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(Please excuse my English)

The James Webb space telescope(JWST) will be launched in 11 days. I've always wondered, how do we fix the JWST if it breaks? The JWST will be too far away. According to Wikipedia, it will be about 1.5 million km from Earth, circling about 370,000 km from the Sun-Earth L2 point.

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    $\begingroup$ This probably should belong on Space SE. $\endgroup$
    – WarpPrime
    Dec 10, 2021 at 18:47
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    $\begingroup$ @pdh0710 space.stackexchange.com Space Exploration Stack Exchange. But search to see if this was already asked before you ask. I found a somewhat related question on refueling, in theory, in 5-10 years when it runs out of fuel. space.stackexchange.com/questions/38389/… $\endgroup$
    – userLTK
    Dec 10, 2021 at 21:28
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    $\begingroup$ mentioned above is Is it possible to refuel the James Webb Space Telescope? but also see The JWST - What happens if/when it breaks? and How will JWST be serviced? and all answers concur with ProfRob's answer below who said it perfectly; "There are no plans in place to fix JWST. It has to work." $\endgroup$
    – uhoh
    Dec 10, 2021 at 22:44
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    $\begingroup$ However in the not so distant future there will certainly be space repair capabilities provided by one or more commercial ventures, either refueling robots that tap into fuel lines not originally designed for it, or strap on propulsion/navigation/attitude control units that will "take over" those functions. Serving the JWST is a much, much bigger challenge than say a communications satellite because the telescope has to be rock-solid when observing yet regularly point in different directions on demand, so it would be a far bigger challenge for a space telescope than a coms satellite. $\endgroup$
    – uhoh
    Dec 10, 2021 at 22:46
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    $\begingroup$ @uhoh Yes. The questions(and answers) you mentioned can be the answer to my question. There are too many forums in StackExchange, So it is difficult to find a suitable forum, like the Space SE forum. $\endgroup$
    – pdh0710
    Dec 10, 2021 at 22:58

3 Answers 3

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There are no plans in place to fix JWST. It has to work.

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    $\begingroup$ Oh, my... I hope the JWST will work without problem. $\endgroup$
    – pdh0710
    Dec 10, 2021 at 18:51
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    $\begingroup$ @pdh0710 That's what the entire field of astronomy&astrophysics is hoping... $\endgroup$ Dec 10, 2021 at 18:54
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    $\begingroup$ @pdh0710 that's also one of the cost drivers in space business: quality assurance. The other is documenting that so that the blame can be assigned, if it doesn't work ;) $\endgroup$ Dec 10, 2021 at 23:34
  • $\begingroup$ @planetmaker you mean armies of lawyers to ensure contracts are written so as to ensure no blame can be assigned $\endgroup$
    – jwenting
    Dec 11, 2021 at 4:02
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Fix covers many things. Once the telescope is on station it will be too far away for a manned mission to fix it unless that is judged extremely important and worth developing new capabilities. There are a surprising number of things that can be fixed by command or by uploading new software. Without some definition of what needs to be fixed there is no better answer. NASA has spent a lot of effort trying to ensure that nothing needs to be fixed.

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    $\begingroup$ Yes. fix is a easy but ambiguous word. I wanted to express my question in easy and everyday language. Because I think the JWST will be of ordinary people's interest, like the Hubble space telescope. $\endgroup$
    – pdh0710
    Dec 11, 2021 at 9:25
  • $\begingroup$ You make a good point! There are several kinds of remote fixes, from reprogramming to a series of instructions with the goal of "shaking something that's stuck loose". But changes of crewed repair are low. However there are also robotic repairs imaginable in the future. $\endgroup$
    – uhoh
    Dec 11, 2021 at 11:53
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    $\begingroup$ Except they designed jwst knowing it won't be repairable. A robotic mission would have a very hard time doing anything. Also any craft approaching jwst is likely to damage the extremely fragile sunshield with its thrusters. $\endgroup$
    – Rob
    Dec 11, 2021 at 14:14
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The same question applies to practically all space sondes that leave a near Earth orbit. If one of the Mars rover breaks, you can't call your local service station to send an engineer and get it repaired. Or think of New Horizons that went to Pluto.

Basically, the sondes get designed and tested as much on Earth as possible to withstand all the forces of the launch and the environmental conditions in space (radiation, heating/cooling cycles etc) and by now the engineers have a lot of experience.

A sonde consists of many different components and some are redundant (i.e. there are several systems that can take over each other's function), so when something fails, the sonde is still usable with possibly limited functionality. It's not like a car breakdown where you're really stuck in the middle of nowhere and can't move until it's repaired.

For space sondes, there are certain ways of "repairing" by uploading new software that e.g. disables malfunctioning instruments or modifies their function.

In the past, there are many examples of sondes where certain components failed and the sonde continued to be used, often long beyond the original design life.

An instructive example is the Kepler space telescope. Two of the four reaction wheels failed in 2012; they stabilise the sonde so that it points precisely in the desired direction (this is called "attitude control"), so their failure is a serious problem for a space telescope as it becomes useless if you can't point it to where you want.

So they put it into powersaving mode for a while and designed a new mission using the remaining capabilities, which started 2013 and run for another 5 years until 2019, so this is pretty good for a failure that is almost as bad as you can get. For this second mission, they didn't need any actual "repairs" but uploaded software so it could be stabilisied with the two remaining reaction wheels (by turning it such that the radiation pressure from the sun provided some force). It was more restricted and could only observe certain parts of the sky, but it still delivered a lot of data even the failure.

So for Webb, we hope it works of course, but if something breaks, they will analyse the problem and work around the failed components to continue a useful mission.

A more frequent experience is that sondes have worked perfectly fine for the planned design life, and still continued to work afterwards with perhaps some components failing one after another many years after the original design life. So the missions get extended, redesigned and modified to get as much out of the instruments as long as they are somewhat functional. I guess the record are the Voyager and Pioneer sondes that were designed for 3 or 4 years but worked (at least) ten times as long.

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  • $\begingroup$ Good point! I was worried about the failure of Webb telescope compared to the case of Hubble telescope. Now I can understand that Webb telescope should be compared with the case of Kepler telescope. $\endgroup$
    – pdh0710
    Dec 11, 2021 at 22:22
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    $\begingroup$ Yeah, the lens in Hubble was quite something. But engineers (hopefully) also learned from this mistake and improved the test procedures to catch such mistakes before the launch. $\endgroup$
    – uUnwY
    Dec 11, 2021 at 23:47

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