[HN Gopher] The James Webb space telescope has arrived at L2
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The James Webb space telescope has arrived at L2
Author : lanna
Score : 61 points
Date : 2022-01-24 19:18 UTC (3 hours ago)
(HTM) web link (www.jwst.nasa.gov)
(TXT) w3m dump (www.jwst.nasa.gov)
| eminence32 wrote:
| 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?)
| evgen wrote:
| 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.
| CompuHacker wrote:
| 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.
| eminence32 wrote:
| 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.
| spookthesunset wrote:
| 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".
| thehappypm wrote:
| 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.
| air7 wrote:
| 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...
| jacquesm wrote:
| 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.
| cwillu wrote:
| 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.
| IvyMike wrote:
| 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...
| asavadatti wrote:
| >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
| dshpala wrote:
| Kudos to NASA for not giving up James Webb name. The success (and
| the press around it) of the telescope is a giant flying
| (orbiting?) finger to all "activists" that wanted to rename it.
| makerofthings wrote:
| 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?
| csb6 wrote:
| 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.
| frosted-flakes wrote:
| 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.
| belter wrote:
| "...Webb will orbit around L2, not sit stationary precisely at
| L2..."
|
| https://webb.nasa.gov/content/about/orbit.html
| zabzonk wrote:
| Good BBC article at https://www.bbc.co.uk/news/science-
| environment-60116475
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(page generated 2022-01-24 23:17 UTC)