The James Webb Space Telescope

[Office_Shredder]

I would normally respond with something about the lower cost of unmanned stuff but JWST costa packet.

https://www.planetary.org/space-policy/nasa-budget

Funding varies year-to-year, but generally about 50% of NASA's annual budget is spent on human spaceflight activities, 30% on robotic missions and scientific research, and the remainder split between aeronautics, technology development programs, staff salaries, facilities management, and other overhead.

 

[BWV]

I would normally respond with something about the lower cost of unmanned stuff but JWST costa packet.

Well it would have been considerably more expensive to put a crew on the JWST

Seriously, do contingency plans exist for a manned mission out to L2 for emergency servicing? Hubble had to be repaired by a human back in the 90s.

 

[Borg]

Well it would have been considerably more expensive to put a crew on the JWST

Seriously, do contingency plans exist for a manned mission out to L2 for emergency servicing? Hubble had to be repaired by a human back in the 90s.

No - see my post above. There may be a robotic mission someday to replenish fuel but that would be more than 10 years from now.

 

[sophiecentaur]

but that would be more than 10 years from now.

Apparently, they used up less fuel than anticipated and the latest estimate is a 20 year lifespan. Fab or what?

 

[hutchphd]

This calibration capability was built specifically because of the previous Hubble mirror flaw.

I believe it was required because of the nature of the novel segmented berylium miorror. Do you have a reference for this particular assertion?

 

[Borg]

I believe it was required because of the nature of the novel segmented berylium miorror. Do you have a reference for this particular assertion?

It's from the video that I referred to in my earlier post. It depends on how you interpret that. Saying that it was specifically built because of the Hubble mirror flaw was my wording and may be a bit over-stated. However, the video goes into great detail about the issue with Hubble and how the engineers did not want a repeat of that since they wouldn't be able to repair this one with a mission.

 

[JLowe]

Also, remember that the problem with Hubble was that the mirror was fabricated incorrectly, and the problem wasn't caught because it was never completely tested on the ground. That's one of the reasons Webb was delayed so long, because they did extensive testing on the ground.

Yes, and I'm glad they did. I'm only saying, that's a lot of moving parts that have to work.

 

[valenumr]

When you throw a stone up in the air, it goes slower and slower as it approaches the high point. That experiment on Earth is very nearly under constant gravitational force and the variation of the g field near L2 is a bit more complicated but the same principle applies; Kinetic Energy turns into Gravitational Potential Energy and, at L2, the GPE changes are very small so KE has to be low.
So why not manoeuvre JWST into place more aggressively? The design choice was to avoid retro thrust, which would involve facing back towards the Sun and frazzling equipment. When you can only 'push' you have to be very gentle with these of the engines and make it arrive in the desired spot traveling verrryy slowly (= a long time to get there).

It's quite comparable to rolling a ball up a hill, only we don't want to hit the peak at L2. If it drifts past, it will just continue to drift further out. The propellent will be used now and again to roll Webb back close to the peak from time to time (among other things) as it falls back toward earth. Perhaps we should call it Sisyphus (this is pretty standard for the many other observatories at L2).

 

[sophiecentaur]

It's quite comparable to rolling a ball up a hill,

Or the old pub game of shove ha'penny; just one go to get the coin up to the line and not a nanometre over it.

 

[hutchphd]

Except this game is 3D. I likethe graphics here
https://en.wikipedia.org/wiki/Halo_orbit

 

[mfb]

Mirror alignment has begun. All actuators work, now it's a matter of getting all of them to the right positions.

 

[Oldman too]

Some great links on upcoming programs.

https://www.cmu.edu/mcs/news-events/2021/1220_webb-telescope-walker.html

https://www.stsci.edu/jwst/science-execution/program-information.html?id=2608

https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-go

https://www.scientificamerican.com/...r-of-extraordinary-science-has-been-revealed/

 

[mfb]

T+3 weeks

Where is Webb added a mirror position tracker. The launch position was -12.5 mm, the nominal position is 0 mm. The mirrors move by about 1 mm per day, except A6 and A3 which will be moved separately later. About a week to go for most segments.

The cold side is now below -200 C.
Edit: I found an expected cooldown timeline

 

[sophiecentaur]

The mirrors move by about 1 mm per day,

That's some very low gearing, I think. But things, in space, tend not to be intuitive.

 

[Grinkle]

https://blogs.nasa.gov/webb/2022/01/13/mirror-mirroron-its-way/

"At full speed, it takes about a day to move all the segments by just 1 millimeter. It’s about the same speed at which grass grows!"

 

[mfb]

That's some very low gearing, I think. But things, in space, tend not to be intuitive.

The actuators move in steps of a few nanometers - the precision needed for the mirror alignment. A centimeter is millions of steps. To keep the operation simple they move one at a time, too.

Sure, it would have been possible to add some extra actuators for the coarse alignment, but it would have been useless extra complexity. Cooling down JWST will take longer anyway, two weeks to move the mirrors is not an issue.

-5 mm

 

[BWV]

The actuators move in steps of a few nanometers - the precision needed for the mirror alignment. A centimeter is millions of steps. To keep the operation simple they move one at a time, too.

Sure, it would have been possible to add some extra actuators for the coarse alignment, but it would have been useless extra complexity. Cooling down JWST will take longer anyway, two weeks to move the mirrors is not an issue.

-5 mm

so when fully deployed, the viewing time is scheduled based to some extent on the direction the telescope points as the Earth orbits the sun? What about pointing the thing perpendicular to the orbital plane? That would take weeks to accomplish?

 

[phyzguy]

so when fully deployed, the viewing time is scheduled based to some extent on the direction the telescope points as the Earth orbits the sun? What about pointing the thing perpendicular to the orbital plane? That would take weeks to accomplish?

