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Comment by asmithmd1 | Hacker News Reader

The original sin is the lack of will to make the Webb nuclear powered. Since it is solar powered, it has to be in the sun. Since it has to be in the sun, it needs a huge shield. Since it is solar powered, it must be passively cooled to save power. Since it is passively cooled, it needs the science experiment - 5 layer shield deployed with 50 different pulleys, cables, telescoping poles and who knows what else.

The power limit also requires that science data be downloaded with an experimental 26Ghz radio that only the three 100' diameter Deep Space Network dishes can receive. The projected science data download will require using the DSN for 8 hours a day, everyday for 5-10 years.

They should have started over using nuclear power and then they would have had a chance.

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The need to be in the sun isn't about power. Wherever JWST is, it has to be exposed to either the sun or to a planetary body that's shielding it.

At the low temperatures involved, Earth's night side puts out enough infrared radiation from its own heat to cause problems. JWST is all the way out at Sun-Earth L2 not for power reasons, but so that it will never be in a position where it needs to keep off heat from two different directions.

Nuclear does not help with this. In fact, by having a nearby penetrating radiation source and a high-temperature core mechanically connected to the spacecraft, it may be worse from a thermal perspective.

JWST is a strange spacecraft with very strange design constraints. Your intuitions about planetary/Earth-science or higher-frequency astronomy will not serve you well here.

I am not sure how you know what my intuitions are. My insights come from reading the "design trades" papers about Webb published over the last 20 years.

Here is a good place to start: https://www.spiedigitallibrary.org/journals/Journal-of-Astro...

Yes, insulating an RTG from the IR sensors would have been a challenge, but one that could be completely and accurately tested on earth. The lack of sufficient power directly drove the passive cooling requirement. Which in turn drove the fraught 5 layer shield requirement - which has never been and can't be "Test as you fly" tested on earth.

The lack of power also drove the decision to use the experimental 26 Ghz Ka band downlink radio instead of a higher power and more robust X band radio.

This is good thinking. However, it seems that Earth doesn't quite eclipse the sun from L2, so it might have been complicated anyway.

My calculations tell me this is not true for Mars though. The next time we do this, we should send it to the L2 of Mars.

> The original sin is the lack of will to make the Webb nuclear powered.

There is a broader point about space exploration in general here. The science that can be delivered by nuclear powered probes is well beyond that of solar ones. Higher instrument power, better comms, longer life, more predictable power supply. It's just better. There's one place in America that makes the plutonium for RTGs.

Plutonium is the ideal RTG fuel from an engineering point of view. But controlling and securing it jacks up the costs. What we need is lots of research into less-politically-fraught fuels. Americium is looking really promising.

https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_...

Arguably, this thing should have been installed on the far side of the Moon, with a relay orbiter to get data back to Earth. Then it could be serviced.

(Of course the "dark side" isn't actually dark when the Sun hits it, so that would limit the available observing time.)

The near and far side of the moon spent the same amount of time in darkness, so there's no advantage gained by putting it on the far side.

Edit: I just realized this idea comes from radio telescopes. Those would actually benefit from sitting on the far side of the moon, because that way they'd be shielded from the radio emissions of human civilization. That concept doesn't transfer to visible light or IR telescopes though.

The moon is tidally-locked, so the near side always faces Earth. It'd be a good place for a spy satellite, maybe, but less desirable for any kind of sensitive astronomical instrument.

The Earth might as well be as big as the Sun from the perspective of the IR instrumentation on JWT. They'd prefer to keep it entirely out of their field of view, which is why it's being positioned at L2. Placing the telescope on the far side of the moon would have the same effect at least part of the time, and we'd potentially be able to carry out maintenance and upgrades in the future.

> Since it has to be in the sun, it needs a huge shield.

I don't think there's any place in the solar system you can put things so that they won't be in the sun. If you're going to be in the sun anyway, better use it.

You can hide behind a planet

Lagrange Points can offer deployment in the shadow, or by using orbits with long times spent in the shadow while being sun synchronous.

I thought it is already designed to be at L2? But because of diffraction it still cannot stay perfectly in the darkness. It also requires propellant to stay at L2, so it’s actually only designed to be operational for 5-10 years. It’s kinda sad to see some gadgets going out faster than it takes to build it.

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