James Webb Telescope

[RecoveringYuppy]

"This orbit (which takes Webb about 6 months to complete once) keeps the telescope out of the shadows of both the Earth and Moon. "

Yeah, but not for solar power reasons. Again, you could read the sources cited.

 

[RecoveringYuppy]

Huh. If that's true, I wonder why they chose solar panels as a power source. Couldn't they have gone with a nuclear source?

It's not true. L2 gets sunlight and they could have designed it be right at L2 if there was a good reason to. They want it where it is for consistent temperature. Getting more sunlight is a secondary concern.

 

[Mike Helland]

It's not true. L2 gets sunlight and they could have designed it be right at L2 if there was a good reason to. They want it where it is for consistent temperature. Getting more sunlight is a secondary concern.

Consistent sunlight leads to consistent temperature, and consistent power.

 

[Puppycow]

So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?

ETA: and also it would give off waste heat, and that might be a problem for some of the instruments that have to be kept very cold?

 

[RecoveringYuppy]

So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?

Sort of. Both nuclear and solar would have very long life times. So the reason nuclear wouldn't help extend the lifetime is because solar potentially lasts forever too.

Thruster fuel is the planned (or should I say hoped for) limitation, though.

ETA: and also it would give off waste heat, and that might be a problem for some of the instruments that have to be kept very cold?

Possibly. But it would presumably be on the "bus" side of the heat shield, not the instrument side. Not sure how big a problem that would actually present.

 

[Puppycow]

This video is about the engineering behind the telescope. It has a click-baity title, but the video itself is quite good.

https://www.youtube.com/watch?v=aICaAEXDJQQ

 

[Puppycow]

This one is called "JAMES WEBB TELESCOPE Orbit & Trajectory Explained - Where Is It Flying To?" A 6-minute video.

https://www.youtube.com/watch?v=Dxl3nXbOJzQ

 

[Mike Helland]

A little over 12 hours in, and it completes an important step:

https://blogs.nasa.gov/webb/2021/12/25/the-first-mid-course-correction-burn/

At 7:50 pm EST, Webb’s first mid-course correction burn began. It lasted 65 minutes and is now complete. This burn is one of two milestones that are time critical — the first was the solar array deployment, which happened shortly after launch.

This burn adjusts Webb’s trajectory toward the second Lagrange point, commonly known as L2. After launch, Webb needs to make its own mid-course thrust correction maneuvers to get to its orbit. This is by design: Webb received an intentional slight under-burn from the Ariane-5 that launched it into space, because it’s not possible to correct for overthrust. If Webb gets too much thrust, it can’t turn around to move back toward Earth because that would directly expose its telescope optics and structure to the Sun, overheating them and aborting the science mission before it can even begin.

Therefore, we ease up to the correct velocity in three stages, being careful never to deliver too much thrust — there will be three mid-course correction maneuvers in total.

After this burn, no key milestones are time critical, so the order, location, timing, and duration of deployments may change.

 

[Samson]

I really wish NASA would stop doing this prototype-only stuff. If we can justify designing a single sun-shaded telescope operating at a lagrange point a million kilometers from Earth, why not send three and have a bit of redundancy?

I think this is right.
Eta an early thread post and the emergence of trillion dollar tech companies says this current approach with a small budget is tragic. I hope to be very wrong.

 

[BowlOfRed]

Not a conspiracy, but kind of eye opening:

https://www.space.com/16000-spy-satellites-space-telescopes-nasa.html

That was several years ago. One will be used for the Nancy Grace Roman telescope.

https://www.nasa.gov/feature/goddard/2020/primary-mirror-for-nasas-roman-space-telescope-completed. Probably didn't end up saving much (if any) money, but there was a lot of pressure to try to come up with a use.

 

[BowlOfRed]

So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?

Both for occasional stationkeeping and for angular desaturation. Magnetic fields are too weak to use magnetic torque (like Hubble), so it has to be thrusters.

 

[Mike Helland]

Both for occasional stationkeeping and for angular desaturation. Magnetic fields are too weak to use magnetic torque (like Hubble), so it has to be thrusters.

So, how many times in a month will the thrusters fire?

 

[BowlOfRed]

So, how many times in a month will the thrusters fire?

Only a couple. The spacecraft is constrained in where it can be for everything to work. It is planned to fire them every 21 days.

The better the orbit is modeled, the less fuel will be used. But the radiation pressure on the sun shield complicates things. And even the planned observation targets (which change the orientation of the shield) are not expected to be known that far in advance. So fuel use is expected to be greater than than what would be required for an "optimal" orbit.

https://ntrs.nasa.gov/api/citations/20140007519/downloads/20140007519.pdf

 

[Pixel42]

https://mobile.twitter.com/marinakoren/status/1474367236244750345

Unlike Hubble, Webb isn't designed to be fixed by astronauts. But it can be refueled robotically. Zurbuchen says that "once this telescope is deployed, I'm going to put all the effort towards developing that technology, and so within the 10-year lifespan, we can go refuel it."

A few days ago I asked Bill Ochs, JWST's project manager at NASA, what he would say if Elon Musk offered to try to refuel JWST someday, which would be a very Musk thing to say. Ochs said, "Yeah, go for it."

 

[Puppycow]

Well if it's working as it should, a refueling mission might be well worth it.

If there are other problems than simply needing more fuel though, it might not be worth it.

 

[welshdean]

Wasn't it stated upthread that there is no 'dock'? How can it be refuelled if that is the case?

 

[Darat]

Would have thought better to start on a JW Mk 2 than refuel a 10 year old Mk 1.

 

[Ziggurat]

Yeah, but not for solar power reasons. Again, you could read the sources cited.

No, actually solar power is relevant here.

https://en.wikipedia.org/wiki/Lagrange_point#L2

"For example, the angular radius of the sun as viewed from L2 is arcsin(695.5×10³/151.1×10⁶) ≈ 0.264°, whereas that of the earth is arcsin(6371/1.5×10⁶) ≈ 0.242°. Looking toward the sun from L2 one sees an annular eclipse. It is necessary for a spacecraft, like Gaia, to follow a Lissajous orbit or a halo orbit around L2 in order for its solar panels to get full sun."​

So yeah, you still get sunlight at L2, but you get a lot less.

Note also that while L2 is unstable, it's a saddle point instability, not a maximum. If you displace from L2 perpendicular to the sun-earth-L2 direction, then the net gravitational force pushes you back towards L2, it's only displacements along the direction of sun-earth-L2 which are unstable. So you can orbit L2 for free, as long as that orbit is perpendicular to the sun-earth-L2 direction. Which is what they're doing, and which then allows them to get full sunlight. You only need fuel to correct for variations along that direction, which you would still need even at L2.

 

[Ziggurat]

Would have thought better to start on a JW Mk 2 than refuel a 10 year old Mk 1.

If the cost of a refueling mission is, say, 10% of the cost of the JW, seems like a refueling mission would be worth it. And given the cost and complexity of JW, I wouldn't be surprised if it's in that ballpark. The launch cost is a minority of the total cost.

 

[RecoveringYuppy]

Wasn't it stated upthread that there is no 'dock'? How can it be refuelled if that is the case?

It's presumably got something that can be latched on to even if it's not a traditional "dock" such as the ISS would have. Presumably some of the structure that mates it to the launch vehicle is sturdy enough to support a docking.

What I can't find is a reliable statement that the fuel tanks have a robotic compatible way of refilling. There is an alternative that something could dock and then take over the positioning of the satellite. I don't know what complications there might be with that approach.

And sending a human presumably changes the cost by a lot, not to mention the risk.