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Hubble, besides the initial high-profile COSTAR, was upgraded many times giving essentialy a new instrument with the same heavy/expensive glass. It was set in LEO for that purpose even though it has some significant drawbacks.

Now JWST is going to be at L2 of the Earth, nearly a million miles out.

How will it be serviced and maintained?

This point is dynamicly unstable, so it will use up fuel to stay put. Will it use the last of its fuel to come back, so it can be cheaply refilled? I recall that it's easy to get between L1 and L2 but that doesn’t help with the necessary ∆v needed to park in LEO.

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  • $\begingroup$ As mentioned below, it won't be serviced (under current plans). However, we're getting more and more powerful rockets, and new spacecraft designed to go beyond LEO... it seems plausible that, well before JWST is expected to reach the point of complete failure, something could occur... and that by then it will make more sense to send a spacecraft (manned or otherwise) to repair the fault than it does to replace or give up on the telescope. A F9H or similar could get a lot of mass to L2, no problem. $\endgroup$ – CBHacking Jul 9 '16 at 21:38
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    $\begingroup$ Would (maybe someday) plans include a fuel hatch that's not glued shut? $\endgroup$ – JDługosz Jul 10 '16 at 12:13
  • $\begingroup$ Fair point (about it not being built for serviceability, not about a glued fuel hatch as I'm sure that could be worked around if needed). Much of the maintenance on Hubble was unrelated to fueling but did probably take advantage of servicing being a design consideration, which it's not on JWST. That makes it a lot less likely that something will fail which can be safely serviced without unduly risking the rest of the satellite, and is worth the cost and risk of sending a servicing craft. $\endgroup$ – CBHacking Jul 10 '16 at 20:41
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It will not be serviced. When it runs out of fuel or there's a problem, it's End of Mission.

Because Webb, like virtually every satellite ever constructed, will not be serviceable it employs an extensive seven year integration and test program to exercise the system and uncover any issues prior to launch so they might be remedied. Unlike Hubble, which orbits roughly 350 miles above the surface of Earth and was therefore accessible by the Space Shuttle, Webb will orbit the second Lagrange point (L2), which is roughly 1,000,000 miles from Earth. There is currently no servicing capability that can be used for missions orbiting L2, and therefore the Webb mission design does not rely upon a servicing option.

They tried to increase reliability of the mechanical components:

The gyroscopes on HST and Chandra are mechanical devices dependent on bearings for their function, and they face problems typical of such designs. Webb has adopted a different gyroscope technology. The "Hemispherical Resonator Gyroscope" (HRG) uses a quartz hemisphere vibrating at its resonant frequency to sense the inertial rate. The hemisphere is made to resonate in a vacuum, and the hemisphere's rate of motion is sensed by the interaction between the hemisphere and separate sensing electrodes on the HRG housing. The result is an extremely reliable package with no flexible leads and no bearings. The internal HRG operating environment is a vacuum, thus once the gyroscope is in space any housing leaks would actually improve performance. The HRG eliminates the bearing wear-out failure mode, leaving only random failure and radiation susceptibility of the electronics (which all such devices share, and which can be mitigated by screening and shielding). Stress analyses of HRGs show this design has a "mean time before failure" of 10 million hours. As of June 2011, this type of device had accumulated more than 18 million hours of continuous operation in space on more than 125 spacecraft without a single failure.

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  • $\begingroup$ Interesting about the gyros. I recall laser loops of optic fiber being a better replacement for mechanical ones. $\endgroup$ – JDługosz Jul 9 '16 at 11:58
  • $\begingroup$ They make it sound like gyros with no moving parts is a brand new thing for JWST. We have been using ring laser gyros for many, many years now. $\endgroup$ – Mark Adler Jul 9 '16 at 15:11
  • $\begingroup$ yeah, it's a puzzling addition to the FAQ. $\endgroup$ – Hobbes Jul 9 '16 at 15:30
  • $\begingroup$ This article is paywalled, but the first few sentences are intriguing: aviationweek.com/new-space/… $\endgroup$ – uhoh Nov 9 '16 at 10:07

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