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 $bn 8.8 telescope. Even if JWST is not designed to be upgraded, a moving part gotten stuck might be moved manually during an EVA. The upper stage would then bring it to L2. I do think some other spacecrafts have orbited Earth before having set off to their interplanetary destinations. Why won't JWST deploy in LEO where it is potentially serviceable? (like Viking?).
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1$\begingroup$ 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. $\endgroup$– Mark AdlerCommented Oct 16, 2015 at 19:21
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$\begingroup$ @MarkAdler I don't mean that anything has been orbited in LEO for service potential, but for other reasons, before setting off beyond Earth. $\endgroup$– LocalFluffCommented Oct 16, 2015 at 19:28
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$\begingroup$ about servicing with current tech: space.stackexchange.com/q/10629/6642 $\endgroup$– AntziCommented Sep 8, 2016 at 16:08
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$\begingroup$ It;s an infrared instrument, it needs to be away from Earth. $\endgroup$– FattieCommented Feb 2, 2018 at 1:08
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5 Answers
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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.
answered Oct 16, 2015 at 19:28
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1$\begingroup$ 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. $\endgroup$ Commented Oct 16, 2015 at 19:42
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1$\begingroup$ 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. $\endgroup$– TristanCommented Oct 16, 2015 at 21:20
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1$\begingroup$ That plot indicates that's it's about the same at 1600 km as 400 km. $\endgroup$ Commented Oct 16, 2015 at 21:31
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1$\begingroup$ Skylab had a jammed solar panel, though due to insulation being ripped away on launch. $\endgroup$ Commented Oct 17, 2015 at 17:47
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1$\begingroup$ @AtmosphericPrisonEscape I think they meant Hubble. $\endgroup$ Commented Sep 23, 2018 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:
answered Oct 16, 2015 at 18:47
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3$\begingroup$ @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. $\endgroup$– TristanCommented Oct 16, 2015 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.
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2$\begingroup$ 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. $\endgroup$ Commented Oct 16, 2015 at 17:34
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1$\begingroup$ The Moon also radiates in the thermal infrared. The JWST sun shield also acts as an Earth and Moon shield. $\endgroup$ Commented Oct 16, 2015 at 17:37
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2$\begingroup$ 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. $\endgroup$– PearsonArtPhoto ♦Commented Oct 16, 2015 at 18:33
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3$\begingroup$ 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. $\endgroup$– HopDavidCommented Oct 18, 2015 at 17:05
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1$\begingroup$ 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". $\endgroup$ Commented Dec 28, 2021 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
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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.
