I was wondering why the JWST did the unfurling of the sunshade, deployment of mirrors, before getting to its stable orbit at L2.

Does the relative motion of the moving parts not affect the orbit/path to L2, and so could it have been done at either time?

I have tried looking around, but I only seem to find information about the stages themselves, not why they chose to do the stages before reaching a stable orbit.

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    $\begingroup$ Got nothing else to do for a few weeks. Now with a price of several billion, and an optimistic lifetime of a thousand weeks, each week is worth several million $. So getting started earlier is a good deal. $\endgroup$ Jan 22, 2022 at 20:19
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    $\begingroup$ Moving parts inside Webb cannot affect trajectory in the vacuum for an isolated system like Webb. Solar wind (different for furled/unfurled telescope) may have a certain effect which I'm sure has been accounted for. $\endgroup$
    – almaz
    Jan 23, 2022 at 20:47

6 Answers 6


Some deployments (solar panel and antenna) had to be made shortly after start. For the rest of the deployments the answer is:"why not?"

It's not like this is powered flight to L2. Sure, there were 2 correction burns but they were done with the same type of thrusters that will be used during the mission for attitude maneuvers and orbit keeping anyway. We've basically thrown the JWST up to L2 (or just a bit "below" it) and if nothing was done it would just fall back to earth in something resembling a parabola (I'm not sure if it's a parabola if we can't just ignore the change of gravitational pull). So the JWST has microgravity basically the whole flight from earth to L2.

Deploying earlier is easier in most cases because moving parts tend to become stuck after some time in the vacuum of space. Metals tend to cold weld together, lubricants tend to evaporate, and prolonged exposure to low temperatures does not exactly make part moving easier. So deploying early has a bunch of benefits and no downsides.

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    $\begingroup$ Re Sure, there were 2 correction burns but they were done with the same thrusters that will be used during the mission anyway. The two thrusters used for MCC-1a and MCC-1b will never be used again. The deployment of the asymmetric sunshield means that the thrust from those two thrusters no longer passes through the vehicle's center of mass. A different pair of thrusters, identical to the two used for MCC-1a and MCC-1b, will be used for injection into the L2 pseudo-orbit and for orbit maintenance. $\endgroup$ Jan 21, 2022 at 8:07
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    $\begingroup$ Oh. Thank you for the correction. I'll correct it to "type of thrusters". My Intention was to make clear, that there is not more accelleration on the structure than in standard operations. $\endgroup$
    – TrySCE2AUX
    Jan 21, 2022 at 9:31
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    $\begingroup$ Not a deciding factor but probably easier doing things with lower speed of light delays as well. $\endgroup$ Jan 21, 2022 at 10:28
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    $\begingroup$ @GremlinWranger good point! The light travel time to L2 is alsmost 5 seconds (one way). So Roundtripp 10s. I don't think that this poses a huge issue at the speed of which the deployments workd. But it is for sure easier to work with shorter delays. $\endgroup$
    – TrySCE2AUX
    Jan 21, 2022 at 14:07
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    $\begingroup$ There is also the benefit that by deploying now they can make sure everything works and try resolve any problems before the JWST gets to where it's going. It doesn't currently have anything better to do, might as well do everything you can to make sure it is operational before its planned mission actually starts. $\endgroup$
    – Seth R
    Jan 21, 2022 at 15:56

It's a bit of a wrong picture to consider L2 as a destination. This is not some kind mountaintop on which we need to set up the telescope. In fact, JWST will never even reach L2 itself (only halo-orbit around it).

Really, the thing that matters is just that

  1. JWST has been launched into microgravity, in direction away from both Earth and Sun (so that they remain hidden behind the sunshield).
  2. It will remain reasonably close to Earth, to facilitate communication of all the data.

These conditions don't require getting to L2 per se. Only, that's the point where you can maintain the desired configuration over a long time. But as far as the telescope is concerned, it has already reached its destination, so there's nothing to wait for before unfolding, calibrating or even starting to do science properly.


The commissioning process involves 130+ steps, in a specific order, which takes months. If you never start, you never finish. There is no advantage to delay.

Deployment has almost no effect on trajectory. The sunshade has a small effect from solar pressure. But opening the sunshade so cool-down can begin is a critical path event, so do it early.

A journey of a thousand miles begins with a single step. So get off your duff.

  • $\begingroup$ Is there a lot of dwell time required actuations? Or do some actuations just proceed very slowly? $\endgroup$
    – DKNguyen
    Jan 21, 2022 at 22:38
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    $\begingroup$ @DKNguyen, they just proceed slowly. For example, moving the mirrors from their launch position to their operating position took nearly nine days, since the actuators are designed to move with nanometer precision. $\endgroup$
    – Mark
    Jan 22, 2022 at 0:10

That is a great question! kruemi has some good answers about deployment in space. The reason to deploy the shield as soon as possible really centers around temperature.

