Outburst and Splitting of Interstellar Comet 2I/Borisov

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, was caused by the expulsion of solid particles having a combined cross section ∼100 km2 and a mass in 0.1 mm sized particles ∼2 × 107 kg. The latter corresponds to ∼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 3 weeks after the outburst), having a cross section ∼0.6 km2 and corresponding dust mass ∼105 kg. The secondary was absent in images taken on and before March 28 and in images taken on and after April 3. 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.