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Near-Sun Comet C/2019 Y4 (ATLAS) is the first member of a long-period cometgroup observed to disintegrate well before perihelion. Here we present ourinvestigation into this disintegration event using images obtained in a 3-day{\it Hubble Space Telescope} (\hst) campaign. We identify two fragment clustersproduced by the initial disintegration event, corresponding to fragments C/2019Y4-A and C/2019 Y4-B identified in ground-based data. These two clustersstarted with similar integrated brightness, but exhibit different evolutionarybehavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B, andshowed signs of significant mass-loss and changes in size distributionthroughout the 3-day campaign. The cause of the initial fragmentation isundetermined by the limited evidence but crudely compatible with either thespin-up disruption of the nucleus or runaway sublimation of sub-surfacesupervolatile ices, either of which would lead to the release of a large amountof gas as inferred from the significant bluing of the comet observed shortlybefore the disintegration. Gas can only be produced by the sublimation ofvolatile ices, which must have survived at least one perihelion passage at aperihelion distance of $q=0.25$~au. We speculate that Comet ATLAS is derivedfrom the ice-rich interior of a non-uniform, kilometer-wide progenitor thatsplit during its previous perihelion. This suggests that comets down to a fewkilometers in diameter can still possess complex, non-uniform interiors thatcan protect ices against intense solar heating.

Disintegration of Long-Period Comet C/2019 Y4 (ATLAS): I. Hubble Space Telescope Observations