Comet C/2019 Y4 (ATLAS) is the first long-period comet group member observed to disintegrate well before perihelion. During the disintegration event, we conducted a 3-day observational campaign using the Hubble Space Telescope WFC3 camera. We identify two fragment clusters produced by the initial event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters started with a similar integrated brightness, but exhibit different evolutionary behavior. The cause of the initial fragmentation is crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of sub-surface super-volatile ices, either of which would lead to the release of a large amount of gas, as inferred from the significant bluing of the comet observed shortly before the disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a somewhat extreme perihelion distance of q=0.25 au. We speculate that Comet ATLAS is derived from the ice-rich interior of a non-uniform, kilometer-wide progenitor that split during its previous perihelion.
This research is supported by NASA through grants HST-GO-16089 and 16111 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.