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.) - 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
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