Free Astronomy Magazine July-August 2021

42 JULY-AUGUST 2021 ASTRO PUBLISHING instrument on ESO’s VLT, located in northern Chile, to study 2I/Borisov in detail using a technique called po- larimetry. Since this technique is reg- ularly used to study comets and other small bodies of our Solar Sys- tem, this allowed the team to com- pare the interstellar visitor with our local comets. The team found that 2I/Borisov has polarimetric properties distinct from those of Solar System comets, with the exception of Hale–Bopp. Comet Hale–Bopp received much public in- terest in the late 1990s as a result of being easily visible to the naked eye, and also because it was one of the most pristine comets astronomers had ever seen. Prior to its most re- cent passage, Hale–Bopp is thought to have passed by our Sun only once and had therefore barely been af- fected by solar wind and radiation. This means it was pristine, having a composition very similar to that of the cloud of gas and dust it — and the rest of the Solar System — formed from some 4.5 billion years 2I/Borisov may be the most pristine comet ever found by ESO - Bárbara Ferreira N ew observations with the Eu- ropean Southern Observa- tory’s Very Large Telescope (ESO’s VLT) indicate that the rogue comet 2I/Borisov, which is only the second and most recently detected interstellar visitor to our Solar Sys- tem, is one of the most pristine ever observed. Astronomers suspect that the comet most likely never passed close to a star, making it an undis- turbed relic of the cloud of gas and dust it formed from. 2I/Borisov was discovered by amateur astronomer Gennady Borisov in August 2019 and was confirmed to have come from beyond the Solar System a few weeks later. “2I/Borisov could repre- sent the first truly pristine comet ever observed,” says Stefano Bag- nulo of the Armagh Observatory and Planetarium, Northern Ireland, UK, who led the new study published in Nature Communications . The team believes that the comet had never passed close to any star before it flew by the Sun in 2019. Bagnulo and his colleagues used the FORS2 T his image shows an artist’s im- pression of what the surface of the 2I/Borisov comet might look like. [ESO/M. Kormesser]