Outburst and Splitting of Interstellar Comet 2I/Borisov

<p>We present Hubble Space Telescope observations of a photometric outburst and splitting event in interstellar comet 2I/Borisov. &#160; 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&#160; 2e7 kg. &#160;The latter corresponds to 1e-4 of the mass of the nucleus, taken as a sphere of radius 500 m. &#160;A transient&#160; double nucleus was observed on UT 2020 March 30 (about three weeks after the outburst), having a cross-section 0.6 sq. km and corresponding dust mass 1e5 kg. &#160;The secondary was absent in images taken on and before March 28, and in images taken on and after April 03. &#160;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. &#160;Overall, our observations &#160;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.</p> <p>Journal: The Astrophysical Journal Letters, Volume 896, Issue 2, id.L39</p> <p>&#160;</p>