Free Astronomy Magazine January-February 2022

17 JANUARY-FEBRUARY 2022 ASTRO PUBLISHING than the other, and determined the length of the asteroid to be about 270 kilometres or about half the length of the English Channel. In a second study, also published in Astronomy & Astrophysics and led by Miroslav Brož of Charles Univer- sity in Prague, Czech Republic, the team reported how they used the SPHERE observations to find the cor- rect orbits of Kleopatra’s two moons. Previous studies had estimated the orbits, but the new observations with ESO’s VLT showed that the moons were not where the older data predicted them to be. “This had to be resolved,” says Brož. “Because if the moons’ orbits were wrong, everything was wrong, in- cluding the mass of Kleopatra.” Thanks to the new observations and sophisticated modelling, the team managed to precisely describe how Kleopatra’s gravity influences the moons’ movements and to deter- mine the complex orbits of AlexHe- lios and CleoSelene. This allowed them to calculate the asteroid’s mass, finding it to be 35% lower than previous estimates. Combining the new estimates for volume and mass, astronomers were able to calculate a new value for the density of the asteroid, which, at less than half the density of iron, turned out to be lower than previ- ously thought. The low density of Kleopatra, which is believed to have a metallic composition, suggests that it has a porous structure and could be little more than a “pile of rubble”. This means it likely formed when material reaccumulated fol- lowing a giant impact. Kleopatra’s rubble-pile structure and the way it rotates also give in- dications as to how its two moons could have formed. The asteroid ro- tates almost at a critical speed, the speed above which it would start to fall apart, and even small impacts may lift pebbles off its surface. Marchis and his team believe that those pebbles could subsequently have formed AlexHelios and CleoSe- lene, meaning that Kleopatra has truly birthed its own moons. The new images of Kleopatra and the insights they provide are only possible thanks to one of the ad- vanced adaptive optics systems in use on ESO’s VLT, which is located in the Atacama Desert in Chile. Adaptive optics help to correct for distortions caused by the Earth’s at- mosphere which cause objects to appear blurred — the same effect that causes stars viewed from Earth to twinkle. Thanks to such correc- tions, SPHERE was able to image Kleopatra — located 200 million kilometres away from Earth at its closest — even though its apparent size on the sky is equivalent to that of a golf ball about 40 kilometres away. ESO’s upcoming Extremely Large Telescope (ELT), with its ad- vanced adaptive optics systems, will be ideal for imaging distant aster- oids such as Kleopatra. “I can’t wait to point the ELT at Kleopatra, to see if there are more moons and refine their orbits to detect small changes,” concluded Marchis. T his processed image, based on observations taken in July 2017, shows the two moons of the asteroid Kleopatra (the central white object), AlexHelios and CleoSelene, which appear as two small white dots in the top-right and bot- tom-left corners of the picture. Kleopatra’s moons are difficult to see in the raw images — which were taken with the Spectro-Polarimetric High-contrast Exo- planet REsearch (SPHERE) instrument on ESO’s VLT — owing to glare around the asteroid, inherent to this kind of adaptive-optics observations. To achieve this view, the images of Kleopatra have been processed to remove the glare and re- veal the moons. [ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)] !