The James Webb space telescope has arrived at L2 | Hacker News Reader

The James Webb space telescope has arrived at L2

jwst.nasa.gov

95 pointslanna21 comments

Related, NASA has a big album of photos related to the preparations & launch. So many awesome shots in here: https://www.flickr.com/photos/nasawebbtelescope/albums/72157...

>The remaining five months of commissioning will be all about aligning the optics and calibrating the scientific instruments

Plenty of time for the mirror to cool down.

It is interested that we have 2 projects with huge solar shields; JWST and the Parker probe

Basically, the mirrors are still glowing white-hot, for a value of "white" that matters to an eye that sees in a spectrum consisting mostly of infrared.

But things should be cooling down fast. There's a lot of different bits that all have different temperatures; here's a 2019 projected timeline of where we expect each bit to be. (We should be around day 30 today)

https://www.reddit.com/r/jameswebb/comments/s4yfs8/jwst_temp...

Serious question, how does it stop when it gets to the right place? Does it have to thrust in the opposite direction or does it use some gravitational effect to slip into orbit?

The telescope slows down over time due to Earth’s gravitational pull. IIRC the trip to L2 was all driven by momentum after the rocket released it. The rocket boosted it in the right direction/speed so that gravity would slow it to a stop at the right place.

Picture rolling a ball up a hill. If you don't roll it fast enough it will roll back down before reaching the top. If you roll it too fast it will go over the top and roll down the other side. Just right, and it will stop at the top of the hill. But being at the top of the hill, it's an unstable orbit, and any small drift will end up with the ball eventually rolling back down the hill. That's why occasional course corrections are required, and why the lifetime of the satellite is determined by the amount of fuel it has.

"...Webb will orbit around L2, not sit stationary precisely at L2..."

https://webb.nasa.gov/content/about/orbit.html

It's like bowls, or curling (the sports). It's a million mile curl. At the end of the curl, the telescope has close to zero radial velocity with respect to Earth (that is, its orbit around the Sun is parallel to Earth's and has the same period). At that point there will be an orbit injection burn to put it into a small elliptical orbit around the Lagrange point.

Good BBC article at https://www.bbc.co.uk/news/science-environment-60116475

I actually thought we chose L2 so that JWST could be shielded from the Sun by the Earth to keep it ultra cool. I didn’t realize it was solar powered.

No. I thought so too, but it's orbit around L2 is bigger than the Moon's orbit around the Earth. It's never in Earth's shadow, but it's position allows it to block both the Sun and the Earth consistently with one shield without moving it. At least that's my current understanding...

At the L2 distance you'd have passed out of Earth's shadow cone even if exactly at L2, at best you'd be in the penumbra. But Webb has an orbit of 800,000 km radius around L2, which would put it will outside of that regardless.

This gives some more appreciation for how large the sun is.

Wow! That’s pretty cool.

Does anyone know why JWST will orbit around L2 instead of being parked at L2? (Is it even possible to be parked directly on top of L2?)

L2 is unstable so sitting there is not a win. By orbiting (and orbiting at quite a long distance from the actual L2 point) the JWST can ensure its solar panels get maximum sunlight.

JWST is solar powered. Sun-Earth L2 is in permanent eclipse and may have unstable lighting or heating conditions. A halo orbit about L2 is extraordinarily boring and always in full sunlight.

Maintaining L2 or an L2 halo requires propellant.

Ah, great answer, thank you. Do you happen to know the approximate radius of JWST's halo orbit?

(Edit below, to answer my own question via wikipedia)

> The telescope will circle about the Sun-Earth L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius varying between about 250,000 km (160,000 mi) and 832,000 km (517,000 mi), and take about half a year to complete.

According to wikipedia it "[has] a radius varying between about 250,000 km (160,000 mi) and 832,000 km (517,000 mi), and take about half a year to complete".

If anyone wants to build a fun intuition of how Lagrange orbits work, I recommend trying Kerbal Space Program with the Principia mod (n-body gravity and selectable reference frames).

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