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JWST will be orbiting L2 which is unstable equilibrium thus will require propellant for station keeping. JWST will carry propellant for 5-10 years.

What will happen once it runs out of propellant? Will it drift in a random direction? Is some direction more probable than others?

PS: why is JWST not parked at L4/L5 instead? They are stable equilibriums and JWST would not require much propellant to stay there indefinitely. Yes, it would require extra shielding from Earth but to a layperson that seems negligible compared to the sun shield it already carries.

uhoh
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daniel.sedlacek
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    For your PS: It will be at the Sun-Earth L2, about 1.5 million km from Earth. Sun-Earth L4/L5 are 1AU from Earth, or about 100 times farther away. It would not require "extra shielding from Earth" at SEL4/L5, but it would be a lot more trouble trying to maintain high-speed comms for all of the data it will generate. – uhoh Oct 03 '17 at 13:48
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    Pure speculation; if it does run out while it is still producing useful science, it will get refueled. See Is Landsat-7's propellant resupply port “robot-ready”? (Restore-L mission). Also see Besides HST, JWST and stations, are there any examples of satellites designed for service in space?. ...its launch interface ring will carry optical targets that could be sought out by a servicing robot’s machine vision as it approaches to attach itself in order to make ... – uhoh Oct 03 '17 at 17:15
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    Another comment for your PS: As uhoh already mentioned L4/5 is much farther away. So yould need many orbital manouvers to reach this point and the JWST has a.) not enough fuel for this and I don't know if there is a rocket thats large enough to bring an even bigger JWST into orbit. b.) It would take years to bring it in position c.) It can be refueled if we develop the technology within the next years. But docking in space is already hard and docking at L4/5 would mean that we need fully automated approaching and connection due to signal delays – CKA Jun 21 '18 at 11:06

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This Northrop Grumman video (starting at 09:31) illustrates JWST's orbit in a non-rotating (normal) frame. It's really in an orbit around the Sun about 1% farther than Earth's, but the weak tug of Earth pulls it along a bit faster so that it remains in 1:1 resonance with the Earth. The orbit is called a "Halo orbit" because in a rotating frame it looks like it's a ring around the L2 point.

Regular, but very small station keeping propulsive maneuvers keep it in this otherwise unstable configurations. The more frequent the adjustments, the lower overall fuel consumption per year. According to this and James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism and Station Keeping Monte Carlo Simulation for the James Webb Space Telescope there will be a small propulsive station-keeping event every 21 days. Wikipedia says this will consume 2-4 m/s of delta-v per year from a budget of 150 m/s, so the lifetime could conceivably be much longer than 5-10 years, although I believe about half of that (~67 m/s) will be used in mid-course corrections on it's way out to the Halo orbit.

If/when station-keeping maneuvers were to stop, JWST will wander away from this resonance and go into a more (or less) chaotic orbit around the sun. It will not be unpredictable though. The understanding of the orbital mechanics is well developed, and if there is no refueling, the final orbit will be chosen and the final propulsive maneuvers will be carefully executed to put the JWST in it's chosen final path into Heliocentricity.

However, there is talk about making the JWST approachable for service missions, and this might make refueling possible. See for example Besides HST, JWST and stations, are there any examples of satellites designed for service in space?, refueling technology for spacecraft is also discussed in Is Landsat-7's propellant resupply port “robot-ready”? (Restore-L mission).

NASA's Propellant Transfer Technologies, Robotic Refueling Mission, and RRM Task : Refueling webpages have more photos and information.

Considering that the Hubble Space Telescope is still highly productive after 27 years, it's hard to imagine that there is no contingency for extending the JWST's lifetime beyond ten years, even though it many not be a formal part of it's current mission. Throwing away a billions-of-dollar science instrument simply for want of some propellant (or other consumables) is hard to imagine.

In JPL Horizons there is a sample calculation of a Halo orbit for JWST that was done in 2014. It orbits for about eleven years, and presumably includes small station-keeping maneuvers to do so. This real-world orbit is more complex than a pure CR3BP (Circular Restricted Three-body problem) halo orbit because the Earth is in an elliptical orbit around the Sun, and the Moon adds further perturbations.

The first figure below shows the complete, eleven-year orbit with it's final wander away from L2 when station keeping ends. This is a "top-down" view of the Ecliptic plane, with the Sun to the left.

