Why is the Webb Space Telescope placed at L2?

[brajesh]

Why is the Webb placed at L2 (Lagrange point 2)?

I read some articles that said it would be perpetually shaded being on the opposite side of the sun and moon, but that makes no sense when the Webb orbit is so huge?

So why is it at L2?

 

[anorlunda]

This is definitely a case where pictures are far better than words. This video may explain it.

 

[brajesh]

This is definitely a case where pictures are far better than words. This video may explain it.

That's a really nice animation. But that's exactly my point.
It's not being shaded by the Earth or the moon.
Why does it have to be at L2?

 

[anorlunda]

What other place could it be where:

1. It is not shadowed by Earth or Moon
2. The communication antenna always faced toward Earth.
3. It stays roughly the same distance away from Earth.
4. It scans most of the visible universe without motors to aim it.
5. It can always keep one side away from the Sun to make it cold enough.

?

The L2 orbit can satisfy all of those. If there were other solutions, we would have heard them discussed.

 

[brajesh]

What other place could it be where:

1. It is not shadowed by Earth or Moon
2. The communication antenna always faced toward Earth.
3. It stays roughly the same distance away from Earth.
4. It scans most of the visible universe without motors to aim it.
5. It can always keep one side away from the Sun to make it cold enough.

?

The L2 orbit can satisfy all of those. If there were other solutions, we would have heard them discussed.

Wow, these are beautiful reasons.
I had to ponder each one and I can see the advantages to each.
It seems like some people are misunderstanding point 5 to mean the Earth and moon are providing the shade, and that's what I believed was the reason. Now the reason is much clearer.
Thank you @anorlunda

 

[berkeman]

Why is the Webb placed at L2 (Lagrange point 2)?

See this related thread in the Classical Physics forum:

https://www.physicsforums.com/threads/why-does-webb-orbit-l2-is-it-because-of-the-moon.1010563/

 

[FactChecker]

Why is the Webb placed at L2 (Lagrange point 2)?

I read some articles that said it would be perpetually shaded being on the opposite side of the sun and moon, but that makes no sense when the Webb orbit is so huge?

So why is it at L2?

The orbit around L2 is large enough that the direct sunlight is never blocked by the Earth or Moon, but small enough that the single sun-shield can shield the telescope from the Sun, Earth, and Moon all at once. So the satellite can always get solar power on the sunny side of the shield but never get heated on the shady side of the shield. It is important to keep the temperatures very stable and to always have power.

 

[russ_watters]

What other place could it be where:

1. It is not shadowed by Earth or Moon

Flip that over: it's not being illuminated by the Earth and Moon...or heated as much.

 

[brajesh]

Flip that over: it's not being illuminated by the Earth and Moon...or heated as much.

Hmmm, I never imagined that reflected light from the Earth or moon could make a difference?
Or would it have made more of a difference if Webb was nearby instead of a million miles away?

 

[FactChecker]

Hmmm, I never imagined that reflected light from the Earth or moon could make a difference?
Or would it have made more of a difference if Webb was nearby instead of a million miles away?

The requirement for the infrared telescope is that there be absolutely minimum energy from outside sources, including the Earth and Moon. Don't forget that full moonlight is bright, now imagine Earth-light. Either would be too much. In fact, a lot of the electronics of the satellite itself are placed on the sunny side of the sun-shield so that it does not mess up the telescope on the other side.

 

[sophiecentaur]

Hmmm, I never imagined that reflected light from the Earth or moon could make a difference?

It's always on the cooler side of both objects. IR from a 'nearby' warm object will contribute to Noise for an IR sensor on the telescope. They're chasing signals right down in the noise so every little helps.

 

[bland]

Why is the Webb placed at L2 (Lagrange point 2)?

I read some articles that said it would be perpetually shaded being on the opposite side of the sun and moon, but that makes no sense when the Webb orbit is so huge?

So why is it at L2?

It needs to be perpetually shaded and it is by the tennis court size sun shade.

 

[anorlunda]

It needs to be perpetually shaded and it is by the tennis court size sun shade.

Clarification. The mirror side needs to be shaded. The solar panels need sunlight to provide power.

 

[sophiecentaur]

Clarification. The mirror side needs to be shaded. The solar panels need sunlight to provide power.

And there's no need to worry about the eclipse situation either. Saves on batteries.
PS Arthur C Clarke would have loved us using L2.