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The James Webb Space Telescope will deploy (unfold mechanically) while on the way to L2. Couldn't it do so in LEO, where it is potentially serviceable? Starliner CST-100 and Dragon are planned to soon allow affordable crewed missions to LEO, and Soyuz could maybe already do it. At least Orion is designed to allow for an EVA, and it is a $bn 8.8 telescope. Even if JWST is not designed to be upgraded, a moving part gotten stuck might be moved manually during an EVA. The upper stage would then bring it to L2. I do think some other spacecrafts have orbited Earth before having set off to their interplanetary destinations. Why won't JWST deploy in LEO where it is potentially serviceable? (like Viking?).

TildalWave
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LocalFluff
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    Like Viking?? Do you mean the bio-shield jettison? There was neither significant risk, nor any opportunity to remedy. It had to immediately depart on the Centaur, which operated on batteries and had cryogenic propellant boiling off. – Mark Adler Oct 16 '15 at 19:21
  • @MarkAdler I don't mean that anything has been orbited in LEO for service potential, but for other reasons, before setting off beyond Earth. – LocalFluff Oct 16 '15 at 19:28
  • about servicing with current tech: http://space.stackexchange.com/q/10629/6642 – Antzi Sep 08 '16 at 16:08
  • It;s an infrared instrument, it needs to be away from Earth. – Fattie Feb 02 '18 at 01:08

5 Answers5

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JWST is being launched on an Ariane V with a cryogenic upper stage. That upper stage has to be used immediately to launch it on a trajectory to the Sun-Earth L2. The stage operates on batteries, and the cryogenic fuel is boiling off. So there would be no time to do anything even if you deployed the telescope before departure.

Furthermore, the deployed mirror and sunshade would have to be designed to take the loads from the thrust of the upper stage, which would increase their mass and complexity. Pretty much needlessly increasing the mass for the entire lifetime of the telescope for a 20-minute event.

The only way to do this would be to have an electric propulsion upper stage. Then you could have it in a low-Earth orbit for a short time, short enough to mitigate the debris, and still accessible to Orion or Dragon. You could use the electric propulsion to raise it above most of the debris while waiting for servicing (more than 2000 km), and lower it for the servicing mission. A few dings in the mirrors is better than no telescope at all. However you'll have the van Allen belts to consider in the waiting orbit. The electric propulsion would be very low thrust, so the deployed telescope would hardly notice, and it could wait as long as desired before departing. The telescope would also have to be designed to be serviceable, which it currently isn't.

What would be really nice is if it had an electric propulsion system with enough propellant to take to L2, and bring it back to LEO. Then you could service it, refill the Xenon tank, and send it back off.

Alas, all of that would just increase the cost of an already rather costly telescope, in order to mitigate a risk that can and has been mitigated through design and test.

Mark Adler
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    Maybe what we could do is, if there is a problem once it gets to L2, would be to design an SEP system to go to L2, grab it, and bring it back. – Mark Adler Oct 16 '15 at 19:42
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    I commented on another answer, but there's a misconception here that the debris environment gets better when you go higher. That doesn't begin to happen until well above 2000 km altitude. In fact, the debris environment at 1600 km is on the order of 1,000 to 10,000 times WORSE than the debris environment at 400 km. – Tristan Oct 16 '15 at 21:20
  • Yes, it is about equivalent to 400 km at 1300 km, where I was thinking. The peak is at about 800 km due to some big collisions that happened around there. The web page on Webb has limited information, but they were also complaining about junk emanating from ISS. The orbital debris environment at ISS altitude is actually not too bad. – Mark Adler Oct 16 '15 at 21:30
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    That plot indicates that's it's about the same at 1600 km as 400 km. – Mark Adler Oct 16 '15 at 21:31
  • @Tristan Do you have access to more recent and/or more accurate data than the wiki chart? I don't know if you can share any of it, since it relates to your work, but if you could that would probably help. – called2voyage Oct 16 '15 at 21:39
  • @MarkAdler That graph relates to tracked objects only (10 cm and larger). See http://hdl.handle.net/2060/20150003495 (NASA presentation, released publicly via NTRS) for some plots of smaller size particles. Note that even those sizes plotted are still larger than the smallest size we would be concerned with, since we are dealing with optical surfaces. – Tristan Oct 16 '15 at 21:44
  • Ok, then at 1300 km it's about a factor of five worse than ISS. It's back to ISS level at 2000 km. From that package, I'm not seeing your 1000 to 10000. If I go to the local maximum at 1500 km, that is about 40 times the density at 400 km. Is there some other metric you're using? – Mark Adler Oct 16 '15 at 21:52
  • It isn't as promising as I thought, so I will edit the answer. – Mark Adler Oct 16 '15 at 21:52
  • People in the business seem to have confidence in mechanically self-erecting systems. And the only serious failure I come to think of is Galileo's main antenna 25 years ago. – LocalFluff Oct 17 '15 at 07:15
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    Skylab had a jammed solar panel, though due to insulation being ripped away on launch. – Mark Adler Oct 17 '15 at 17:47
  • @MarkAdler There's more to what makes a debris environment bad than just "how much stuff is there?" Particle sizes, material densities, approach angles and velocities, etc, all matter. You will probably not find all of that detailed information publicly available, but you may have luck if you look into ORDEM 3 (NASA) and MASTER-2009 (ESA). – Tristan Oct 19 '15 at 16:23
  • Yep, about 20 years ago I used a similar model, Neil Divine's solar system micrometeoroid model, to calculate Cassini's vulnerability, especially its main engines, to the flux, dependent on velocity, direction, and particle size. – Mark Adler Oct 19 '15 at 17:33
  • I really like this answer, but could I ask for an upgrade? Specifically the part of OP's question comparing JWST to Viking servicing in orbit deserves a little comment. Was Viking services in orbit before launch? And if so, what's the main difference? – AtmosphericPrisonEscape Sep 23 '18 at 11:23
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    @AtmosphericPrisonEscape I think they meant Hubble. – Mark Adler Sep 23 '18 at 19:20
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The James Webb Space Telescope will not be deployed in Low Earth Orbit because there is too great a risk of the optics being damaged by debris.

