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Comment by Maxburn | Hacker News Reader

See where Webb is on the way to L2 orbit; https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html Fun to see where it is on deployment.

It's also interesting to me that the mirror appears to be fully exposed to space unlike Hubbell. If anyone knows something about that design choice I've like to hear it.

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Hubble needed to hide mirror because it is passing from sunlight into shadow every 47 minutes and mirror has to be protected from thermal stress. Also Earth emits some light even on night side that would interfere with observations.

JWST's mirror is always in shadow and stray light from sides will not be a problem at L2.

That makes a lot of sense, thanks.

If you're interested in a more indepth view of how it's supposed to work, Real Engineering did a video about it recently.

No idea if what they're saying is true however, I've no knowledge about the domain.

https://youtu.be/aICaAEXDJQQ

Fascinating that it's already covered 40% of the distance, but it's only on day 4 out of 30. That makes sense once you realize that its radial velocity needs to slow to zero when it reaches L2. I'd love to see the shape of how the velocity curve slows.

They link to this which shows the gravity gradients in play. I could have sworn they charted the velocity involved but I can't find it now.

https://upload.wikimedia.org/wikipedia/commons/3/36/Lagrangi...

I've been glued to that page since the launch. The temperature delta just came online so you can now see the 'top' and 'bottom' temperature.

In the end it will be several hundred degrees difference, which given the distances involved is extremely impressive.

The explore all deployments section is worth a read too. Wealth of info on the whole site and it's designed really well. Clearly catering to the education / general public good will with this site. https://jwst.nasa.gov/content/webbLaunch/deploymentExplorer....

Factors I can think of:

1. Bigger mirror = better. Enclosing a mirror requires mass and volume, limiting mirror size.

2. The L2 orbit is unstable. Far less risk of being hit by debris than low Earth orbit.

3. Webb's sun shield blocks the primary light pollution angle. Hubble would need more protection from all angles due to the Sun, Earth, and Moon.

4. Any variations in temperature in the mirror array affect geometry/optics

I was just reading that after L2 insertion it's going to take five months to get everything cooled off and aligned properly. Amazing.

Cooldown has already started with the deploying of the sunshield from it's launch position, even before it is unfolded fully and stacked properly the delta is already a whopping 60 degrees Celsius across a bit over two meters.

Maybe we will get lucky and find out something like this announcement.

Did not realize the light pollution angle, that's a major one.

I also had no idea L2 was unstable and found a little discussion on it. https://space.stackexchange.com/questions/284/why-should-the...

Here is a good video about the stability of L1 - L5 points

https://youtu.be/7PHvDj4TDfM

Main reason I heard was that enclosing it would be preferable, but then it wouldn't fit in any currently in use fairing.

If the optical path was enclosed the JWST would probably be too big to fit on any existing rocket.

i've been checking for the past few days and it looks like JWST is available now in NASA's little 3D space model. you can literally it in the solar system model now too!

https://solarsystem.nasa.gov/solar-system/our-solar-system/o...

Another neat site from NASA, thanks!

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