[yshklarov]

To nitpick with the grammar in the quote: It's capable of measuring to the accuracy of 120 μm at 1000 km. So it cannot accurately measure the diameter of a human hair (which ranges from around 20 to 200 μm) at that distance, but only to the accuracy of a human hair.

[perihelions]

You're right: this precision is hundreds of times below the diffraction limit of even the James Webb telescope. It can't possibly measure the width of an object that finely; rather, only the relative displacement of its centroid position between two points in time. (And it's a seriously confusing physics miracle that that much is possible).

[jdhwosnhw]

For point source astrometry, there are a few ways to beat the diffraction limit. For instance, some observers will purposely defocus their optics to spread the target photons over a larger number of pixels, which with sufficient SNR lets you gain a better lower bound (from a Cramer Rao perspective) on precision. I think Gaia actually does something similar to this but “blurs” through time, rather than across space, by purposely not perfectly tracking stars so that they drift at sub-pixel rates across the FPA.

[UltraSane]

Yes, Gaia slowly spins to that the stars move across its CCD array.