  We present Hubble Space Telescope observations of a photometric outburst and
splitting event in interstellar comet 2I/Borisov. The outburst, first reported
with the comet outbound at 2.8 AU (Drahus et al.~2020), was caused by the
expulsion of solid particles having a combined cross-section about 100 sq. km
and a mass in 0.1 mm sized particles about 2e7 kg. The latter corresponds to
1e-4 of the mass of the nucleus, taken as a sphere of radius 500 m. A transient
``double nucleus'' was observed on UT 2020 March 30 (about three weeks after
the outburst), having a cross-section about 0.6 sq. km and corresponding dust
mass 1e5 kg. The secondary was absent in images taken on and before March 28,
and in images taken on and after April 03. The unexpectedly delayed appearance
and rapid disappearance of the secondary are consistent with an origin through
rotational bursting of one or more large (meter-sized) boulders under the
action of outgassing torques, following their ejection from the main nucleus.
Overall, our observations reveal that the outburst and splitting of the nucleus
are minor events involving a negligible fraction of the total mass: 2I/Borisov
will survive its passage through the planetary region largely unscathed.
