JWST@L2
January 25, 2022 8:46 AM   Subscribe

 
Every time JWST executes another critical phase of the mission I relax a little. So glad the L2 orbital insertion went smoothly. None of the links seem to discuss this; how much energy or ∆v was required for this burn? My guess is it's pretty small in comparison to getting out to this region of space but I have no idea.

Reading about this yesterday inspired me to consider firing up Kerbal Space Program to play with something similar myself. Unfortunately KSP's physics are simplified in such a way that they can't simulate Lagrange points. (They only simulate two bodies and Lagrange points are the balance of three.) The Principia mod does allow for Lagrange points but it seems complicated enough I gave up my plan.
posted by Nelson at 9:03 AM on January 25 [4 favorites]


According to Wikipedia it's about 3.43 km/s, once you get to low Earth orbit. That's lower than the burn required for a geostationary orbit -- I assume because you don't have to change the orbit inclination to equatorial, you can approach L2 from any inclination.
posted by credulous at 9:17 AM on January 25


So in my head L2 was somewhere between the Earth and the Sun...because I know nothing about orbital mechanics and just assumed it was somewhere between the two bodies with the Earth tugging on one side and the Sun on the other. If I understand anything (a dubious proposition at best) I was thinking of L1. So the animation linked on that page really helped. Still can't say I understand the forces but at least I can kind of understand where it is.....and I guess L2 being further from the Sun makes sense given the optics and what not....

NASA’s James Webb Space Telescope is at L2. What’s next?
L3 which has a super hard boss...just use lots of ground kicks and hide behind the barrels when its special powers activate
posted by inflatablekiwi at 9:18 AM on January 25 [17 favorites]


how much energy or ∆v was required for this burn?

The final insertion burn was 1.6m/s which is about a walking pace of 3.6 miles per hour.

The burns were so shorter than expected because Ariane 5 basically targeted the thing perfectly and very little fuel was required for the mid course corrections. We could expect multiple years of extra time with the telescope as L2 requires active station keeping even to keep to a halo orbit.
posted by Your Childhood Pet Rock at 9:32 AM on January 25 [18 favorites]


NASA’s James Webb Space Telescope is at L2. What’s next?

The inner Oort cloud.
posted by The Tensor at 9:37 AM on January 25 [9 favorites]


NASA’s James Webb Space Telescope is at L2. What’s next?
L3 which has a super hard boss...just use lots of ground kicks and hide behind the barrels when its special powers activate

And since it's on the other side of the sun you're basically fighting blind unless you have the neutrino upgrade
posted by achrise at 9:45 AM on January 25 [7 favorites]


Here's a pretty good Sky and Telescope article that goes into detail about JWST's position and what it is doing now. Interestingly, saying the JWST is 'at' L2 is kinda the same as saying that the Moon is 'at' the Earth i.e. the JWST is in a 232,000 mile (374,000 km) wide orbit around L2. Short video showing the L2 orbit.
posted by achrise at 10:00 AM on January 25 [7 favorites]


The 18 mirror segments each have 7 actuators, so that's 126 motors that need to be adjusted to within a few nanometers of their ideal positions. A friend from college (he and I were 2 of the 3 students in an astrophyics course) is the leader of that "wavefront sensing and control" team. He said on Facebook on 1/19 that "the work begins in about a week". He's also said that deploying the mirrors "should take about 10 days. Then, after another 45 days, we should have the first diffraction-limited image."
posted by neuron at 10:01 AM on January 25 [19 favorites]


Heh. I was wondering just last night where Webb was. Very cool!
posted by Thorzdad at 10:01 AM on January 25 [1 favorite]


The burns were so shorter than expected because Ariane 5 basically targeted the thing perfectly and very little fuel was required for the mid course corrections
I’ve seen this reported a number of places and TBH I don’t fully understand it.

From my understanding, the big risk with the JWST mission (and one of the things that led to the selection of Ariane 5) was that overshooting would have been extremely bad and basically nonrecoverable – JWST’s thrusters can only safely point one way. Therefore, the launch was designed to slightly undershoot, while making the rest up with the onboard thrusters.

