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Comment by miohtama | Hacker News Reader

How many single points of failure left after getting Webb on its way to L2? :)

Here are most of the big ones, with timing: https://planet4589.org/space/misc/webb/time.html

This is very helpful. Looking at the table, my guess is that the scariest moments will be over by "launch +10 days". The sunshield is unfurled by "launch +8 days".

A lot less than there was 40 minutes ago ;)

Also a lot less explosive fuels, shaking and G stress as well :)

Several hundred (mostly related to unfolding stuff)

And cooling stuff.

Serious question, if the sun shield does fail for some reason, would the telescope still operate but just at reduced sensitivity? Asking to the infrared astronomers out there…

Not an IR astronomer, but this is a 'no'. The delta between the 'hot' and the 'cold' side of the telescope is very impressive if the sunshield is working, without that the mission would be a failure because the IR sensors need to be that cold to work. You might still get some visible light work done but the IR part of the mission would be over because the sensors would effectively be blind.

For reference:

https://www.inverse.com/article/42894-sunshield-rips-keep-ja...

Not an IR astronomer, but enthusiast astrophotographer. As I understand it there is a bit of leeway in the sunshield. They added strips of "duct tape" going across layers to prevent potential tears from spreading and telescope can still achieve full science power with few tears in the sunshield, depending on their location and sizes. With bigger tears it might be possible to still carry out observations in visible spectrum and near-IR but not in mid-IR as thermal radiation would crush the signal. Mission would still be considered failure in that case.

Think of it as the body of a camera cracking. Some amount of light will leak in and expose the film/sensors. Just depends on how much.

That analogy only works for a small part, the light would not do damage if it didn't fall on the film or sensors directly or through reflection, whereas with IR it doesn't really matter where it falls on the telescope it will cause the whole thing to warm up and the rejection mechanisms can only cope with so much of it.

I read some more about the effect of sunlight on the telescope structure and the optics. They would actually be damaged beyond use by direct sunlight, which is one of the factors that they had to take into account when launching the JWST:

"The James Webb Space Telescope is launched on a direct path to an orbit around the second Sun-Earth Lagrange Point (L2), but it needs to make its own mid-course thrust correction maneuvers to get there. This is by design, because if Webb gets too much thrust from the Ariane rocket, it can’t turn around to thrust back toward Earth because that would directly expose its telescope optics and structure to the Sun, overheating them and aborting the science mission before it can even begin. Therefore, Webb gets an intentional slight under-burn from the Ariane and uses its own small thrusters and on-board propellant to make up the difference."

This is a very delicate machine.

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