Why the James Webb Space Telescope needs propellant

[James Demers]

TL;DR Summary

The orbit of the JWST around its Lagrange point is unstable, and has to be tweaked if the telescope is to remain on station. What happens when the propellant runs out?

Orbits around the Lagrange point are not stable, and you need to tweak your trajectory every now and then to remain in that orbit. Simulations of a non-accelerated body give spectacularly weird results:

When the propellant runs out, is this what the JWST will do?

 

[A.T.]

I assume this includes only Earth's and Moon's gravity. The Sun and other planets will probably mess those nice periodic patterns up.

 

[DaveC426913]

It is not scheduled for refuel, so when its propellant runs out, it will drift out of the L2 point and find its own orbit.

Someone has run a simulation:
https://space.stackexchange.com/que...falls-off-the-l2-or-l1-point-where-will-it-go

Their conclusion, which I cannot vouch for, suggests that it may end up in a wide Earth orbit, beyond the Moon (IOW, still orbiting Earth but no longer synced to the Moon's revolution). I'm just guessing here but I imagine it might end up in a resonant orbit with the Moon (like Jupiter's Galilean satellites)