So… hearing “we used a lot less fuel than we thought we would” makes me think that they may have gotten uncomfortably close to the other safety margin.
posted by schmod at 10:06 AM on January 25 [4 favorites]


My sense of it is they undershot almost exactly as much as they wanted to (more accurately than they expected), not that they overshot the undershoot, as it were.
posted by tclark at 10:08 AM on January 25 [8 favorites]


It's been such an exciting month for JWST! As a member of the science teams for one of the cameras on board the telescope, it's been incredible to see the hard work as the whole observatory has unfolded, deployed, and parked itself in the halo orbit around L2.

But now I keep seeing articles saying that everything from here on out should be "fairly routine." It's awesome that the observatory is out at L2, but we need to do months of slow and detailed commissioning as we turn the instruments on, focus the telescope, make sure every mode works as it should. Before we can even think about science, we need to make sure that this incredible telescope can point where we want it to point, can reach and stay at the cold temperature it needs to be, can take images and spectra, can move all the internal filter wheels and various optical elements, can reach the desired sensitivities. This is one of the longest commissioning periods of any space telescope because of the raw complexity of the observatory.

So, for those champing at the bit for pretty images and science from JWST...well patience is recommended.
posted by RubixsQube at 10:35 AM on January 25 [44 favorites]


The 18 mirror segments each have 7 actuators, so that's 126 motors that need to be adjusted to within a few nanometers of their ideal positions.

How exactly does the Webb focus on things? Or is it just assumed everything is so dang far way that the focus is set to +INF and it's left there?
posted by JoeZydeco at 10:45 AM on January 25


RubixsQube > As a member of the science teams for one of the cameras on board the telescope…

Which camera? What does it do? Please tell us more.
posted by cenoxo at 11:06 AM on January 25 [1 favorite]


I work on NIRCam, which is the primary imager for the telescope. It's designed to image the sky in the near-infrared (hence *NIR*Cam), from roughly 0.7 - 5.0 microns, just outside of what we can see with our eyes. NIRCam's field of view is rectangular, because it's actually two identical (well, mirror image) cameras brought together, so it will take two 2.2' x 2.2' images next to each other at once. This is 2.2 "arcminutes," where an arcminute is 1/60th of a degree, which is 1/360th of a circle. An arcminute is about the resolution of the human eye. So, it might seem like a small field of view, but it's pretty big as compared to Hubble's equivalent near-IR instrument, WFC3. The other cool thing about NIRCam is that it has a "dichroic beamsplitter" in each module, which sends short and long wavelength light to two different detectors, so it can take a short and a long wavelength image simultaneously. This will allow NIRCam to be an incredible machine for taking a lot of data all at once, which is great when you want to maximize your science!

While there are a lot of very, very exciting science projects being planned for all of the JWST instruments, I think NIRCam is going to be one of the most exciting for the public, as it will be NIRCam images that will inspire once commissioning is done. BUT I AM PRETTY BIASED
posted by RubixsQube at 11:15 AM on January 25 [83 favorites]


JoeyZydeco: When you focus a camera, you move the lens in and out so that the image is formed on the sensor, and not in front of or behind it (which gives you a blurry image). You are not modifying the lens itself.

For JWST, it's like there are 18 separate "lenses" -- actually mirrors -- each of which can be distorted and moved. The tricky bit is to move and distort all 18 mirror segments to the positions that one large perfect mirror would have. (To get technical, each wavefront of light from each segment needs to arrive with the same phase at the sensor, hence "phasing".)
posted by phliar at 11:16 AM on January 25 [3 favorites]


What's Next

We'll be able to get our first images of the Cowboy Universe.
posted by RonButNotStupid at 11:18 AM on January 25 [3 favorites]


NASA’s James Webb Space Telescope is at L2. What’s next?

As long as we don't set up a permanent colony there, we should be fine.
posted by Glegrinof the Pig-Man at 11:35 AM on January 25 [1 favorite]


An L2 orbit is unstable, so JWST needs to use propellant to maintain its halo orbit around L2 (known as station-keeping) to prevent the telescope from drifting away from its orbital position.

Where will it go when it runs out of fuel in (hopefully) 20 years?
posted by little onion at 11:43 AM on January 25


More of RubixsQube’s JWST comments are in dancestoblue’s earlier MF post, The Webb Space Telescope Will Rewrite Cosmic History. If It Works.

More about NIRCam (WP).

