Free Astronomy Magazine November-December 2019

21 NOVEMBER-DECEMBER 2019 SPACE CHRONICLES A n animation of Comet 2I/Borisov. [NASA, ESA, D. Jewitt (UCLA)] T his illustration shows the path of comet 2I/Borisov through our Solar System. This visitor came from interstellar space along a hyperbolic trajectory. It is only the second known intruder to zoom through our Solar System (the interstellar object ‘Oumua- mua was detected in 2017). As the graphic shows, the comet’s straight path across interstellar space is slightly deflected by the gravitational pull of our Sun. The comet is travelling so fast, at over 150,000 kilometres per hour, it will eventually leave the Solar System. The panel on the right shows the comet’s position relative to Earth when the NASA/ESA Hubble Space Telescope ob- served it on 12 October 2019, when it was 420 million kilometres from Earth. [NASA, ESA, J. Olmsted, F. Summers (STScI)] the beginning of the discoveries of interstellar objects paying a brief visit to our Solar System. There may be thousands of such in- terstellar objects here at any given time; most, however, are too faint to be detected with present-day tele- scopes. Observations by Hubble and other telescopes have shown that rings and shells of icy debris encircle young stars where planet formation is underway. A gravitational interac- tion between these comet-like ob- jects and other massive bodies could hurtle them deep into space where they go adrift among the stars. Future Hubble observations of 2I/Borisov are planned through Jan- uary 2020, with more being pro- posed. that it came from interstellar space. Until now, all catalogued comets have come either from a ring of icy debris at the pe- riphery of our Solar System, called the Kuiper belt, or from the Oort cloud, a shell of icy ob- jects which is thought to be in the outermost regions of our Solar System, with its inner- most edge at about 2000 times the distance be- tween the Earth and the Sun. 2I/Borisov and ‘Oumuamua are only International Astronomical Union’s Minor Planet Center computed an orbit for the comet which showed !