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I remember being excited when ESA launched Hipparcos in 1989 and was in operation until 15 August, 1993. Later, Gaia came into operation in 2013.

Reading What is the overlap between the Gaia and the Hipparcos catalog? I started wondering whether any of the Hipparcos data is still being used and for what. Could somebody please guide me with a concise explanation?

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The Hipparcos results can be used as an earlier epoch to improve the astrometry produced by Gaia. This was the linchpin of data release 1 (DR1), but less important in DR2 and DR3, because the astrometry of Gaia alone is so much better in general.

An exception to this is bright stars where much data is still missing in DR3, but can be found in the Hipparcos catalogue.

Apart from precision-based concerns, the earlier Hipparcos data, combined with Gaia, can be used to look for changes in proper motion: i.e. accelerations (see Brandt 2018).

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  • $\begingroup$ Thanks for the very insightful summary. Your note on the bright stars is interesting, do you have some reference for that? Differently put: If you say "still missing" - (when) will the bright stars be included in DR3? $\endgroup$
    – B--rian
    Commented Feb 3, 2021 at 8:37
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    $\begingroup$ @B--rian From cosmos.esa.int/web/gaia/faqs#BrightStar Some stars are too bright for Gaia to observe. Some of these bright stars are being observed with a special mode on board the spacecraft, requiring to predict when it passes the focal plane and then telling the on board computer to record the relevant pixels around the position of the star. These data should be processed eventually but cannot be handled automatically at this stage. $\endgroup$
    – PM 2Ring
    Commented Feb 3, 2021 at 9:39

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