No. The whole telescope/sunscreen rotates driven by reaction wheels. This can happen quickly. At any point in time it can see nearly half the sky. See the pictures I posted in post #117.

 

[mfb]

Most mirrors have reached their nominal position, A3 and A6 have moved by half of the distance.
NASA's 1 mm/day estimate was very conservative.

 

[Borg]

And soon, the L2 insertion burn will be occurring - 94% of the way there today.

 

[Oldman too]

https://ntrs.nasa.gov/citations/20190002647
150 Pages of "fun facts" on Space Mirror Tech and related cool stuff.

https://blogs.nasa.gov/webb/ :)

 

[russ_watters]

So, what is Webb doing today? Where is Webb says it arrives at L2 with a velocity of 0.135 mi/s around 2pm EST. I've googled and found conflicting answers about whether there is a burn. Is there one?

It would need to have a thruster pointing "up" to do a burn to slow down, but wouldn't that be generating a lot of heat on the "cold" side?

https://www.republicworld.com/scien...-before-orbit-insertion-read-articleshow.html

https://space.stackexchange.com/questions/57702/james-webb-orbit-insertion

 

[Grinkle]

I see this from NASA.

https://blogs.nasa.gov/webb/2022/01/21/webbs-journey-to-l2-is-nearly-complete/

"On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short."

 

[Oldman too]

Houston, we have L2 Orbit. Flawless!

 

[russ_watters]

I see this from NASA.

https://blogs.nasa.gov/webb/2022/01/21/webbs-journey-to-l2-is-nearly-complete/

"On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short."

Thanks, so my questions are:

1. What direction relative to the spacecraft was the burn? (where was the thruster?)
2. Did it generate heat that could be problematic for the cold (cooling) instruments?

In addition:
"Where is Webb" still shows a cruising speed of 0.1255 mi/s (presumably the orbital speed?) and quite a large curve to orbit insertion (graphical license or freefall path?). And what's the orbit diameter? What are the corrections going to look like (perpendicular delta-V?)

 

[mfb]

It would need to have a thruster pointing "up" to do a burn to slow down, but wouldn't that be generating a lot of heat on the "cold" side?

It came in sideways, the thrust was somewhat in the general direction of the Sun. It's a three-dimensional problem and thinking of just slowing down/speeding up isn't representing the geometry of the orbit. Here is a good 2D projection. The displayed velocity is probably the velocity relative to L2, which is non-zero as it orbits L2 now.

 

[russ_watters]

It came in sideways, the thrust was somewhat in the general direction of the Sun. It's a three-dimensional problem and thinking of just slowing down/speeding up isn't representing the geometry of the orbit. Here is a good 2D projection. The displayed velocity is probably the velocity relative to L2, which is non-zero as it orbits L2 now.

You might have been writing that while I was writing the prior post. That's the graphic I was referring to. So, it's fairly accurate? Googling for the topography of the Lagrange points, it is difficult to see how it is pulled into a roughly circular if unstable orbit (per the graphic). For example:
https://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_points2.svg

Anyway, it hadn't occurred to me that the reported speed and distance weren't along the same line. Though I guess at first they were closer to it.

 

[phyzguy]

This site says that, "Webb's orbit is represented in this screenshot from our deployment video (below), roughly to scale; it is actually similar in size to the Moon's orbit around the Earth! This orbit (which takes Webb about 6 months to complete once)...". If we assume that the "orbit" around L2 is circular with a radius of 400,000 km (moon's orbit SMA is 380,000 km), and that it takes 180 days to complete, then the speed would be 0.16 km/sec, and its current speed is 0.20 km/sec. So it all makes sense.

 

[Oldman too]

 

[sophiecentaur]

1. What direction relative to the spacecraft was the burn? (where was the thruster?)

In the same way as getting to the right distance without retro firing, starting off (early deployment) can give the craft the right 'lateral' velocity so that it will be doing 30 days' worth of 'corkscrew' motion on the way. It will ideally not lose any of that translational / orbital energy as it slows down due to (radial with respect to Earth and Sun) potential energy. If the orbit around L2 takes six months (?) then it will have done around 360/6 = 60 degrees of corkscrew on the journey and arrived 'hanging there' and gently moving around in a large ellipse.

So it all makes sense.

(Yep. After a bit of brain ache and arm waving.)
Navigating is space does have some advantages in that there's no wind, air pressure or rain to deal with so it's more predictable.

 

[mfb]

You do have radiation pressure and solar wind. They can lead to larger trajectory uncertainties, especially with uncontrolled spacecraft . An old Falcon 9 upper stage might hit the Moon in March. It launched a NASA spacecraft to L1, somewhat similar to Ariane 5's launch of JWST to L2. People are studying it to get a better estimate. If it hits then the impact will be used to study the Moon, just like the many deliberate impacts in the past.

 

[sophiecentaur]

Ah yes. Solar wind does vary too. It will always be outwards tho’ and must affect station keeping a bit.

 

[Devin-M]

The way I heard it described was the launch rocket gave it slightly less velocity than was needed to reach the final orbit, so rather than turning around and thrusting to slow down (which would expose the mirror and instruments to the Sun’s heat), the final burn had to add a small amount of speed in the direction it was already moving (the telescope has been slowing down continuously relative to Earth since launch).

“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. “

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

 

[Arjan82]

You know, it just dawned on me that I finally understand the Dune song:
"
I was sent to
outer space
To find another happy place
Now I'm left here all alone
Million miles away from home
"
It was about Webb!

 

[Arjan82]

There is a new video out from Launch Pad Astronomy explaining the Web orbit really well (I think):