L2 is unstable. There are stable Lagrange points that would be much better for this purpose if that was the reason.

The unique thing about L2 is that it's the only Lagrange point where the Sun is always blocked by the Earth. That's what they're going for here.

Not entirely true. Spacecrafts orbiting L2 do not stay still with respect to the Earth; rather, they follow a Lissajous orbit, which is quite efficient given the gravitational properties of the L2 point. Therefore, they will observe the Sun quite often. (The apparent size of the Sun from L2 is roughly the same as Earth's, so eclipsing the Sun would be too complicated.) And this is not bad, because otherwise solar panels could never produce the power necessary for the spacecraft to work!

The real reason why L2 is so interesting is that it violates Kepler's third law: despite being farther from the Sun than Earth, the orbital period is still the same as Earth, because of the gravitational attraction of Earth itself. This allows the spacecraft to keep the same distance from Earth during the year, which eases communications with ground stations.

(Disclaimer: I have been part of the Core Team of the ESA Planck mission, which flew around L2 like JWST will do.)

The spacecraft would also keep the same distance from the Earth during the year in Earth orbit, so there has to be another reason they picked L2.

The real reason for L2 is that the sun, moon, and Earth are all the same direction from the telescope when it is at L2, so the sunshield can block them all simultaneously. Yes, the spacecraft will receive insolation at L2 (yay solar power), but it will always be from the same direction and therefore the instrument can be persistently shielded.

EDIT: from the the horse's mouth:

> To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.

> This is why the telescope will be out at the second Lagrange point.

https://jwst.nasa.gov/orbit.html

I don't think that's true? At the exact L2 point, the forces are zero, anywhere off that point, they are non-zero, away from the point, from what I understand.

Ref: https://www.space.com/30302-lagrange-points.html "L1, L2 and L3 are all unstable points with precarious equilibrium."

That is incorrect, L2 is not a stable lagrange point, and will require station keeping adjustments to remain in position

They need to use fuel to station keep, there is a hard time limit to this.mission

Is it known how long it would last? Wikipedia doesn't say anything about this issue

The mission is minimum of 5 years with maximum 10 years based on the fuel.

https://jwst.nasa.gov/faq.html#howlong

After 10 years it will probably need more than just fuel. And doing maintenance in orbit, especially way out in the L2 point is easier said than done. Probably easier to build a new satellite with even more capability to replace it, except of course that the JWT is famously behind schedule and over budget and a more sophisticated replacement probably won't be any easier to build.

A replacement might actually easier to build. The detector is no small feat, but the major problems with JWST come from the foldable heat shield and the segmented mirror. Both of which are due to size constraints during launch. Basically it has to fit a 20 x 14 m heat shield and a 6.5 m mirror into the 6.4 by 4.6 m fairing of an Arianne 5.

Targeting a rocket with a larger fairing diameter (even at same payload mass) would make the design much simpler. An upper stage with 20 m length and 9 m diameter on a Falcon Super Heavy would allow to have a fixed mirror and heat shield that is only folded once, instead of the crazy origami that JWST is. But of course such an upper stage doesn't exist yet. SpaceX was only started 6 years after the work on JWST started...

What a shame. The mission won't be operational as long as its delays. After all that I'd at least hope there could be an option for re-fueling, but I bet there isn't.