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I recently had to delete my answer to Why doesn't James Webb have a larger supply of thruster propellant? because @NgPh noted:

As the last update, JWST weights 6200 Kg. jwst.nasa.gov/content/about/faqs/facts.html

Which basically invalidates the argument I was making. I then went searching for other sources to verify this and arrived at a clashing list of stakeholders/sources (as of Dec. 16 ~15:00 UTC):

6,500 kg Camp: 6,200 kg Camp: Other (~6,200 kg):
Northrop Grumman ("main industrial partner") ESA (Last Update: 15 December 2021) JWST Mission Operations Concept Document (Jan. 13, 2006): "The current Observatory mass is 6,194 kg (MR-99)"
Canadian Space Agency (Date modified: 2021-12-15) NASA 1 James Webb Space Telescope Project Mission Requirements Document (Oct. 17, 2007) MR-99: "The JWST Observatory wet mass shall not exceed 6,159 kilograms"
NASA 2

(Note NASA 1&2 are literally the same website)

How much does JWST actually weigh?

BrendanLuke15
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    What is the mass of the JWST might be a better question, particularly once it is in orbit... – Jon Custer Dec 16 '21 at 15:27
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    It seems that its exact mass, as well as its exact launch epoch, determine its likelihood of reaching the right orbit ... – Ng Ph Dec 16 '21 at 15:40
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    To throw in a third value, this paper quotes 6330kg (not clear if wet or dry mass). – Andrew is gone Dec 16 '21 at 23:34
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    The values from 2006 and 2007 can be discarded for two reasons. One is that 6194 kg, 6159 kg and 6200 kg are essentially the same number. The other reason is that those figures are from early on in the construction of the spacecraft. Many things have changed since then. However, mass limit oftentimes is one of the things that remain constant throughout a spacecraft's design and implementation. – David Hammen Dec 17 '21 at 02:08
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    Note that ESA says 6500 kg at its eoportal.org web site. – David Hammen Dec 17 '21 at 02:10
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    I see two possible sources for the discrepancy, and there might well be others, so these are just guesses. Guess number one: The sites that say 6200 kg haven't updated since 2007. Yes, those sites say they've been updated recently, but that's mostly for launch dates. If true, this would not be the first time that NASA or ESA has carried forward numbers that are fifteen years out of date. – David Hammen Dec 17 '21 at 02:15
  • Guess number two: The sites that state the lower mass value are state that the mass is approximately 6200 kg, "including observatory, on-orbit consumables and launch vehicle adaptor." They do not say that this includes the fairing mass. The launch vehicle adaptor is inside the payload fairing. It could well be that the sites that say the payload mass is approx. 6500 kg at launch includes the mass of the fairing. The fairing is part of the mass the Ariane 5 has to contend with, but is not part of the mass the upper stage has to contend with as the fairing is dispensed with soon after launch. – David Hammen Dec 17 '21 at 02:24
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    @David Hammen, the eoportal site's link must be older than Dec 2013 (since it mentions a CDR planned for 12/2013). Therefore, the 6500 Kg reported there must be as old as this. The paper from Greenhouse pointed by Andrew is recent (2020) and gives a number (still estimated) closer to 6200 than 6500. Regardless, if there is a lack of rigor in tracking changes, from both NASA and ESA, it is a recipe for catastrophy. And that would be really a triumph for Mr Murphy. – Ng Ph Dec 17 '21 at 09:19
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    It's certainly not guess #2, which I'll live up for humble pie reasons. The fairing is an order of magnitude more massive than the ~300 kg difference between the two reported numbers. – David Hammen Dec 17 '21 at 11:28
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    @NgPh Mass properties are amongst the most closely carefully tracked and configuration managed of information pertaining to a spacecraft. All analyses involving mass is done through numbers obtained from a configuration managed database or spreadsheet. Unfortunately, NASA (and now apparently ESA) has dumbed down their outward-facing websites to the extent that they are nearly worthless. This dumbing-down of outward-facing websites has been on-going for 20 years. Those outward-facing websites are maintained by public outreach or authorized personnel. Funding is low, and the results are sad. – David Hammen Dec 17 '21 at 11:49
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    @NgPh Another dirty little secret about spacecraft: Requirements and initial design data typically don't get updated after a certain point. It almost never happens for changes made after CDR. Those post-CDR changes instead reference memoranda, detailed analyses, and sometimes even email chains. The mass properties database / spreadsheet on the other hand is constantly modified and is regularly checked with the launch provider and with many other organizations. – David Hammen Dec 17 '21 at 12:04
  • @NgPh One last thing, with regard to And that would be really a triumph for Mr Murphy. I believe you are referring to Dr. Murphy, who has multiple PhDs and multiple postdocs in fields related to chaos generation and mismanagement. – David Hammen Dec 17 '21 at 12:09
  • @David Hammen, I was referring to the well-known Edward A. Murphy and the multiple laws bearing his name. For ex. this one: if you leave the freedom to a fool to pick a number out of two possibilities and use it in a program, he/she will pick the wrong one. – Ng Ph Dec 17 '21 at 12:55
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    This Media Launch Kit gives m=13700 lbs=6214 Kg (page 15) – Ng Ph Dec 17 '21 at 13:33
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    Arianespace have now put out a press kit for the launch, which gives 6173kg as the "maximum mass at launch". – Andrew is gone Dec 22 '21 at 23:40
  • it seems to me that the various stakeholders talk to a mass value that is germane to their particular aspect of participation. (no news here!) We've had dry weight, wet weight, JWST with adapter, etc etc. If I correctly read the interest here... it is "JWST mass at halo orbit" that is wanted. Of course, this mass value will 'start' at a maximum following the Jan 24 2022 MCC-2 insertion burn and slowly decrease from that max with every station keeping burn. Does this about sum it up?? – BradV May 15 '22 at 14:32
  • @BradV I was thinking launch mass (w/o adapter) – BrendanLuke15 May 15 '22 at 14:35
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    OK... got it. Your question is not really 100% aligned with my interest. I wonder how much mass (if any) was shed after all deployments were completed. – BradV May 15 '22 at 14:48
  • As @DavidHammen said, somewhere inside NGC, NASA, Ariane, is a controlled document that has the exact mass and all related assumptions and uncertainties. That document may or may not have informed one of the many public-facing sources. However those public-facing sources will never actually call out the document. Unfortunately that means that it will likely remain impossible to provide an exact and authoritative answer to the question. – Carlos N May 17 '22 at 15:59
  • Mark Twain once said "“There are three kinds of lies: Lies, Damned Lies, and Statistics”. You can manipulate statictiscs. Some inlcude the mass of the propellant, others don't. Some use wet mass, other use dry. The list continues. So, whose right? All of them. They all answer slightly different questions. @BrendanLuke15 – Starship - On Strike May 18 '22 at 11:00
  • I might trust this tweet from Spaceflight Now editor Stephen Clark at 6161.4 kg – BrendanLuke15 May 18 '22 at 12:13
  • super info in tweet! I assume that weight includes water molecules that migrate into permeable materials (such as the kapton sheeting used for sunshield). This water eventually migrates to space, slightly decreasing JWST mass at halo. – BradV May 18 '22 at 16:37

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