Free Astronomy Magazine January-February 2022

46 JANUARY-FEBRUARY 2022 ASTRO PUBLISHING sity suggests the asteroids’ composi- tion varies significantly, giving as- tronomers important clues about their origin. “Our observations pro- vide strong support for substantial migration of these bodies since their formation. In short, such tremen- dous variety in their composition can only be understood if the bodies originated across distinct regions in the Solar System,” explains Josef Hanuš of the Charles University, Prague, Czech Republic, one of the authors of the study. In particular, the results support the theory that the least dense asteroids formed in the remote regions beyond the orbit of Neptune and migrated to their current location. These findings were made possible thanks to the sensitivity of the Spec- tro-Polarimetric High-contrast Exo- planet REsearch (SPHERE) instru- ment mounted on ESO’s VLT. “With the improved capabilities of SPHERE, along with the fact that little was known regarding the shape of the largest main belt asteroids, we were able to make substantial progress in this field,” says co-author Laurent Jorda, also of the Laboratoire d’As- trophysique de Marseille. Astronomers will be able to image even more asteroids in fine detail with ESO’s upcoming Extremely Large Telescope (ELT), currently under con- struction in Chile and set to start operations later this decade. “ELT observations of main-belt asteroids will allow us to study objects with di- ameters down to 35 to 80 kilome- tres, depending on their location in the belt, and craters down to ap- proximately 10 to 25 kilometres in size,” says Vernazza. “Having a SPHERE-like instrument at the ELT would even allow us to image a sim- ilar sample of objects in the distant Kuiper Belt. This means we’ll be able to characterise the geological his- tory of a much larger sample of small bodies from the ground.” A stronomers have used the ESO’s Very Large Telescope in Chile to image 42 of the biggest main-belt asteroids. Meet some of the 42 in this video sum- marising the discovery! [ESO] question of life, the Universe, and everything. “Only three large main belt asteroids, Ceres, Vesta and Lute- tia, have been imaged with a high level of detail so far, as they were visited by the space missions Dawn and Rosetta of NASA and the Euro- pean Space Agency, respectively,” explains Pierre Vernazza, from the Laboratoire d’Astrophysique de Mar- seille in France, who led the asteroid study published in Astronomy & As- trophysics . “Our ESO observations have provided sharp images for many more targets, 42 in total.” The previously small number of de- tailed observations of asteroids meant that, until now, key charac- teristics such as their 3D shape or density had remained largely un- known. Between 2017 and 2019, Vernazza and his team set out to fill this gap by conducting a thorough survey of the major bodies in the as- teroid belt. Most of the 42 objects in their sam- ple are larger than 100 km in size; in particular, the team imaged nearly all of the belt asteroids larger than 200 kilometres, 20 out of 23. The two biggest objects the team probed were Ceres and Vesta, which are around 940 and 520 kilometres in diameter, whereas the two small- est asteroids are Urania and Auso- nia, each only about 90 kilometres. By reconstructing the objects’ shapes, the team realised that the observed asteroids are mainly divided into two families. Some are almost perfectly spherical, such as Hygiea and Ceres, while others have a more peculiar, “elongated” shape, their undis- puted queen being the “dog-bone” asteroid Kleopatra. By combining the asteroids’ shapes with information on their masses, the team found that the densities change significantly across the sam- ple. The four least dense asteroids studied, including Lamberta and Sylvia, have densities of about 1.3 grams per cubic centimetre, approx- imately the density of coal. The highest, Psyche and Kalliope, have densities of 3.9 and 4.4 grammes per cubic centimetre, respectively, which is higher than the density of dia- mond (3.5 grammes per cubic cen- timetre). This large difference in den- !