Hacker News Reader

Comment by willis936 | Hacker News Reader

Actually JWST orbits around L2 at appreciable altitude. The unique aspect of the orbit is that it is never in the shadow of the Earth or moon. It guarantees consistent solar power and thermal load.

https://jwst-docs.stsci.edu/jwst-observatory-characteristics...

View on HN

One of the fascinating bits is how much engineering has gone into keeping the detectors cold. Because JWST is (primarily?) taking images in the mid-IR region, it is crucially important to keep the detectors cool otherwise you'd just be swamped in background noise. Keeping things cold in space is despite it being a cold place actually extremely difficult, because you can't use convective cooling. If something sits in the sun "all-day" it gets warm, so the whole reason for the sunshield is to keep the instruments cool. On top of that they are using active cooling for the (IIRC) first time on a satellite. I saw a presentation from one of the engineers two years ago and it's absolutely fascinating stuff.

Wow. I’m an astronomer and somehow I never learned this! Thank you for the link!

Of course a solar powered satellite can’t be parked in the shade. Can’t believe I never thought about that.

It can, it has batteries, but you don’t want to mess with thermals on that thing.

My first thought is, "Why didn't they eschew solar power entirely and 'simply' park it in the shade?"

Surely they considered that, and the wins would have been massive -- no sunshields needed. So I'm sure the downsides must have been massive as well. Not enough plutonium fuel, or perhaps it just wouldn't provide enough power over the life of the mission. And of course the radioactive fuel would generate its own heat as well.

Now I need to find out more about that decision....

edit: Another commenter mentioned, "The earth's shadow never reaches L2 anyhow - it's only penumbra at that distance since the angular size of the earth is smaller then the angular size of the Sun." If that's correct, then there was never truly an option of "parking it in the shade" anyway.

The dark side of the Earth is very bright in IR. Getting far away from the Earth and moon makes them dimmer. Also, going for L2 makes sure the Earth, moon, and Sun are all always in the same direction, so the sun shield will block all significant IR sources.

Given that this does not really give the orbit any real significance in terms of being shaded by the Earth from the Sun, is there another reason it's there?

I guess it gives you a very stable position for observations, but why not just put it in solar orbit then? Then again this is kind of a solar orbit that happens to stay close to the Earth at all times so that's a plus for comms.

That's exactly what L2 is. Otherwise the orbits of JWST and Earth would have different periods. It's handy to be able to have constant high speed communications and not need large antennas with blackouts.

It also is handy to keep the closest IR sources (Earth and moon) in the same direction as the sun at all times so there is never a point where you have a significant IR source above the sun shield.

Ah yes I hadn't considered the IR sources aspect, that makes a lot of sense.

That explains the animation! I just thought NASA was just adding some more realism into the orbit animation by adding what looked like a wobble. Derp.

Reply on news.ycombinator.com