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JW Space Telescope is designed for a 5-10 year mission duration. Seeing as Hubble and other space missions have paved the way for JWST for reliability issues, It even has zero friction gyro's with 100+ years mean time before failure. Is it reasonable to think that JWST can last longer than Hubble?

JWST will orbit a Lagrange point in earth's shadow and will be 4 times further than the moon, around 10 days of travel away, and NASA states that JWST won't be serviceable. Why?

Why are JWST's fuel limitations so short? Are there no provisions for refueling? even a refueling craft? How many gallons of dinitrogen tetroxide ($N_2O_4$) as oxidizer and hydrazine ($N_2H_4$) will it contain?

Surely NASA are secretly hoping for a 20 year mission?

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  • $\begingroup$ The JamesWebb hardware will probably remain operational for approximately a zillion years :-). The usual weight vs. lifetime rules apply to the amount of propellant sent out on this mission. $\endgroup$ – Carl Witthoft Dec 9 '16 at 12:23
  • $\begingroup$ Note that 'no plan for refueling' isn't equivalent to 'cannot become economical within 20 years or less'. $\endgroup$ – user2338816 Dec 9 '16 at 13:49
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Hubble was in low earth orbit, and was always intended to be serviceable. In fact, the original plan for Hubble was to have the space shuttle carry it down from orbit and take it back up, but they decided that was too risky compared to servicing in orbit.

JWST, on the other hand, will be at the Earth/Sun L2 Lagrange point, like WMAP and Planck before it. The distance to that point is 0.01 astronomical units, which is about 1.5 million kilometers from Earth's center. The Moon's orbit around the Earth is about $1/4^{\mathrm{th}}$ of that. So, a manned servicing mission would be an unprecedented project (beyond Apollo), and an unmanned mission, also unprecedented, would face automation challenges from the 5 second light travel time (10 second round trip).

Because the L2 Lagrange point is only meta-stable (stable to offsets in two directions, unstable along the Earth-Sun line), JWST will have to expend fuel on station-keeping just to stay there. It needs to stay there because it can only be as cool as it needs to be by using the Earth as a sun-shade - those tennis court sized shades built in are actually to shade JWST from the thermal radiation of Earth & the moon.

Put simply, it would be cheaper to build and launch another JWST than to service the existing one.

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  • $\begingroup$ Thanks! If JWST does survive it's intended duration, then they should definitely have planned a refueling craft and some refueling valves to keep it running. It's cheaper to send a bit of fuel out than an entire new telescope. When there's billions of dollars of equipment in space it's good to refuel and to swap the sensors on it. $\endgroup$ – com.prehensible Dec 9 '16 at 8:02
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    $\begingroup$ @comprehensible there are conceptual plans right now (Elon Musk, for one) for unmanned refueling missions. I suspect going out to L2 will take a little longer. $\endgroup$ – Carl Witthoft Dec 9 '16 at 12:22
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    $\begingroup$ "It's cheaper to send a bit of fuel out than an entire new telescope" Hmm, I doubt that would be true. Note that it's tremendously cheaper to buy an iPhone ($100), than it is to make an app for an iPhone ($100,000) - if that analogy makes sense. Or, note that it's incredibly cheaper to just buy a new aircraft, than invent and implement a system of in-air refueling. The idea of "refueling something at L2! robotically!" just seems insanely challenging. $\endgroup$ – Fattie Dec 9 '16 at 12:36
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    $\begingroup$ Japan uses about 400 million to land on an asteroid, take samples and return. The JWST cost 8 billion. iphones are mass produced, so it's diffrenent than the webb telescope. If they had made only one iphone 7, it would have cost 1-5 billion, and the app would have cost 1 thousandth of that. It's also a cost of man hours and labour, if they can refuel it for an 8th of the price, it saves a lot of resources. A PC chip costs $100 to change round, but to make a single intel core7 processor cost 4 billion for the fab and many times that for the previous generations of intel processors. $\endgroup$ – com.prehensible Dec 9 '16 at 13:26
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    $\begingroup$ @comprehensible good points all. however the system to automatically refuel something at L2 would be one of the examples of something that is only cheap when it is mass-produced - it would cost alot to develop such a system. add to that it's now a single point of failure for the extended mission that the refuel would enable, and it becomes more difficult still. if this were done, it also wouldn't solve the problem of replacing broken / worn out components other than fuel, although it would make it possible to deliver them. seems sensible to design the mission without relying on refuelling. $\endgroup$ – jammypeach Dec 9 '16 at 15:09

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