[T]he environment around the ISS is not suitable for the exposed optics that JWST has and would have had the possibility to damage or contaminate the optics. The deployment of JWST happens far above Low Earth Orbit and the debris that resides there.

Source:

called2voyage
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    @MarkAdler The problem is that there isn't a higher low-Earth orbit with far less debris. Until about 1600 km altitude, the higher you go, the worse the environment. You'd have to go well past 2000 km altitude to find an environment comparable to the ISS orbit. At that point, you're not really accessible anymore. – Tristan Oct 16 '15 at 21:17
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Location, location, location.

Earth is too hot for a telescope that needs such low temperatures to operate. They are building a 5 layered sunshield to protect the JWST from the heat of the Sun. By 'hovering' in Sun-Earth L2, a million miles from Earth, they can avoid essentially all of the radiated heat from Earth.

The goal is to keep it as cold as possible passively to minimize the active cooling needed, to allow it to work longer.

Mark Adler
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geoffc
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  • But L2 is only 1% further away from the Sun, wouldn't the sunshield work in LEO too? Is the reflected heat from Earth really too much for it to be exposed for once before it sets off to start operations? Won't it be about +50 to +90 F at Cape Canaveral average temperatures at launch anyway? – LocalFluff Oct 16 '15 at 17:02
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    The reflected heat from the sun is de substantial in LEO, about 20% of the solar radiation. Additionally, the earth gives off its own black body radiation. – Rikki-Tikki-Tavi Oct 16 '15 at 17:34
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    The Moon also radiates in the thermal infrared. The JWST sun shield also acts as an Earth and Moon shield. – David Hammen Oct 16 '15 at 17:37
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    This doesn't really explain why, but I think you've hit the reason. Try explaining why LEO would be worse for the lifetime of the spacecraft than L2 will be. – PearsonArtPhoto Oct 16 '15 at 18:33
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    From LEO, earth is a 300 K heat source that takes up nearly half the sky. From SEL2, earth, sun and moon all remain in the same region of the sky and can all be blocked with a relatively small shield. That leaves most the 4 K sky the scope can radiate heat into. – HopDavid Oct 18 '15 at 17:05
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    The question was not about permanent deployment to LEO. The question was "why not unpack all the tricky stuff in LEO but then move it to L2 only after everything checks out and is fixed if necessary". – Aaron Frantisak Dec 28 '21 at 22:32
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There are so many reasons why an L2 orbit was chosen for the JWST. Though the specific advantages over LEO include;

  • It can use Infrared instruments since the heat from the Earth and Sun can be radiated away from its field of view.
  • It will have a constant, uninterrupted view of deep space.
  • It will require less orbital corrections throughout its lifetime.
  • Obviously, the risk of space debris in LEO is a growing problem.
  • L2 has been a favoured orbit for space observations by astronomers for a long time.

Due to the delicate nature of the telescope's deployment, yes, it would be great if we could get people there in case something goes wrong. However, this is also the reason why it cannot be deployed in LEO and then sent to L2 as it simply isn't structurally designed to withstand that sort of journey and would have to pass through altitudes of even greater levels of debris.

This website talks about the advantages of the L2 orbit for the JWST: http://www.nasa.gov/topics/universe/features/webb-l2.html

Ruth Wrigley
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My guess is that timing played a role. JWST was planned long before Dragon and CST-100 were. At the time of its planning the shuttle was still in service (I think) but people knew the end of the shuttle was coming.

Nathan Tuggy
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Sam
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