So far, so good. Godspeed, JWST!
posted by cenoxo at 11:47 AM on January 25 [1 favorite]


To add to what phliar said, when JWST first takes its images of a star for "focusing" the telescope mirrors, you could imagine that right now the mirrors aren't even close to looking in the right direction, so we'll get an image with multiple stars, each a reflection of the same star by a different mirror. It may be bad enough that some of the reflections aren't even on NIRCam's field initially. At this point, there's an entire crack team devoted to moving the individual mirror segments and taking images to determine which star image corresponds to which mirror segment, and then slowly moving everything into place such that the mirror segments are properly aligned. It's an incredibly complicated task, but a few years ago, while JWST's mirror and instruments were in Houston, this entire procedure was tested to make sure it works.
posted by RubixsQube at 12:11 PM on January 25 [7 favorites]


Still can't say I understand the forces but at least I can kind of understand where it is.....and I guess L2 being further from the Sun makes sense given the optics and what not....

You have to factor in the speed things are moving at. Without any other forces acting on a satellite, e.g. in truly empty space, it will keep moving in a straight line. Gravity obviously acts as a force that does change a satellite's path. If you throw a ball sideways, it falls in an arc; the force you threw it with keeps it moving, gravity pulls it down (bending the path), and eventually it hits the ground. If you threw it hard enough (ignoring the air) it would be going so fast the arc never actually hits the ground. Throw it really really hard, and it would escape from earth's gravity and fly away entirely; not enough, and it eventually arcs lower and lower, and crashes. In between, it would be in orbit, i.e. going in a stable elipse or circle round the earth; fast enough to keep going forever, but too slow to actually escape. Getting into orbit is mostly about going really fast, rather than going 'up' (that's the easy bit) - just as when you throw a ball straight up, it just comes straight back down again.

Obviously we don't put satellites in orbit too near the surface, because air resistance does in fact exist, and you need to keep thrusting to keep your speed up and avoid having your orbit decay. In fact, the space station in low-earth orbit does actually slow down a bit precisely because of the small amount of air resistance at that height - about 2km slower a year - which means it needs periodic boosts to speed it back up. I believe it's usually done from thrusters on supply craft, though the Zvezda service module also has some; the ISS has to change its orbit a bit periodically to avoid chunks of debris also in orbit.

For the JWST, it's effectively orbiting the sun, not the earth; but earth's gravity is also sufficiently large that it goes from being a two body problem (satellite+earth) to a three body one; sun+earth+satellite. The maths gets really complicated really fast, but the lagrange points for the earth&sun are basically where the three things pretty much cancel out, i.e. the gravity from both bodies, plus the very high speed and direction you're going gives a stable(ish) orbit around the sun. In the case of L1, that lies between the earth and the sun so is great for sun observatories. L2 is further out, so it needs to be going faster - but the path it needs to follow is longer, so it balances out, and it stays in the same place relative to earth as the earth rotates around the sun, which is handy for communication. This is handy for JWST, because the infrared detectors need to be REALLY cold - a few degrees above absolute zero. The sun is very, very hot, but there's also heat reflected from the earth (and moon); at L2, the JWST can keep the big sunshield facing the sun AND the earth/moon at all times so the instruments on the other side can be really cold, and really sensitive to incredibly faint signals with a great view of deep space.

L1, L2 and L3 are metastable; move away, and you need to thrust to get back or you just keep going away, and they move around a bit not least as the solar system does of course have more than just the earth and sun in it, though they're the biggest players. JWST is actually orbiting the L2 point itself, as that's easier to do than try and stay exactly on it. And it does need to thrust periodically to adjust that orbit, and stay there. And of course, to point the thing at where you want to look at.

Where will it go when it runs out of fuel in (hopefully) 20 years?

It will fall away from the L2 point. It will likely remain in a solar orbit - kinda hard not to - but it will no longer be stable relative to earth. It will also be unable to rotate itself to keep the ultra cold half pointing away from the sun&earth, at which point the detectors will be blinded permanently. If it's still being useful before that point, a robotic fuel resupply mission is doable, though not currently planned for.
posted by Absolutely No You-Know-What at 12:33 PM on January 25 [9 favorites]


One other piece I picked up from videos is that the precise center of L2 is in the earth's shadow at all times. Which is not good for a solar powered instrument! So part of why the halo orbit is so wide around L2 is to let the solar panels on JWST generate power. I'm a bit confused about this, because I've also read JWST wants shading from the earth to help it stay cool (along with its own shielding). Does the Earth have a sort of fuzzy shadow it's hanging out in or something?
posted by Nelson at 12:37 PM on January 25


Does the Earth have a sort of fuzzy shadow it's hanging out in or something?

