Yes, I’m afraid C/2019 Y4 (ATLAS) looks as if it will soon be an ex-comet. Only a week ago I gave our first webinar talking about prospects for this object. Now I think it is probably a cloud of rubble. As Comet section Director I’m not having much luck with comets!
In my images over the last few nights I have not seen any fragments in the tail. What I have seen is a gradual reduction in the peak pixel ADU count and a flattening/broadening of the downtail coma brightness profile. This plot shows a cut through the photocentre aligned on the tail PA (positive offsets are tailward) for five nights from March 25 to April 7. You can see that peak pixel ADU falls from around 8000 to 1200 in that time (a fade of around 2 magnitudes) and the profile is broader with a more gradual tailward slope in the later images. It looks to me as if the nucleus has completely fragmented and what we are seeing is a cloud of rubble migrating down the tail. This explains the large astrometric residuals in RA since the photometric centroid is no longer aligned with the original nucleus.
If this is correct the future of this comet is as a fainter and fainter diffuse rubble pile. Let’s see what happens and keep this under observation as long as possible. We have a ringside seat to watch the distruction of a Solar System object.
