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Comment by zabzonk | Hacker News Reader

> Instead of building one huge short life-span spacecraft

Short life-span? How long has the Hubble been up there?

webb needs to use thrusters to stay in position. there’s 5 years worth of fuel onboard

they did design the fuel in a way that potentially could be refilled… but the refilling craft doesn’t exist. and webb timescales make that sound impossible

And it's at L2 so not that easy to get to

I take Starship could, and should be operational in some form by then.

Question is whether it'll be financially / commercially viable. I think that greatly depends on how much data the telescope gathers or whether it needs more time. Also keep in mind that the Hubble got multiple upgrades over its lifespan, which will be much more difficult with the Webb - is it feasible to do spacewalks that far out? Has any astronaut ever done a spacewalk outside of Earth orbit?

Are you referring to the protection by Earths magnetic field?

If so, would it matter much, given we (allegedly) shot humans in not more than slightly pressurized tin cans to the moon multiple times?

And they survived.

Yeah, if it has done it set out to do, then we don’t need to. Also depends how much of a moon presence we have in five years. Launching from the moon might make it feasible for sure, considering the billions the JWST has cost!

According to the NASA website there is fuel for 10 years, and the five years is just the "minimal" mission time for which it's tested.

https://jwst.nasa.gov/content/about/faqs/faq.html#howlong

Question if anyone knows: ~9.5 years any reason to not push it out of orbit into space and continue to get signals back for years longer as it floats off? Like the voyagers?

To push it out in to space like Voyager requires a lot of thrust, so I bet that's the reason.

As far as I can find, it's kept in the L2 point mainly for temperature reasons.

Not an expert, but this is what I gathered from https://space.stackexchange.com/q/23238/13952 and https://space.stackexchange.com/q/38408/13952

From what I can tell I don't think that it's very uncommon that these things are designed for 5-10 year missions and that everything else is just a nice bonus. I don't think anyone expected Hubble to survive as long as it did for example, and even the Voyager program was originally designed to be finished in the 80s and I don't think anyone expected it would last this long either.

JWST isn't a probe like Voyager. It's job isn't to go to other worlds and see what's there. Its a space observatory. It does exactly the same job a telescope on earth would do, just in space. The reason why is to avoid light pollution and the interference from earth's atmosphere.

So that fuel isn't used for going places, its for aiming the telescope and keeping it steady. Once the fuel is gone it can no longer control where its pointing. If you cant aim a telescope then its useless.

You can aim it with reaction gyros which rotate the telescope and require only electricity. What you cannot do with electricity is translate the telescope along a vector to compensate for orbital decay and keep it on station. That's what fuel is for.

I understand it's not designed for exploration, but my thinking is given the other alternative is burn it up in the atmosphere isn't it better to get a bit longer (few years?) use as it drifts off vs burnt up.

With the you need to aim it, could they sent it to a slow spin/wobble and time image capture as focus would occasionally pass by things of interest? Not sure if it needs to be relatively stable for that long timed exposure.

There is probably good reason for not doing this and trying to understand why.

Don't worry, JWST isn't going to burn up for a few billion years. Until the Sun expands into a red giant at least.

They are sending it to the L2 lagrange point which is further out that the moon. That's also why they are so careful about making sure the origami sun shield works right the first time. If it screws up the whole mission is lost because there's no chance to fly some astronauts out there to fix it like we did with Hubble.

As for exposures while its spinning, I was under the impression these are really long exposures and require it to be precisely aimed for quite awhile.

> why they are so careful about making sure the origami sun shield works right

They cannot really test it on Earth, as it must work in free-fall. Given the past problems with unfolding these things, I have a horrible suspicion that it won't work, and we won't be able get to L2 to fix it.

When you consider how long this thing was in development/production, 5 years doesn't seem very long.

It’s the telescope of Theseus, though.

Because some component bays designed to be swapped out were swapped out? Far as I know they haven’t replaced the 2.4m main mirror or the housing, you know, the “ship”

Yes, that is the joke I was making. Thank you for explaining it.

I think their point was your joke doesn't make much sense when only a small subsection of the entire telescope was ever replaced.

In my opinion that's part of what makes it funny, and is also a core issue in the Ship of Theseus problem -- who's to say how much it can be changed and is still "the same thing"? Maybe the thing that was replaced was very small but was very important to the identity.

That’s not what the Ship of Theseus is. The thought experiment is: if you replace 100% of the original object then is it the same object. Not like, if you change the batteries in the remote is it the same remote.

Sort of. The broken optics famously got fixed in the first HST shuttle mission, and there have been other replacements over time. A lot of it is the same, though.

And of course, the HST is in LEO, which means it is reachable with current rocket tech, which the James Webb, being situated at L2 won't be. But that's not to say you could not have a large platform at L2 (or another Lagrange point) that could be serviced by robots, or even manned craft, if it was designed that way.

But Hubble wasn't a one-off design. It shared a lot of design elements with other space telescopes that are more earth-focused :) So it had a lot more benefits of scale than JWST.

I guess my point was: without that servicing it would’ve been a pretty short life span (literally 0?). The swarm approach helps with this.

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