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Comment by ashika | Hacker News Reader

it's wild to me, given all the delays and complexity and risk, that the mission length is only 5-10 years max. but even if it blows up on the launchpad we've learned a ton, if only about the difficulty of manufacturing such devices in the 21st century. i am praying it does work, though, and that we get 10 years of amazing data from it before eagerly deploying a replacement.

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Is 10 years a hard max (like does it crash into the moon or something?) or is it just a projected max timeframe?

I wonder that mostly because we've managed to use a lot of our other space equipment well past their their mission lengths. I'd be interested if JWST is possibly the same.

It's a hard limit due needing fuel to maintain its orbit. It's in a lagrange point (https://en.wikipedia.org/wiki/Lagrange_point) which requires occasional orbital corrections.

Well it's not strictly a hard limit but it's currently planned to be a hard limit. If SpaceX can pull off even a fraction of what they claim with Starship, it's not unrealistic to think that it'd be financially viable to attempt a refuelling of the JWST.

That's an event I'd like to see!

IIRC, it was also not designed to be serviceable.

It has a docking ring for potential service mission.

-ish. They have no firm plans for servicing it, but it does have a docking adapter and the fuel/coolant connections are designed to be usable in space.

Basically, because there is no reasonable way to service something in L2, they can't really plan for it, but it's expensive enough that they made sure there is the capability if someone in the future would, say, build a spaceship that is orbitally refuelable and designed so it can take crew that far out.

Yes and no. Fuel is the limiting factor, but it could go beyond a decade. See here: https://space.stackexchange.com/questions/55309/james-webb-t...

a good video about lagrange points https://www.youtube.com/watch?v=Gu4vA2ztgGM

The limit is propellant in the tank, which needs to be used for station-keeping.

5.5yr is the minimum, 10 sounds probable (stated goal), while 20-40yrs is the best guess with expected fuel usage.

https://space.stackexchange.com/questions/55309/james-webb-t...

Can't they design the tank system modularly to be replaceable? Like (also projecting how SpaceX etc are also making space cargo much cheaper) having a rocket carry payload that would replace the tank cartgridge with a new one, giving, say, another 5 years' worth of propeller.

I'm pretty much 100% sure NASA knows this better than me of course, but I'd love to see the reasoning behind planning to retire such an expensive project after a (relatively) short ~10 years.

1) its more complex to design a re-fuelable fuel system

2) no vehicle exists/existed at design that could support a re-fuel system.

Unlike Hubble, since JWST will need to be stable and orbiting around L2, this is cited as the reason for it being a finite mission:

Edit after someone corrected me.

Please refer to this comment: https://news.ycombinator.com/item?id=29490291

I’m sure one of our manned moon missions can swing by and top her off

/kidding

//a little

The article you linked says absolutely nothing about the helium cooling medium.

Three of the four imagers on the telescope are passively cooled and will work as long as they don't succumb to radiation, diffusion, etc. The fourth one (MIRI) has a cryocooler that uses liquid helium, but it will leak out very slowly and mechanical wear and electronics lifespan is expected to be the limiting factor there. [0, 1]

As stated in other comments, the primary driver of lifespan is a combination of how stable the telescope orbit is, and the resulting amount of fuel needed to keep the telescope in a stable orbit. Depending on how things go it has enough fuel for somewhere between 5.5 and 40 years of operation. Assuming nothing else goes wrong. :)

"Webb is designed to have a mission lifetime of not less than 5-1/2 years after launch, with the goal of having a lifetime greater than 10 years." [2]

0: https://jwst.nasa.gov/content/about/innovations/cryocooler.h... 1: https://www.nasa.gov/feature/jpl/how-cold-can-you-go-cooler-... 2: https://jwst.nasa.gov/content/about/faqs/faq.html

You are right. The source for my statement above is this link: https://www.americanscientist.org/article/jwsts-limiting-fac...

At the end of the link is the clarification:

Drs. Heng and Winn respond:

As pointed out to us by Drs. Jason Kalirai and Jason Tumlinson at the Space Telescope Science Institute (STScI), as well as Mr. Sykes, our article misstated the reason for the finite lifetime of the upcoming James Webb Space Telescope. The mission duration of 5.5 to 10 years is not limited by the supply of liquid helium, as we stated. Rather, it is limited by the supply of hydrazine fuel needed to maintain the spacecraft’s orbit.

Thanks for the correction, will edit my parent reply.

Does this mean an ion thruster or solar sail could have significantly increased the service life? Or would something else give out shortly after the fuel runs out?

It is due to orbit in L2, in eternal shade of earth. So no solar power

No, that's not true. It will be orbiting around L2 and not stay at L2, so it will have access to sunlight, which powers the solar array that faces the sun. The actual observatory and the mirror are shielded by the sunshield.

Here is Dr. John Mather explaining it: https://youtu.be/4P8fKd0IVOs?t=1321

The Opportunity rover had a planned mission duration of ~93 Earth days. It went on to serve for ~5,500.

I wonder how credulous I've been about those estimates. Underpromise, overdeliver is an old tool for managing expectations. I wonder what NASA really expects for these projects.

(The projects are still amazing; I'm not complaining about the engineering or performance!)

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