First, you want to move physical structures while they are still relatively warm. In fact, some parts have heaters that are turned on to warm up the part before moving it. As mentioned, metal parts in space tend to cold weld and then cannot be moved. Unfurling the shield (which is the size of a tennis court) was a multi-part process of carefully deploying the support structures and then tensioning it to produce the 5 layers of shielding. Here is a good URL: https://jwst.nasa.gov/content/observatory/sunshield.html

The second reason is that the optics and instruments need to cool down and stablize before they can be operational. Most of the instruments onboard work ideally for Webb at 37 kelvin(K) which is -393F. The Mid-infrared Instrument(MIRI) has a desired operational temperature that is below 7K (-448F!) to suppress infrared background "noise." It has a cryocooler that takes it down to 7K, but the cooler body needs to be below 40K to work properly. It takes a very long time for the bus and payload to cool down in the vacuum of space. Currently, after 27 days it has cooled to -340F (66K).


Because it has a finite lifespan

JWST has a finite lifespan. It has enough fuel to keep itself in orbit around L2 (it is not a stable orbit) for at least 5 years, probably 10, maybe 20. Its best instruments require coolant which will run out after which they will no longer operate at peak efficiency, or at all. And it will just slowly wear out. So it's best to get started as quickly as possible.

Because there's a lot to do

30 days spent just coasting to L2 means 30 days less doing observation.

It will take six months before JWST is fully deployed, calibrated, and tested.

  • +10 days the Sun Shield is fully deployed.
  • +14 days the mirror is fully deployed.
  • +28 days the mirror is calibrated.

And then five months of calibration and testing.

Getting to L2 takes 30 days. Using those 30 days knocks a month off the setup time.

Because its deployment is terrifying

If something goes wrong, they'll want to know about it early so they can start devising a fix and lose as little time as possible.

Because it saves \$161 million

JWST cost $10 billion. Given a 5 year nominal lifespan that's about \$5.5 million per day. At that rate, deploying while traveling to L2 recovers \$164 million of value.

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    $\begingroup$ I love the "time is money" part. I think NASA would do the calculation a very different way, but your way is interesting to consider too. Your method depends on the currently unknown mission duration, so the deployment during cruise would save \$328M if the observatory broke at 2.5 years, or "only" \$82M if it operated for 10 years. $\endgroup$
    – giardia
    Jan 24, 2022 at 5:52
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    $\begingroup$ The coolant should not be a pbroblem because JWST is using a closed loop cooling system (like your freezer, just a "wee bit cooler"). As long as there is no leak and nothing breaks JWST should be able to cool the instruments as long as there is power (which comes from solar panels, so should not run out anytime soon). See webb.nasa.gov/content/about/innovations/cryocooler.html $\endgroup$
    – TrySCE2AUX
    Jan 24, 2022 at 14:32
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    $\begingroup$ @kruemi True, however... "as long as there is no leak" There's always leaks. Though I don't know what the estimates are. $\endgroup$
    – Schwern
    Jan 24, 2022 at 17:35

Does the relative motion of the moving parts not affect the orbit/path to L2

No. This is conservation of momentum, if you want to.look it up. Basically, springs latches and extensions could.open and close, motors could turn, telescopic parts could move, radio could turn on its gimbal to lock onto earth, and none of this would change the telescope's path through space.

Why? Same reason that if you jump off a diving board, then (ignoring air friction which doesn't really exist in space), you can move your hands,legs,body, you can spin and extend or curl up - and your centre of mass (centre point of your body for gravitational purposes) will still follow the exact same path to the water it would have done anyway. You might land the other way up, but you'll follow the same path.

Basically if a latch opened on James Webb, one end moved one way, the other end moved the other way, no net change overall. If a motor spin its spindle, the things attached to the spindle turned one way, the motor housing turned the other way, no net change either.

The only change would be if it altered the direction it was pointing in space, not its position or path. Then, when you fire a thruster, it might be in the wrong direction. But that can be addressed quite easily with care, and it's clear they did.

and so could it have been done at either time?

Yes, it could. It could have done it any time outside earths atmosphere. But there's no advantage to waiting, and as other answers say, there are disadvantages - space can be a harsh environment.

Better to unfold sooner, less passage of time for anything to develop an issue if it was going to.

  • $\begingroup$ However, thespace around the Earth is not completely empty and not without solar radiation ans solar wind. $\endgroup$ Jan 24, 2022 at 8:37
  • $\begingroup$ But that's not at all the question, so while interesting, and relevant to JWST generally, they weren't relevant or asked in this question, which was simply about unfolding and its impact on/by timing and path to L2. Let's focus on what was actually asked. $\endgroup$
    – Stilez
    Jan 24, 2022 at 16:03
  • $\begingroup$ Well, it was you who bolded "and none of this would change the telescope's path through space". $\endgroup$ Jan 24, 2022 at 16:19
  • $\begingroup$ True. And also correct. No "relative movements" of its "moving parts" will change its path. That was the question. Of course, the sunshield will pick up solar wind and radiation, but it was clear, that's not actually what the OP is asking. So it wasn't salient here. $\endgroup$
    – Stilez
    Jan 24, 2022 at 17:48

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