Thread by @markmccaughrean

Huge day ahead for the NASA/ESA/CSA #JWST, with the scheduled deployment of the so-called midbooms, which extend out to the side of the spacecraft.

These will pull out the five-layer, tennis court-sized sunshield, critical to the cooling of the observatory.

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By the way, the clips I’m posting each day of the deployment come from this full video by NASA:



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Until now, the sunshield has been carefully folded up in a zig-zag fashion, held down under the sunshield covers that were rolled back yesterday & in the pallets that were folded down away from the telescope earlier in the week.

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Here’s a picture of the full observatory, with the sunshield pallets folded up against the primary mirror, during a trip to CSG Kourou last month.

The purple lower surface of the pallet is part of the sunshield, & is silicon-doped metallised Kapton.

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The full sunshield has five layers of metallised Kapton & these are incredibly thin.

The first, sun-facing side is 0.05mm or 50 micrometres, about the thickness of a human hair. The other four layers are just 0.025mm / 25 micrometres thick.

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Each of the plastic Kapton layers is coated with 100 nanometres of aluminium to make it reflective, & the first two, the hottest ones on the sun side, have an additional 50 nanometres of doped silicon. That makes them purple, while the other layers are silver in colour.

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The first layer reflects most of the sunlight falling on it, but a small fraction is absorbed, heating the layer. It re-emits some of the energy at longer wavelengths into the gap to the next layer. It also reflects most of the light, but again, some is absorbed & heats it.

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So, layer by layer, the amount of light & heat making its way through is reduced.

Overall, the 5 layers reduce the total incident power on the sunlit side by a factor of ~1 million, lowering the max temperature on Layer 1 from over +110°C to a minimum on Layer 5 of -237°C.

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To ensure that the reflected light & heat at each step doesn’t get trapped between the layers, they are splayed like separated fingers, so the light bounces out & ultimately into space.

This is where the sunshield tensioning comes in a bit later in the deployment process.

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Starting from the sunlit side, each of the layers is slightly smaller than the one above, to avoid any part of a colder layer seeing anything other than the one above. And of course the layers can’t touch each other, to avoid conduction. Again, it’s all in the tensioning.

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But first the layers need to be pulled out from their folded & stowed configuration on the pallets by the midbooms, & that’s what’s happening today.

To avoid them becoming a tangled mess during launch & in microgravity, the layers have been held in place.

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This involves many small pin devices that go through the layers. As the midbooms pull out, these “non-explosive actuators” need to retract their pin, & releasing a section of sunshield in a controlled way. There are about 140 of these – they all need to work.

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These mechanisms have redundancy, multiple ways of making them retract & release the sunshield. But still.

Key side point: the holes where the pins go through the five layers have been designed so that they don’t align & let light through when the sunshield is deployed 👍

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As well as the actuators, there are many cables, pulleys, motors, & other mechanisms involved in this key deployment, & you can read more here:

nasa.gov/feature/goddar…

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There’s a lot more information on the sunshield, including how it has rip-stop features to prevent possible tears during deployment or micrometeoroid holes from propagating, here:

webb.nasa.gov/content/observ…

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Finally, a reminder why we need this huge, complex sunshield in the first place.

To see light from the first galaxies born in the Universe, or to study young stars & planets in nearby nebulae, or to measure molecules in exoplanet atmospheres, we need an infrared telescope.

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That’s because of the effects of redshift, dust absorption, & low temperatures, respectively.

The infrared lies redward of visible light & is a kind of heat. When a fire cools down to a point where it’s no longer visibly glowing, you can still feel the IR hitting you.

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Problem is, if your telescope is also warm, it’s emitting in the infrared, at the same wavelengths you’re trying to observe very faint stuff in deep space. That IR glow adds a bright background & makes it hard to discern your faint astronomical targets.

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As an analogy, IR astronomy with a warm telescope is like trying to observe in the visible in broad daylight with a telescope made of light bulbs. Possible, but you won’t see faint things very well 🤷‍♂️🙂

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Hence JWST’s sunshield. By putting the telescope behind it out at L2, with the Sun & Earth always hidden from view, it can cool into the blackness of space to about 40K or -233°C, where it stops glowing at the wavelengths we’re interested in & we can do amazing science.

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That’s why the sunshield is so critical: if it doesn’t deploy & tension fully, the telescope won’t get cold & we will be blinded by the bright background.

In fact, if it doesn’t get cold, the science instruments won’t even turn on … but that’s for another thread.

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