  Near-Sun Comet C/2019 Y4 (ATLAS) is the first member of a long-period comet
group observed to disintegrate well before perihelion. Here we present our
investigation into this disintegration event using images obtained in a 3-day
{\it Hubble Space Telescope} (\hst) campaign. We identify two fragment clusters
produced by the initial disintegration event, corresponding to fragments C/2019
Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters
started with similar integrated brightness, but exhibit different evolutionary
behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B, and
showed signs of significant mass-loss and changes in size distribution
throughout the 3-day campaign. The cause of the initial fragmentation is
undetermined by the limited evidence but crudely compatible with either the
spin-up disruption of the nucleus or runaway sublimation of sub-surface
supervolatile 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
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 suggests that comets down to a few
kilometers in diameter can still possess complex, non-uniform interiors that
can protect ices against intense solar heating.
