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Thinking about this question, I wanted to look at historical data. Then I noticed that while Celestrak does provide Archived TLE data for Salyut 7, Mir, Kvant-1, Kvant-2, Kristallm, Spektr, Priroda, and ISS (Zarya), it does not provide data for the recent Tiangong-1 and Tiangong-2 stations, nor for Skylab which was in orbit from 1973 to 1979.

Why not?

I'm wondering if there are any governing international agreements, policies, courtesies or considerations involved, or if this is just random happenstance.

edit: For this particular question which has remained unanswered for a month, I'll be interested in thoughtful speculation, if that's what it takes to address this.

Celestrak Archived TLEs

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    $\begingroup$ No idea about the Tiangongs, but I note that Celestrak was established in 1985, at which point Salyut 7 was up, but Skylab was not. The Archived TLE data page says 1980-2004, but there's a link to request earlier data. $\endgroup$ Commented Aug 22, 2018 at 3:37
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    $\begingroup$ Also, took the liberty of correcting your Skylab date. $\endgroup$ Commented Aug 22, 2018 at 3:40
  • $\begingroup$ @RussellBorogove Yikes, thank you! Yes, it was 1979 when I was wearing my "Skylab is Falling" tee shirt. $\endgroup$
    – uhoh
    Commented Aug 22, 2018 at 3:42

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@RussellBorogove's comment is fairly conclusive.

...I note that Celestrak was established in 1985, at which point Salyut 7 was up, but Skylab was not. The Archived TLE data page says 1980-2004, but there's a link to request earlier data.

Skylab reentered the atmosphere before 1980 and Tiangong-1 and Tiangong-2 were both launched after 2004.

And, as also pointed out there; data for them can be obtained by Celestrak's special request feature.

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  • $\begingroup$ first sentence here ;-) $\endgroup$
    – uhoh
    Commented Oct 12, 2018 at 10:26

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