Hubble Space Telescope observations of interstellar comet 2I/Borisov near perihelion show the ejection of large ( \gtrsim 100 \mu m ) particles at \lesssim 9 m s ^ { -1 } speeds , with estimated mass-loss rates of \sim 35 kg s ^ { -1 } .
The total mass loss from comet Borisov corresponds to loss of a surface shell on the nucleus only \sim 0.4 m thick .
This shell is thin enough to be susceptible to past chemical processing in the interstellar medium by cosmic rays , meaning that the ejected materials can not necessarily be considered as pristine .
Our high-resolution images reveal persistent asymmetry in the dust coma , best explained by a thermal lag on the rotating nucleus causing peak mass loss to occur in the comet nucleus afternoon .
In this interpretation , the nucleus rotates with an obliquity of 30 \arcdeg ( pole direction RA = 205 \arcdeg and Dec. = 52 \arcdeg ) .
The subsolar latitude varied from -35 \arcdeg ( southern solstice ) at the time of discovery to 0 \arcdeg ( equinox ) in 2020 January , suggesting the importance of seasonal effects .
Subsequent activity likely results from regions freshly activated as the northern hemisphere is illuminated for the first time .