Preperihelion Outbursts and Disintegration of Comet C/2017 S3 (Pan-STARRS)

A sequence of events, dominated by two outbursts and ending with the preperihelion disintegration of comet C/2017 S3, is examined. The onset times of the outbursts are determined with high accuracy from the light curve of the nuclear condensation before it disappeared following the second outburst. While the brightness of the condensation was declining precipitously, the total brightness continued to grow in the STEREO-A's HI1 images until two days before perihelion. The red magnitudes measured in these images refer to a uniform cloud of nuclear fragments, 2200 km^2 in projected area, that began to expand at a rate of 76 m s^(-1) at the time of the second outburst. A tail extension, detected in some STEREO-A images, consisted of dust released far from the Sun. Orbital analysis of the ground-based observations shows that the comet had arrived from the Oort Cloud in a gravitational orbit. Treating positional residuals as offsets of a companion of a split comet, we confirm the existence of the cloud of radiation-pressure driven millimeter-sized dust grains emanating from the nucleus during the second outburst. We detect a similar, but compact and much fainter cloud (or a sizable fluffy dust aggregate fragment) released at the time of the first outburst. --- The debris would make a sphere of 140 m across and its kinetic energy is equivalent to the heat of crystallization liberated by 100 tons of amorphous water ice. Ramifications for short-lived companions of the split comets and for 1I `Oumuamua are discussed.