Farside Preservation - Protected Antipode Circle

[redliox]

Relatively recently, a few articles have spoken about legally establishing a radio-silent region for the lunar farside.

The most recent one I'll cite here before elaborating: https://spacenews.com/far-side-the-moons-use-as-a-new-astronomical-site/

In short, the idea centers on keeping essentially the deepest part of the farside radio-silent.  This is challenged by how Europe and China (China has Queqiao in a L2 halo orbit currently) want to develop navigation and communication networks to make lunar visits, presumably for both near and far sides, easier to handle.  Radio astronomy versus human development in a way.

Curious who else has heard of this, has more details, and further thoughts on the matter.

[Overtone]

I work on RF spectrum issues including for radio astronomy. 

A radio quiet zone on the far side of the moon is not just an idea. Legally, it is a binding international treaty. Protections for "Radio astronomy in the shielded zone of the moon" were agreed as part of the international radio regulations back in the early 1970s. You can access the latest version of the treaty language at https://www.itu.int/pub/R-REG-RR-2020. I've reproduced the relevant sections, 22.22-22.25, below.

Some things to note:

- The Shielded Zone of the Moon (SZM) is not just a small area in the deepest part of the farside.  It's a substantial fraction of the farside and a cone of space extending outwards from the lunar surface. It's every place that doesn't have line of sight to a 100,000 km radius disc extending from the center of the earth.

- There is no requirement to keep the SZM radio-silent today, at any frequency.  The treaty agreement is to refrain from radio emissions causing harmful interference to radio astronomy and other passive users. There are no telescopes or users out there to be harmed - yet! - so all emissions are permitted.

- Even when a telescope or scientific instrument is deployed in the SZM, there are important exceptions to the protection it gets. Those exceptions include the "radiolocation service", eg. GPS like satellites orbiting the moon, and the "space operation service", e.g. telemetry from spacecraft - as long as the radiolocation and space operation transmissions are in support of "space research". 

There is no case law about how to interpret those exceptions, so we don't yet know how much protection a farside telescope will get. Still, having the agreed protection for the SZM is an excellent first step and an enabler.

This protection has been in the treaty rules for 50 years. It's pretty exciting to think we might be on the cusp of having sufficiently low-cost launch capability, and sufficiently advanced robotics, to actually construct a farside radio telescope.  It would be an amazing observing platform.  See one nice writeup of its scientific potential, and a video of a prototype crater-climbing robot, at https://www.jpl.nasa.gov/news/lunar-crater-radio-telescope-illuminating-the-cosmic-dark-ages.




Section V − Radio astronomy in the shielded zone of the Moon

22.22   1) In the shielded zone of the Moon[31] emissions causing harmful interference to radio astronomy observations[32] and to other users of passive services shall be prohibited in the entire frequency spectrum except in the following bands:

22.23           a) the frequency bands allocated to the space research service using active sensors;

22.24           b) the frequency bands allocated to the space operation service, the Earth exploration-satellite service using active sensors, and the radiolocation service using stations on spaceborne platforms, which are required for the support of space research, as well as for radiocommunications and space research transmissions within the lunar shielded zone.

22.25    2) In frequency bands in which emissions are not prohibited by Nos. 22.22 to 22.24, radio astronomy observations and passive space research in the shielded zone of the Moon may be protected from harmful interference by agreement between administrations concerned.


[31]  22.22.1 The shielded zone of the Moon comprises the area of the Moon’s surface and an adjacent volume of space which are shielded from emissions originating within a distance of 100 000 km from the centre of the Earth.

[32] 22.22.2 The level of harmful interference is determined by agreement between the administrations concerned, with the guidance of the relevant ITU-R Recommendations.

[Zed_Noir]

Don't think there will be a major radio or optical telescope on the Lunar far-side anytime soon or possibly ever. It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift.

[TheRadicalModerate]

Seems to me that the best way to approach this is to get really, really good at optical communication.  Otherwise, you're going to have RF side-lobes slopping all over the PAC.

Space lasers!

A bit of geometry:

The PAC is supposed to be 1820km in diameter (910km radius).  The Moon's mean radius is 1737km, so the PAC has a solid angle of arcsin(910/1737) = 31.6º.  If you then compare the area of a spherical cap with that angle with the total lunar surface area, you get 2πR²(1-cos θ) / 4πR² = 7.4% of the lunar surface.  Not too terrible.

However, let's look at polar lunar orbits, which are (hopefully) going to be awash in RF.  If we draw a line tangent to the perimeter of the PAC, the maximum orbital radius that won't be over the horizon in any part of the PAC is R/sin 31.6º = 3315km.  That's an altitude of 1578km.  So the entire NRHO that's planned for use for the Gateway will be over the horizon of the PAC.

[Phil Stooke]

Zed:  "It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift."

The reason for choosing the far side is to be sheltered from terrestrial interference.  You don't get that at the Lagrange points, except right at the unstable L2.  Also not sure how the desired big arrays can be deployed in space rather than on the surface.  There are good reasons why people want to do this on the far side.

[TheRadicalModerate]

For a planet-sun L2 to work, the angular width of the planet has to be able to completely eclipse the sun, which of course pours out massive amounts of RF noise.  Attachment #1 is a table of the possibilities.

Mars stands out as a pretty interesting case.  However, there are some caveats:

1) You need a modestly stable lissajous or halo orbit around the L2.  It's possible that a sufficiently stable orbit would not be eclipsed.

