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Comment by loudmax | Hacker News Reader

Youtube channel Launch Pad Astronomy discussed some of the challenges for a lunar telescope: https://www.youtube.com/watch?v=QKJY7gH2n9I

One of the problems is moon dust. There's a lot of it. If you build a big parabolic dish on the moon, you'll need some way to either remove dust or prevent it from accumulating in the first place.

Another issue is the change in temperature. Without an atmosphere stabilize the temperature, day times become extremely hot and night times are extremely cold. This means that you need to engineer something that can withstand very significant swings in temperature over the course of the lunar day.

Challenges like these highlight the benefit of placing James Webb space telescope at Lagrange Point L2. The temperature remains constant and there's far less interplanetary dust than moon dust.

That isn't to say that we shouldn't build a lunar telescope, but we should have a clear understanding of the difficulties.

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One of the problems is moon dust.

Well, yes, but at radio wavelengths, it would take an awful lot of dust to significantly impact the performance of the telescope. My gut says centuries worth if not millennia. And the article is proposing a wire mesh primary reflector, so dust will mostly just fall through.

Another issue is the change in temperature.

Similar answer to the above. The wire mesh reflector would expand and contract. I suspect the main impact would be to change the location of the focal point. We can handle that by moving the receiver up and down similar to what we do with telescopes on Earth.

I think the wire mesh expansion and contraction is not such a huge issue for a number of reasons.

First of all, you will probably want to make observations when not illuminated by the Sun, so that the receiver is shielded by the mass of the Moon from the Sun interference.

Second, on the Moon there is no weather. You basically have only two conditions, it either is illuminated by the full sun or it is not (with some time while it is partly illuminated, but most of the time is just basically two temperatures). During night the temperature will be rather predictable for the entire length of the night.

So I think the simplest solution would be to just use the telescope for about 50% of the time when its geometry is stable and is shielded from the Sun.

It is not such a huge issue, the basic fact of this kind of telescope is that you can't steer it so you still rely on orbital mechanics of the body on which you place to move the part of the sky which it listens to.

In this case even restricting to 50% of the time does not change much as it still requires 1 whole year to make observation of entire part of the sky it can observe.

>it would take an awful lot of dust to significantly impact the performance of the telescope.

The dust also picks up charge and levitates, but I suppose that allows you to sweep it up using a charged broom.

> Another issue is the change in temperature.

The changes in temperature are exaggerated. If you cover the telescope during day with some type of cover (like space blanket [1]), then everything that's not in direct light will stay at the same temperature as during the lunar night. For the simple reason that there's no air on the moon to transfer heat, and the regolith is a very good insulator.

So you could end up using the telescope 14 days out of 28 and be done with it.

But what's more, you can set up the space blanket to only protect the part of the telescope that's in direct sunlight. For the rest of the telescope it will appear like it's perfect night, because there's not atmosphere to disperse the rays of the sun. So you can end up using the telescope during the lunar day as well, although not at full exposure.

[1] https://en.wikipedia.org/wiki/Space_blanket

> 14 days out of 28

Out of 29 and a half actually.

I'm curious where the misconception that a lunar month is 28 days comes from. I believed it too but I can't find any good reason for doing so.

You are right, and I was just lazy. 28 is an even number, and it felt nicer to say 14 days out of 28, rather than 14.75 out of 29.5.

Separately though, the misconception comes from the fact that we have 4 phases of the Moon (new, first quarter, full, last quarter), and it's easy to conceptualize that the interval between two is 7 days, or one week, rather than 7.38264725.

Sidereal month (relative to fixed stars) is 27.32 days. Synodic month relative to Earth is 29.53. There are other months that correct sidereal for motion around the sun, eccentricity of the lunar orbit, and tilt of lunar orbit, but the difference is tiny.

https://en.wikipedia.org/wiki/Lunar_month

> One of the problems is moon dust. There's a lot of it. If you build a big parabolic dish on the moon, you'll need some way to either remove dust or prevent it from accumulating in the first place.

Moon doesn't have atmosphere, so once you've built the dish and cleaned it, it should stay clean unless a meteorite strikes nearby.

Solar winds can also kick up moon dust. It would be a long slow process though and I think the reflector would still work under a thin layer of dust. By the time it’s a problem we’ll hopefully be there in person to either dust it off or build a bigger better one.

One my favorite quotes from one of my astronomy professors is: "Everything has an atmosphere, no matter how tenuous."

This is a reminder that he used to make sure we checked our assumptions and always considered constantly changing conditions.

Minor correction but the moon does have an atmosphere although it is largely negligible when compared to earth's.

>"At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume."

>"We think that there are several sources for gases in the moon's atmosphere. These include high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior."

Source: https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphe...

Also interesting, there is a thin film of electrostatically charged dust (regolith) that is visible across the moon's horizon during lunar sunrises and sunsets. The Apollo 17 crew drew a sketch to depict this phenomenon https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#/media/...

It's not a real dish (as obvious in OP), it's a metal spider web. That will accumulate much less dust!

How can there be moon dust if there’s no atmospere? Is it from asteroid collisions kicking up dust?

It’s mostly regolith that’s been pulverised by meteorite impacts. It’s quite abrasive as the particles have jagged sharp edges.

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