2020 April 5
Sudden changes on Comet C/2019 Y4 (ATLAS)
As I said in my recent BAA webinar comets are unreliable things.
Recent astrometry shows significant residuals in RA indicating a fairly large non-gravitational force (NGF) acting on the comet nucleus. The attached plot shows the RA residuals using an orbit solution which uses observations up to March 25. These fit well without any NGFs. The NGFs seem to start after this date. My astrometry from April 5th shows that nuclear magnitude of the comet is around a magnitude fainter than on March 25 and that the point spread function is a lot broader.
The sudden onset of significant NGFs may indicate that the nucleus is very small and is in the process of breaking up or it may simply indicate a significant release of material from an otherwise intact nucleus on or around March 27. Only observations will confirm this.
Astrometry can be performed during strong moonlight so please observe this comet as much as you can and send me any astrometry that you obtain.