Radio observatories on the Moon

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

>> Does anybody here know the right number?

> Yes, the people who wrote the references I provided.
>
> The references I provided discuss the issues in some detail.
>
> It is not a simple matter.
>
> Geostationary communications satellites, for example, extend the
> interference zone considerably several radii beyond the Earth's
> disk.

Yes. And future expansion will extend it to well beyond the orbit of
the Moon. Unless one is willing to assume a perpetual radio silence
enforced beyond the Moon, the problem of "domestic" radio sources
will probably catch up to Lunar farside by the time a radio
observatory can be built.

> The more shielding the better.

Well, I guess I can't much argue with that, except to suggest that
three-million-plus metres of solid rock may be a case of overkill.

> The size of the shield also makes a difference, as does the
> frequencies being shielded. For example, cell phone signals can
> penetrate through several feet of concrete, or a few inches of
> metal. Acutally the signal penetrates a lot further, but the
> usuable S/N ratio cause loss of the data channel, the RF radiation
> is still there.

Which means that as soon as somebody makes a cell call from beyond
the Moon, all those millions of metres of rock might as well not be
there (sounds like a fifties B-movie: Beyond the Moon!).

> In order to reduce the interference to below the senstivity level
> of radio telescopes would need tens of feet of concrete, or a few
> feet of metal (my SWAG).

OK, then. We make it two metres of iron, covered over with a two-
metres coating of silicate. That should block most anything.

> Metal shields would have other problems, at certain frequencies
> they could act as antennas and resonate the signal.

Why I added all that silicate.

> Cell phone frequencies are of great interest to radio astronomers,
> and they are now completely unusable on Earth.

And just as useless on the Lunar farside in a few decades.

> Spread spectrum systems are making things worse. Entire frequency
> bands are wiped out..

Let's face it, there will be no banning of radio in space. So you
need shielding, and not just from Earth.

> The Moon is as big a shield as we can get.

Mars is bigger. Titan is bigger. And besides, you don't need three
million-plus metres.

> But even on the far side of the Moon, interference from satellites,
> present and future, are starting to reduce the area which will be
> in radio shadow.

Yes. This is my point exactly.

>>> 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.

> I read THF, and I did not see any serious analysis in there of radio
> interference issues.

None come immediately to my mind, either.

> Has SSI studied space based telescopes and done a comparison to
> lunar telescopes ?

Now that I do not know. Going to have to check that out. Unless
somebody here knows the answer already.

> The best place for radio astronomy is in the radio shadow of the
> Moon.

Well...

> Shielding does not cut it,

Sure it does. That is what all those millions of metres of rock are
all about. Shielding.

> or would be so massive that the cost of launching, even from the
> Moon, would be significant.

Not as massive as a habitat.

> A 1 km asteroid might be sufficient.

*Might*? I should think that one thousand metres would be PLENTY!

> Such an asteroid would have to be put at a Langrange point, and
> stationkeeping propellant requirements would be considerable.

Why not out in the Kuiper Belt? Or in a polar orbit about Mars?

> Landing a telescope on the Moon would a good deal easier than
> trying to manouver an asteroid.

You take the asteroid apart (or launch sufficient Lunar regolith) and
use it to build your shielding disk. And if it is a *really big*
telescope, then no, landing it on the Moon is not so easy.

> Locations:
>
> There is only one location which is ALWAYS in the radio shadow, and
> that is a zone at the center of the far side of the Moon.

For now.

> There is no orbital location in space which is always in the Moon's
> radio shadow. None of the Langrange points, for example.

So you make your own radio shadow, with that big disk.

> There has been some discussions of constellations of radio
> telescopes in lunar orbit, which each spend a portion of their
> orbit in the radio shadow.

That could be a possibility, but then we are back to 'why not Mars or
Titan?' There is no reason why the shield has to be born (the Moon,
an asteroid, Mars, etc.), rather than made (that big disk).

> This is a trade off between having multiple satellites in orbit,
> versus a single observatory on the luanr surface.

Or a single observatory in free space, with a big honkin' shield.

> This is a fruitful area of research - radio astronomers are
> starting to become more interested in the lunar region, because the
> radio environment on Earth is continuing to deteriorate.

The problem is that they want to find a "pristine" place, and places
tend not to remain pristine.

> I recommend you review the references, especially the Maccone paper.

Well, that's probably a good idea. So here I go. Wish me luck. ^_^

> Best regards,
>
> Charles F. Radley

Xenophile (who has given this some thought, but who has not done much
in the way of looking up refs... and that would help my thought
considerably)