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 \sim 2.8 AU ( Drahus et al .
2020 ) , was caused by the expulsion of solid particles having a combined cross-section \sim 100 km ^ { 2 } and a mass in 0.1 mm sized particles \sim 2 \times 10 ^ { 7 } kg .
The latter corresponds to \sim 10 ^ { -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 \sim 0.6 km ^ { 2 } and corresponding dust mass \sim 10 ^ { 5 } 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 .