At the distance L2 is, the earth isn't quite big enough to completely shade the sun, you have a sort of halo - about 85% of the sun is blocked out, IIRC. Rather like when the moon passes in front of the sun (solar eclipse), you can still see the corona as a 'ring of fire'.
posted by Absolutely No You-Know-What at 12:43 PM on January 25 [3 favorites]


Media Briefing: What’s Next for the James Webb Space Telescope - YouTube - yesterday's NASA teleconference.

It takes six months to orbit L2. They're waiting for things to passively cool down (and keeping some things warm). Then adjust mirrors with a star who's name I forget and then tweak to align the phase. Once that's done they'll release all the data collected during the alignment phase. Station keeping burns are roughly every 21 days as they orbit around L2 on a *gahhh* Pringles shaped gravity well. The burn came a day later than scheduled because they could get a more precise orbit. They didn't even technically have to do the burn and would have gone into L2 orbit anyway.

Rather interesting for a teleconference, they have some funny people on the line.
posted by zengargoyle at 12:49 PM on January 25


On orbiting L2:

The video shows it orbiting 90 degrees perpendicular to the sun, a vertical circle compared to its orbiting place around the sun. How does that work, seeing that part of its orbit it'll be going faster than it's orbiting the sun, and the other half it will be going slower, repeating ad infinitum? Is it using it's jets to keep itself orbiting?

Or, does L2 act like it has gravity itself, like JWST is literally orbiting a point in space which keeps pulling the JWST toward it but it's too fast to actually fall inward?
posted by AzraelBrown at 1:27 PM on January 25


Or, does L2 act like it has gravity itself, like JWST is literally orbiting a point in space which keeps pulling the JWST toward it but it's too fast to actually fall inward?

It's more like the spacecraft is moving too quickly for its orbit. To stay in the L2 area a spacecraft needs to be going around the sun faster than the Earth. If the Earth wasn't there JWST would become heliocentric but its orbit would go outwards. There's a balance of the centrifugal force of the speed of the orbit pushing outward combining with the two sources of gravitational attraction bringing it back in.

Then from the procession of the Earth around the sun the spacecraft is also going to experience a Coriolis force. If I put you on the edge of a merry-go-round and ask you to throw a ball the ball will be straight by your reckoning but if we look down from the top at the path it travels, it's very much curved. Same thing happens here. The Earth is going to be pulling on the craft, the craft accelerates towards the Earth, it comes closer to the Earth but it'll take a curved path inward. Then it's going too fast, centrifugal force starts winning, it takes a curved path back outwards. Bam, a halo orbit.

It is dynamically unstable though so small pertubations eventually cause anything at the L1, L2, or L3 points to eventually lose their equilibrium and drift out of the area. In the cast of the JWST, it only needs a 3 minute burn every 20 days or so to keep in its orbit.
posted by Your Childhood Pet Rock at 1:49 PM on January 25 [7 favorites]


In the teleconference you can hear the mirth in the scientists voices describing the L2 orbit as Pringle shaped. Unstable, but only mildly so. Lagrange point - Wikipedia.
posted by zengargoyle at 5:33 PM on January 25 [1 favorite]


It’s just a tiny white speck, but the Virtual Telescope Project captures a view of the James Webb Space Telescope at its final destination [*], Space.com, Elizabeth Howell, January 25, 2022 – It's orbiting in the bowl of the Big Dipper, from the perspective of Earth.

*James Webb Space Telescope: a new image – 24 Jan. 2022; The Virtual Telescope Project, Gianluca Masi, 01/25/2022 (large image).
posted by cenoxo at 10:02 PM on January 25 [1 favorite]


wow, nircmd really can do everything.
posted by Clowder of bats at 6:41 AM on January 26


Waiting in the wings after JWST is the Nancy Grace Roman Space Telescope (Goddard Space Flight Center). More about its mission at Astrobiology in NASA > WFIRST > Roman Space Telescope:
The Nancy Grace Roman Space Telescope – or Roman Space Telescope, for short – is a NASA observatory designed to settle essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The Roman Space Telescope is currently planned for launch in the mid-2020s. The telescope was initially developed as the Wide Field InfraRed Survey Telescope (WFIRST), and renamed in 2020 to honor Nancy Grace Roman, NASA’s first Chief of Astronomy. Roman has been called the “mother” of NASA’s Hubble Space Telescope.

