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.
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.
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
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.
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).
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.
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.
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.
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.
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.
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.
