[time-nuts] Re: constructing a moon base

[Lux, Jim]

On 9/29/21 10:24 AM, Bob kb8tq wrote:
> Hi
>
>
> My thinking was that radio astronomy / VLBI sort of stuff is something
> folks get interested in. There are some advantages to a lunar location
> (more so one on the far side). Enough interest and maybe there’s funding.
> Clock specs could / might be similar to an earth based station.
>
> Yes, there’s more than a little bit of handwaving there. A lot depends on just
> what is being done and how it is be done. Maybe we’ll have TCXO’s that
> drive VLBI in 40 years time :) ….( I sorta doubt that will happen. )
>
> Bob


Far Side radio observatories, particularly for frequencies blocked or 
distorted by Earth's ionosphere are of great interest for two things:

Cosmology wise - looking at very old deeply red shifted (z=50) Hydrogen 
emissions (1420 MHz) that have been shifted down to 18 MHz - this is 
from before there were stars, much less galaxies. One of the things 
you'd be interested in is how smooth the background radiation is over 
frequency. You're probably familiar with COBE that showed the lumpy 3K 
remnants of the big bang - what astronomers would like to know is that 
if you remove the spatial variability, what does the background look like.

Exoplanets - Earth (and Jupiter, Saturn, etc.) all have aurora, 
resulting from the interaction of solar wind with the strong magnetic 
fields. These AKR (Auroral Kilometric Radiation) are down around 1 MHz 
and are not radiated isotropically (they tend to follow the field 
lines).  The existence of a magnetic field is considered to be "life 
friendly" in that it reduces the charged particle flux on the surface, 
so there is interest in detecting such emissions from nearby "life 
possible" planets, of which dozens have been identified (right distance 
from parent star, right mass, etc.)Â  - say, within 10 parsecs of Earth.

For both of these applications, you need a large array (maybe 10s of 
km), but not enormous (1000s of km).  You don't need the angular 
resolution of a larger extent.  A fairly conventional "send all the 
signals to a common point and digitize them with a common clock" works 
fine at these frequencies.  A terrestrial equivalent is the OVRO-LWA in 
California, or the MWA in Australia, as well as various starts at the 
Square Kilometer Array (SKA) and the big LOFAR array. All of those are 
higher frequency (above 30 MHz or so) because the "seeing" down low is 
so bad from the ionosphere.


There have been proposals (and actual experiments) with VLBI between 
Orbiter (at the Moon, I think) and Earth, but those were limited in 
scale (essentially demonstrations that it could be done).  It's not 
clear that we *need* (or more properly, that we should spend money on) 
the resolution of higher frequency Space/Earth VLBI (that is, existing 
VLBI on Earth does well enough).  Of course, I guarantee that there are 
at least half a dozen astronomers who will say that 300,000km baselines 
are positively essential, because if they can't do their science, then 
civilization is lost.  (perhaps a bit hyperbolic).

What everyone is eagerly awaiting is the new Astrophysics Decadal Study 
from the National Academies, which describes the "big questions" and 
"what's needed", and gives some sort of ranking of importance to the 
science community.  There is some hope that when this comes out (mid 
October is the rumor) that there will be a recommendation for a "probe 
class" (= ~$1B) mission to build a radio telescope on the far side of 
the Moon.

With that lead in, I give you FARSIDE

https://www.colorado.edu/project/lunar-farside/

The final report (which went to the National Academies): 
https://www.colorado.edu/project/lunar-farside/sites/default/files/attached-files/farside_finalrpt-2019-nov8.pdf

https://www.space.com/farside-moon-radio-astronomy-mission-concept.html 
in the "popular press" - it's also been in a variety of other magazines 
(Popular Science, etc.)



and a nice lecture by Gregg Hallinan, one of the PIs

https://kiss.caltech.edu/lectures/2020_Hallinan.html

https://www.youtube.com/watch?v=Zr0Pq7bFD2Q