange orbital locations, that could loosely fall within the defintion of HEO. L5 would be a good choice I think- it's far closer to LEO and the moon. I went back last night to have another look. Unfortunately, the only specific size O'Neill gives for the baffle is the diameter: 2x the diameter of the dish. He does not give a number for the thickness, and merely describes it as a "thin" aluminum skin stretched over a geodesic frame. What geometry ? It sounds like he is assuming a disk. Radio waves are pesky things. Most people think about them as "line of sight", but that is not the case at all. Sidelobes and harmonics are everywhere. Radio signals diffract around corners and edges, The equation for the angle at the edge is 1.22*wavelength/diameter (angle is in radians i.e. 2 pi to a circle, diameter is size of disk); so you might have a problem below a wavelength of about 10m (about 30 Mhz) for a 100 meter diameter disk.
I think that O'Neill assumed double the diameter of the dish, because the dish can't resolve signals smaller than it's own diameter anyway, so you don't care about the low frequencies. getting rid of sidelobes completely, even at large angles, is impossible. The best we can do is employ the inverse square law to reduce the intensity. Up to a point. If you get too far away from the disk, then you hit the diffracted radiation cones. So we want the edge of the shield to be as far away as possible, thousands of kilometres if possible. No. Not correct, see above. A few hundred metres would give some attenuation, but not as much as behind the Moon. No. At a few hundred meters from the edge, reasonably close to the metal of the disk, you have no field for all frequencies below about 30 Mhz.
And incidentally, thickness of this disk need be no more than a millimeter, at the very worst. You just need a good conductor- 1 mm of aluminum gives excellent blocking of radio waves- it just acts as a big mirror. And you wouldn't have very bad interference anyway out at 100,000 km or more anyway- the inverse square law is really well on your side. In fact I need to do some more calculations, it may very well be that baking foil is enough at that kind of distance, that seemed to be what O'Neill was saying. And there is another problem - metals reflect radio waves, so when the receiver tries to focus on a source of interest in the sky, it will have to deal with secondary reflections of the source from the metal shiled behind the receiver. No, the waves never hit the dish at all. Small shields might be a good low cost interim solution for some applications, but they are not the best solution. Terrestrial radio telescope use two methods: 1) terrain shielding, putting the receiver in a valley or crater, e.g. Arecibo. They've got lots and lots of problems you don't have in space. The radio waves are coming in sideways, and diffraction over the edge of the crater is more of a problem, since the angle necessary to reach the disk is low. 2) They construct an "Interference Fence" - I have not seen any detailed descriptions of this approach. Yes, this blocks these radio waves I'm sure. I have no info on the dimensions nor thickness of this type of shield. I hope this discussion illustrates that there are a lot of practical considerations in trying to design radio shields. And in this case, GEO is a terrible place to put a radio telescope for reasons of RFI. You're right about that anyway- you'd tend to see the other satellites around you, and pick up their signals. On top of the existing comm-sat output, SPS would increase the noise environment. Actually, that's not so bad, it's easy to filter away, it's on a well defined wavelength, and narrow bandwidth. Best regards, Charles R. -- -Ian Motto: "You're Not Authorized to Know Our Motto." So, like, how many lives DOES Shroedinger's cat have anyway?

What geometry ? It sounds like he is assuming a disk.

I think the idea was that the robustness of the system to shocks and
upsets would increase with increased numbers and varieties of species. If
there are only humans and three other species in a habitat far from Earth,
and one of the species has an epidemic of disease that decimates its
population, then it might be difficult for the remaining species to ensure
the right gas balances for humans in the habitat. If, on the other hand,
there are hundreds of species in the habitat, then perhaps the loss of one
or two to disease would not be quite so disruptive on the overall gas
balances.

> I think the idea was that the robustness of the system to shocks and
> upsets would increase with increased numbers and varieties of species. If
> there are only humans and three other species in a habitat far from Earth,
> and one of the species has an epidemic of disease that decimates its
> population, then it might be difficult for the remaining species to ensure
> the right gas balances for humans in the habitat. If, on the other hand,
> there are hundreds of species in the habitat, then perhaps the loss of one
> or two to disease would not be quite so disruptive on the overall gas
> balances.