The next three images are just one year's worth, showing the two loops of the six-month period halo orbit. The Blue dot is the Earth, the green torus is the Moon's orbit, and the red line is the orbit of JWST. The coordinates are in the rotating frame of the Earth-Moon barycenter around the Sun.

image image

above left: Top-down view, eleven years. right: Top-down view, one year. (Sun to the left)

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above left: View from the Sun, one year. right: View from the side, one year. (Sun to the left)

uhoh
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  • 'Hubble is still highly productive after 27 years' - That required extremely expensive servicing missions. It could have been cheaper to launch a new every time Hubble. The shuttle launches were about $1-1.5 billion. Note that the NRO doesn't service its spy sats either, but rather replaces them with new ones, and some of those are just as expensive/complicated, others almost certainly more so. Eg, the two Hubble look alikes the NRO donated to NASA. – Hephaestus Aetnaean Oct 06 '17 at 18:39
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    @HephaestusAetnaean "Throwing away a billions-of-dollar science instrument simply for want of some propellant (or other consumables) is hard to imagine." In ten years time after the future launch date for JWST, it may not cost "billions of dollars" to bring some additional propellant for station keeping. Let's keep the focus on the topic of the question. Refueling is not the same thing as replacing subsystems, and in general cost for space operations is going down. – uhoh Oct 07 '17 at 01:07
  • I'm sorry if I've offended you, but I wasn't taking a position (either for or against you). I was adding one additional morsel of context/depth to an already good answer. Further, I'd really rather space.se not devolve into another podium for reflexive counter-arguments and nitpicking. Let's have a conversation to solve a problem, not a contest to defend a position. – Hephaestus Aetnaean Oct 07 '17 at 01:42
  • I'll start: given that costs for space ops are falling (esp. potentially launch costs), do you think it's more feasible to launch a heavy/overbuilt/robust (but cheaper and newer) JWST replacement (and losing the old one), or to launch a (potentially) more complicated refueling mission to save the older (but exquisite/expensive/valuable/proven) JWST? – Hephaestus Aetnaean Oct 07 '17 at 01:53
  • @HephaestusAetnaean This SE question and answer are about the propellant needed for stationkeeping of JWST. I've left a comment to reiterate that constraint. If you'd like to address a larger concept you might consider posting a new question. – uhoh Oct 07 '17 at 02:48
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    After a second inspection, you're right, my mistake. I read too much into the question. Your first comment makes a lot more sense to me now. Cheers. – Hephaestus Aetnaean Oct 07 '17 at 03:06
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    @HephaestusAetnaean you make interesting points, if there is a way to cast them in the form of an SE question I think the discussion would be very interesting and on-topic! – uhoh Oct 07 '17 at 03:28
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    @daniel.sedlacek the answer is great, though I don't believe I was the original asker. – Hephaestus Aetnaean Nov 03 '17 at 23:44
  • @HephaestusAetnaean oops, indeed. Sorry about that! – uhoh Nov 04 '17 at 00:51
  • @Rob the mention was meant to be tangential, but if you think that helps you are welcome to edit this answer and add some things. I've squished the images down to make more room, however It's late here now, I'll check it tomorrow. Or you could consider posting an additional/supplemental answer here instead. Also if any of that applies to this or this question you might consider adding an answer there as well. I'll check back tomorrow, thanks! – uhoh Jun 20 '18 at 19:53
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    Personally, IMHO instead of refueling, which is a very complex operation in space, JWST should have a grapple fixture, and within 10 years a very rudimentary spacecraft - something like common satellites except without the whole "mission-specific payload" should be sent, equipped with a grappling device, dock to the port and serve as an external propulsion module. – SF. Jun 20 '18 at 21:36
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    @SF. Since the center of mass is not going to be at the grapple fixture, a single unit may be "torqish". But if station keeping is primarily in one direction (as it is for some halo orbits) then the complications are minimal. It's certainly a good idea; looking forward to buying stock in GrappleDox LLC soon! – uhoh Jun 21 '18 at 02:41
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    @uhoh - A photo of the JWST's LVIR. PS: Did the edit, minimal changes. – Rob Jun 21 '18 at 19:38
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    Re If/when station-keeping maneuvers were to stop -- The last planned stationkeeping operation is to eject the JWST from its L2 pseudo orbit into an Earth-trailing heliocentric orbit. There will be hearings etc. if this disposal maneuver is not performed. The 5 year figure is a worst case scenario where the launch and upper stage barely "works". The 10 year figure is a best case scenario where everything works nearly perfectly. Reality should be somewhere in between. – David Hammen Dec 11 '21 at 20:57
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    If a refueling mission was to be performed (there is no plan for one), its development would have to be already underway. A complex robotic vehicle that only takes five years from initial concept to deployment is unheard of. – David Hammen Dec 11 '21 at 20:58
  • @DavidHammen yes that certainly sounds right! btw tangentially only, Is Landsat-7's propellant resupply port "robot-ready"? (Restore-L mission) has a bounty. I'm trying to figure out if there really is a refueling mission that's going to launch, cut away insulation, find a fuel line, tap into it an invasively refuel a spacecraft demonstration or not. – uhoh Dec 11 '21 at 21:28
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    @uhoh - What was Restore-L is now called OSAM-1. Planning started in 2016. It was projected to launch in 2020. It is now projected to launch in late 2023, maybe, and is projected to cost over $1.3 billion (US). To make matters worse, OSAM-1 is now a mission to a vehicle in LEO, not necessarily Landsat-7. At some point, (and $1.3 billion is well past that point), the sunk cost fallacy kicks in. – David Hammen Dec 11 '21 at 22:47
  • @DavidHammen YIKES! That seems to be the best and final answer here, please consider posting as such! – uhoh Dec 11 '21 at 22:56