The James Webb Space Telescope

[Jonathan Scott]

We (i.e. I) fond to easy to forget the relevance of some aspects of big projects. The L2 position is not stable so the situation is very different from setting a course for orbiting round a massive object. The inverse law for GPE of a target planet provides a certain amount of help for getting into orbit, once you are near. There is no such central force around L2 so I imagine everything needs much more precision and you have to make your own quasi orbit around L2 (to avoid being eclipsed by Earth).

There are at least quasi-stable "Halo orbits" around L2 as described in the Wikipedia article Lagrange Point. These can be quite extended to avoid eclipsing.

L4 and L5 would probably be less problematical in that respect. L4 and L5 probably have their own disadvantages. Those points are a long way away (many minutes of signal path delay) and a much longer journey time.

The L2 point is relatively close but enables the sun shade to hide all of the sun, the Earth and the moon at the same time.

 

[Borg]

155,000 km, 10.7% - but that's 10% of the distance not the time. The telescope is fast early on and slows down as its distance to Earth increases.

The cruising speed yesterday was in excess of 2.5 miles/second. Today, it's down to less than 1.1. It's going to take a month to get to L2 at this rate.

 

[sophiecentaur]

The cruising speed yesterday was in excess of 2.5 miles/second. Today, it's down to less than 1.1. It's going to take a month to get to L2 at this rate.

Just like chucking a ball into the air!

 

[pinball1970]

Six and a half hours, Tracking Webb!



The shorter version.

 

[hutchphd]

QUERY:
Does anyone know whether the length of the first correction burn (65 minutes) was of an "expected" duration? How much of the total onboard fuel inventory does that represent?

 

[Jonathan Scott]

QUERY:
Does anyone know whether the length of the first correction burn (65 minutes) was of an "expected" duration? How much of the total onboard fuel inventory does that represent?

A Google search on JWST mcc-1a duration gives this text:

The first, MCC-1a, is expected to be a long, continuous burn (potentially up to 3 hours) performed approximately twelve hours after launch. The concept for this maneuver is to execute 95% of the nominal maneuver that would take the observatory directly into the LPO.

Reference:
https://ntrs.nasa.gov/api/citations/20140008868/downloads/20140008868.pdf

 

[Jonathan Scott]

JWST has an unusual constraint that it can't slow down, only speed up, because the delta-V thrusters in the current configuration are only at the back end, and it can't turn round without exposing sensitive components to the sun. So it has to be very careful not to speed up too much.

 

[hutchphd]

A Google search on JWST mcc-1a duration gives this text:
Reference:
https://ntrs.nasa.gov/api/citations/20140008868/downloads/20140008868.pdf

Wow that is a very complete treatment. Thanks much.

 

[valenumr]

QUERY:
Does anyone know whether the length of the first correction burn (65 minutes) was of an "expected" duration? How much of the total onboard fuel inventory does that represent?

I'd like to know as well!

 

[valenumr]

I'd like to know as well!

Oops..
Didn t get to the reference before I posted. Seems like 65 minute burn is really good.

 

[pinball1970]

The cruising speed yesterday was in excess of 2.5 miles/second. Today, it's down to less than 1.1. It's going to take a month to get to L2 at this rate.

1 last night and 0.83 this morning GMT.
29 days to reach L2. 26 to go!

 

[mfb]

417,000 km - it crossed the orbital distance of the Moon. Sunshield deployment should start in about half a day and take several days.

 

[hutchphd]

From NASA yesterday
At 7:20 pm EST – 60 hours after liftoff — Webb’s second mid-course correction burn began. It lasted 9 minutes and 27 seconds and is now complete. This burn is one of three planned course corrections to put the telescope precisely in orbit around the second Lagrange point, commonly known as L2.

 

[sophiecentaur]

it can't turn round without exposing sensitive components to the sun.

Just like the management, it's keeping its backside covered.

 

[hutchphd]

And the sunshield is beginning its fan dance: https://blogs.nasa.gov/webb/2021/12...-beginning-multiple-day-sunshield-deployment/

 

[Imager]

This link below gives distance traveled and a brief summary of what's happening:

Where is JWTS!

 

[anorlunda]

I just saw an announcement that temperature data from JWST is now available.

Does anyone have a link to get access to the real time data?

 

[berkeman]

Does anyone have a link to get access to the real time data?

Different from the "Where is..." link above your post?

ADD -- The start of the info about the temperature data:

 

[anorlunda]

Sorry, I should have linked this.

 

[valenumr]

According to the official blog page, after the two initial mcc burns, there is enough remaining propellent to substantially exceed 10 years of service life!

 

[mfb]

4 days 8 hours after launch:

We already have a large difference between hot and cold side.

Next step is the deployable tower (edit: already in progress), increasing the separation between spacecraft and sunshield before the latter is expanded.

 

[finney]

Webb telescope will be looking deep into the universe (back into earlier universe) so if someone is looking at the milky way from the farthest known galaxy would they think they were seeing an earlier universe?

 

[russ_watters]

Webb telescope will be looking deep into the universe (back into earlier universe) so if someone is looking at the milky way from the farthest known galaxy would they think they were seeing an earlier universe?

Yes, they would be seeing an earlier form of the Milky Way.

 

[anorlunda]

4 days 8 hours after launch:

View attachment 294873

We already have a large difference between hot and cold side.

Next step is the deployable tower, increasing the separation between spacecraft and sunshield before the latter is expanded.

Very cool (pun intended.) Where did you find that data?

 

[mfb]

On the where is Webb? page that has been linked multiple times.Deployable Tower Assembly is in progress.

 

[KurtLudwig]

I tried finding answers to the below questions on NASA's website and on Wikipedia, but was unable to:

1. How is JWST pointed? There does not appear to be any mechanism, as on a terrestrial telescope. Is the whole telescope with heat shield just rotated and tilted a limited number of degrees? I noticed that the heat shield is elongated, allowing it to be tilted up to 90 degrees and still being shielded from the sun. In the specs of JWST the minimum adjustment is 0.1 arc seconds.

2. I read that the JWST is orbiting the L2 Lagrange point. At what angles is that orbital plane relative to the line of centers of the sun and earth? Is the orbit perpendicular to that line of centers or some other angles?

3. What is the radius of that orbit?

 

[hutchphd]

https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Orbit

 

[mfb]

Momentum flap deployed, sunshield cover removed. We are passing many of the single points of failure now.

The cold side has warmed up a bit since my last post, it's now -45 degrees at the mirror and -145 degrees at the instruments. I expect it to cool down again once the sunshield deployment starts.

1. How is JWST pointed?

The telescope part rotates/tilts, the sunshield part stays where it is because it needs to keep shielding the Sun. Reaction wheels take care of that.

An L2 halo orbit has to be perpendicular to the L2/Sun direction because only these two dimensions are an attractive potential. You can't orbit L2 along the L2/Sun direction. About 800,000 km, as you can find on the Wikipedia page.

 

[JLowe]

This has to be nerve wracking for astronomers.

 

[Borg]

NASA Plans Coverage of Webb Space Telescope Deployments​

First up is the coverage of the sunshield tensioning at 2pm EST on Jan. 2nd.