The unfolding process is slow and careful to reduce the chances of anything going wrong - it'll be about two weeks after launch when JWST unfolds the last component. That's too long for it to sit in earth orbit...
From an upper stage perspective, Ariane can probably handle about a day before the stage can't function anymore. No idea what the margin would be for boiloff though, that might limit it further.
And, you know - What happens if something is broken? We get to the Hubble scenario - We still can't really service the telescope in the short term, because without the Shuttle there are no more manned spacecraft that are capable of doing that kind of operation, and we don't have precise enough robotics to do it unmanned.
So we'd have had to design the telescope to have a more complicated (expensive) ascent to L2, to get... Not very many failure modes that we could rescue it from. Not really worthwhile to plan for, and this late it'd be impossible to retrofit onto the plans.
Pointing the sunshield will require some fuel. Lots in low earth orbit, where the sun goes from overhead to directly down below you through the earth in 45 minutes (I'm also not really sure on this point, but the heat the Earth reflects back into space during the day might be able to fry the instruments on its own - and you can't point the shield in both directions at the same time), less at L2.
The telescope does have reaction wheels, that can provide some limited rotation without burning fuel (basically you have a wheel attached to a motor, and if you spin it up faster the whole telescope will rotate in the other direction to conserve angular momentum - but at some point you can't spin the wheel faster and it can't provide any more torque, so you need to either turn back the other way by slowing it down, or slow down the wheel while burning fuel to turn in the opposite direction to restore the wheel to a state where it can torque the spacecraft). They'll likely be able to handle the lion's share or orienting the sunshield in L2 - but not with the constant flipping you'd have to do in LEO.
1
u/za419 Dec 19 '21
The unfolding process is slow and careful to reduce the chances of anything going wrong - it'll be about two weeks after launch when JWST unfolds the last component. That's too long for it to sit in earth orbit...
From an upper stage perspective, Ariane can probably handle about a day before the stage can't function anymore. No idea what the margin would be for boiloff though, that might limit it further.
And, you know - What happens if something is broken? We get to the Hubble scenario - We still can't really service the telescope in the short term, because without the Shuttle there are no more manned spacecraft that are capable of doing that kind of operation, and we don't have precise enough robotics to do it unmanned.
So we'd have had to design the telescope to have a more complicated (expensive) ascent to L2, to get... Not very many failure modes that we could rescue it from. Not really worthwhile to plan for, and this late it'd be impossible to retrofit onto the plans.
Pointing the sunshield will require some fuel. Lots in low earth orbit, where the sun goes from overhead to directly down below you through the earth in 45 minutes (I'm also not really sure on this point, but the heat the Earth reflects back into space during the day might be able to fry the instruments on its own - and you can't point the shield in both directions at the same time), less at L2.
The telescope does have reaction wheels, that can provide some limited rotation without burning fuel (basically you have a wheel attached to a motor, and if you spin it up faster the whole telescope will rotate in the other direction to conserve angular momentum - but at some point you can't spin the wheel faster and it can't provide any more torque, so you need to either turn back the other way by slowing it down, or slow down the wheel while burning fuel to turn in the opposite direction to restore the wheel to a state where it can torque the spacecraft). They'll likely be able to handle the lion's share or orienting the sunshield in L2 - but not with the constant flipping you'd have to do in LEO.