2) Solar radio noise is going to diffract off of Mars.  Whether L2 receives such diffraction, and at what intensity, I don't know.

3) Just because Mars-Sun L2 is shielded from the Sun doesn't necessarily mean that it's shielded from Earth.  I'm too lazy to figure out the geometry here, but I'd guess that at times when Earth's and Mars's orbits are close to 90º or 270º out of phase, terrestrial radio noise will reach the L2.

4) And of course we're talking about a giant spacecraft, likely km in diameter that can't be solar powered, since it's intentionally hiding from the sun.

[Zed_Noir]

Zed:  "It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift."

The reason for choosing the far side is to be sheltered from terrestrial interference.  You don't get that at the Lagrange points, except right at the unstable L2.  Also not sure how the desired big arrays can be deployed in space rather than on the surface.  There are good reasons why people want to do this on the far side.

The proposed starshade structures for future large optical space telescopes can be adapted for RF shielding from Terrestrial sources.

We supposedly has operational antenna dishes of about 100 meters in diameter aboard NRO SigInt satellites launched within the limitations of EELV class payload fairings.

[deadman1204]

Don't think there will be a major radio or optical telescope on the Lunar far-side anytime soon or possibly ever. It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift.

Launch costs are not significant for big space telescopes. Its always ignored that the big space telescopes are so expensive because after we plan it, we need to INVENT the technology to do what we designed. The timeline makes the assumption that it will take X years to discover these things. There is no store where you can buy a dozen MIRI instruments for JWST, or a spare 3.2 gigapixel camera for Vera Rubin. A big part of why JWST was so late was because we had to invent a way to make the instruments.

[Paul451]

Zed:  "It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift."
The reason for choosing the far side is to be sheltered from terrestrial interference.  You don't get that at the Lagrange points, except right at the unstable L2.  Also not sure how the desired big arrays can be deployed in space rather than on the surface.  There are good reasons why people want to do this on the far side.

A mm of aluminium will shield the whole RF/microwave spectrum, the remaining 3475km thickness of the moon is redundant. Lighter, simple mesh can be tuned to shield any preferred part of that spectrum. And if you're at ESL2, for example, your Earth-facing RF-shield is also a sun-facing RF-shield, unlike the lunar far-side which will be exposed to solar RF 2wks out of 4. It also means you can put solar arrays on the sun-side of the RF-shield for power, again unlike on the lunar far-side.

The only reason for a lunar radio telescope is if you are exploiting a crater a suitable diameter to use as the "frame" for a non-steering Arecibo-type dish.

[deadman1204]

Zed:  "It is cheaper and easier to deploy big space telescopes at the Lagrange points if there is cheap space lift."
The reason for choosing the far side is to be sheltered from terrestrial interference.  You don't get that at the Lagrange points, except right at the unstable L2.  Also not sure how the desired big arrays can be deployed in space rather than on the surface.  There are good reasons why people want to do this on the far side.

A mm of aluminium will shield the whole RF/microwave spectrum, the remaining 3475km thickness of the moon is redundant. Lighter, simple mesh can be tuned to shield any preferred part of that spectrum. And if you're at ESL2, for example, your Earth-facing RF-shield is also a sun-facing RF-shield, unlike the lunar far-side which will be exposed to solar RF 2wks out of 4. It also means you can put solar arrays on the sun-side of the RF-shield for power, again unlike on the lunar far-side.

The only reason for a lunar radio telescope is if you are exploiting a crater a suitable diameter to use as the "frame" for a non-steering Arecibo-type dish.

I feel that I'm misunderstanding something, why would shielding be useful? A radio telescope is listening for different frequencies of radio waves coming towards it.
The idea of the farside telescope is that all of earth's noise wouldn't be garbling what its detecting.

[tbellman]

A mm of aluminium will shield the whole RF/microwave spectrum, the remaining 3475km thickness of the moon is redundant. Lighter, simple mesh can be tuned to shield any preferred part of that spectrum. And if you're at ESL2, for example, your Earth-facing RF-shield is also a sun-facing RF-shield, unlike the lunar far-side which will be exposed to solar RF 2wks out of 4. It also means you can put solar arrays on the sun-side of the RF-shield for power, again unlike on the lunar far-side.

The only reason for a lunar radio telescope is if you are exploiting a crater a suitable diameter to use as the "frame" for a non-steering Arecibo-type dish.

I feel that I'm misunderstanding something, why would shielding be useful? A radio telescope is listening for different frequencies of radio waves coming towards it.
The idea of the farside telescope is that all of earth's noise wouldn't be garbling what its detecting.

Paul451 is not suggesting putting up the 1mm aluminium shield in addition to the Moon, but instead of the Moon.  I.e, don't build the radio telescope on the far side of the Moon, but as a free-flyer (e.g. the Sun-Earth Lagrange point 2, SEL2), and build a shield of your own, instead of relying on the Moon as a radio shield.

Yes, you loose the free shield you get by the Moon, but on the other hand:
• you save a couple km/s Δv as you don't need to actually land on the Moon (and more if you use a reusable ship that wants to return to the Earth for the next delivery),
• you can point the telescope more freely,
• you don't need to deal with the lunar night and the accompanying lack of solar power,
• on the far side of the Moon, the telescope will be subject to the radio radiation from the sun two weeks per month; the RF shield of a free-flyer will shield against that as well.

So a radio telescope on the lunar far-side might not be as advantegous as it might appear at first glance, as there are disadvantages to the location as well, not just advantages.