Mission Overview
The Roman Space Telescope [*] will have a field of view that is 100 times greater than the Hubble infrared instrument, capturing more of the sky with less observing time. As the primary instrument, the Wide Field Instrument will measure light from a billion galaxies over the course of the mission lifetime. It will perform a microlensing survey of the inner Milky Way to find ~2,600 exoplanets. The Coronagraph Instrument will perform high contrast imaging and spectroscopy of dozens of individual nearby exoplanets. The Roman Space Telescope is designed for a 6 year mission, and will launch on a EELV out of Cape Canaveral.
Additional details in The End of Cosmic Dark Ages: How NASA’s Roman Space Telescope Could Expand on Hubble’s Deepest View, SciTechDaily, Ashley Balzer/Goddard Space Flight Center, January 21, 2022.

*The wide-angle IR telescope in WFIRST/NGRST was apparently meant for an unbuilt terrestrial spy satellite, but the National Reconnaissance Office donated it (and a second unused telescope) to NASA in 2012. See the article NASA gets two military spy telescopes for astronomy, Washington Post, Joel Achenbach, June 4, 2012 [alternate 12ft.io link].
posted by cenoxo at 9:16 AM on January 26 [1 favorite]


Do other probes and satellites at L2 have enough collective mass to impact each others orbits? Say, to the extent that they would need to reduce their operational lifespans in order to manage their orbit around the L2 point?
posted by They sucked his brains out! at 12:08 PM on January 26


So… hearing “we used a lot less fuel than we thought we would” makes me think that they may have gotten uncomfortably close to the other safety margin.
I haven't heard an explanation for this but my suspicion is that they budgeted for correcting other parameters of the trajectory as well but mostly needed to just correct for the deliberate undershoot.
posted by mscibing at 5:06 PM on January 26


🎵 Home, home on Lagrange 🎵
posted by rhamphorhynchus at 6:21 PM on January 26 [7 favorites]


Do other probes and satellites at L2 have enough collective mass to impact each others orbits? Say, to the extent that they would need to reduce their operational lifespans in order to manage their orbit around the L2 point?

No. The Earth pulls on the JWST with a force about a newton. Two JWSTs within a meter of each other would pull on each other with a force of 0.0025N. At 1000km apart it's 0.000000000000003N. Other satellites that would be up there would be over 100,000km apart the great majority of the time.
posted by Your Childhood Pet Rock at 7:34 PM on January 26 [1 favorite]


Much obliged! Always wondered how they manage to get all these probes into one space without affecting one another.
posted by They sucked his brains out! at 8:27 PM on January 26


Mirror alignment actuators

I didn't find much detail about how the mirror actuators are built to allow such tiny, precise movements, of less than 10 nanometers. How would that work!?

The only information I could find is from this interesting and readable pdf copy.

Cryogenic Nano-Actuator for JWST
(pdf link)

Some of the astonishing requirements, from the pdf:

A fine step size of less than 10 nanometers. There are a million nanometers in a millimeter!
A coarse range of greater than 20 mm (mostly for the unlocking from launch setup, I think.)
Operation at 30 Kelvin temperature.
Size 138.8mm length, 700 grams total weight.

And they were able to use the same motor for both fine and coarse adjustments.

The report mentions "flight actuator fabrication is scheduled to begin in the last quarter of 2006." 144 of them.
posted by jjj606 at 9:06 PM on January 26 [2 favorites]


First (cross-eyed) images, as spoken about excitedly by Dr. Becky.
posted by clawsoon at 9:58 AM on February 11 [2 favorites]


Naff enters my appropriated vocabulary of slang.
posted by zengargoyle at 12:42 PM on February 11


jjj606, you might have seen this since you asked about the mirror actuators, but here's a good walkthrough of how they function.
posted by samw at 7:16 PM on February 11


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