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
Edit after someone corrected me.
Please refer to this comment: https://news.ycombinator.com/item?id=29490291
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
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.
Here is Dr. John Mather explaining it: https://youtu.be/4P8fKd0IVOs?t=1321
/kidding
//a little