These eleven images are of the asteroid Kleopatra, viewed at different angles as it rotates. The images were taken at different times between 2017 and 2019 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s VLT. Kleopatra orbits the Sun in the Asteroid Belt between Mars and Jupiter. Astronomers have called it a “dog-bone asteroid” ever since radar observations around 20 years ago revealed it has two lobes connected by a thick “neck.” Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)
Using the European Southern Observatory’s
This image provides a size comparison of the asteroid Kleopatra with northern Italy. The top half of the image shows a computer model of Kleopatra, a “dog-bone” shaped asteroid which orbits the Sun in the Asteroid Belt between Mars and Jupiter. End to end, Kleopatra is 270 kilometers (~168 miles) long. The bottom half of the image gives an aerial view of northern Italy, with the footprint Kleopatra would have if it were hovering above it. Credit: ESO/M. Kornmesser/Marchis et al.
Kleopatra orbits the Sun in the Asteroid Belt between
This image provides a size comparison of the asteroid Kleopatra with Chile. The top half of the image shows a computer model of Kleopatra, a “dog-bone” shaped asteroid which orbits the Sun in the Asteroid Belt between Mars and Jupiter. End to end, Kleopatra is 270 kilometers long. The bottom half of the image gives an aerial view of Chile, with the footprint Kleopatra would have if it were hovering above the country. Credit: ESO/M. Kornmesser/Marchis et al.
In a second study, also published in Astronomy & Astrophysics and led by Miroslav Brož of Charles University in Prague, Czech Republic, the team reported how they used the SPHERE observations to find the correct 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, including the mass of Kleopatra.” Thanks to the new observations and sophisticated modeling, the team managed to precisely describe how Kleopatra’s gravity influences the moons’ movements and to determine the complex orbits of AlexHelios and CleoSelene. This allowed them to calculate the asteroid’s mass, finding it to be 35% lower than previous estimates.
This 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 bottom-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 Exoplanet 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 reveal the moons. Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)
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 previously thought.[1] 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 following a giant impact.
Kleopatra’s rubble-pile structure and the way it rotates also give indications as to how its two moons could have formed. The asteroid rotates 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 CleoSelene, meaning that Kleopatra has truly birthed its own moons.
This animation shows where the orbit of the asteroid Kleopatra (in red) is in our Solar System. Kleopatra orbits the Sun in the Asteroid Belt, which is located between the orbits of Mars and Jupiter. Credit: ESO/spaceengine.org
The new images of Kleopatra and the insights they provide are only possible thanks to one of the advanced 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 atmosphere which cause objects to appear blurred — the same effect that causes stars viewed from Earth to twinkle. Thanks to such corrections, SPHERE was able to image Kleopatra — located 200 million kilometers away from Earth at its closest — even though its apparent size on the sky is equivalent to that of a golf ball about 40 kilometers away.
ESO’s upcoming Extremely Large Telescope (ELT), with its advanced adaptive optics systems, will be ideal for imaging distant asteroids 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,” adds Marchis.
Notes
The newly calculated density is 3.4 grams per cubic centimeter, while previously Kleopatra was believed to have a mean density of about 4.5 grams per cubic centimeter.
References:
“(216) Kleopatra, a low density critically rotating M-type asteroid” by F. Marchis, L. Jorda, P. Vernazza, M. Brož, J. Hanuš, M. Ferrais, F. Vachier, N. Rambaux, M. Marsset, M. Viikinkoski, E. Jehin, S. Benseguane, E. Podlewska-Gaca, B. Carry, A. Drouard, S. Fauvaud, M. Birlan, J. Berthier, P. Bartczak, C. Dumas, G. Dudzinski, J. Durech, J. Castillo-Rogez, F. Cipriani, F. Colas, R. Fetick, T. Fusco, J. Grice, A. Kryszczynska, P. Lamy, A. Marciniak, T. Michalowski, P. Michel, M. Pajuelo, T. Santana-Ros, P. Tanga, A. Vigan, O. Witasse and B. Yang, 9 September 2021, Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202140874 arXiv:2108.07207
“An advanced multipole model for (216) Kleopatra triple system” by M. Brož, F. Marchis, L. Jorda, J. Hanuš, P. Vernazza, M. Ferrais, F. Vachier, N. Rambaux, M. Marsset, M. Viikinkoski, E. Jehin, S. Benseguane, E. Podlewska-Gaca, B. Carry, A. Drouard, S. Fauvaud, M. Birlan, J. Berthier, P. Bartczak, C. Dumas, G. Dudzinski, J. Durech, J. Castillo-Rogez, F. Cipriani, F. Colas, R. Fetick, T. Fusco, J. Grice, A. Kryszczynska, P. Lamy, A. Marciniak, T. Michalowski, P. Michel, M. Pajuelo, T. Santana-Ros, P. Tanga, A. Vigan, D. Vokrouhlický, O. Witasse and B. Yang, 9 September 2021, Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202140901 arXiv:2105.09134
More information
This research, based on observations with SPHERE on ESO’s VLT (Principal Investigator: Pierre Vernazza), was presented in two papers to appear in Astronomy & Astrophysics.
The team of the paper entitled “(216) Kleopatra, a low density critically rotating M-type asteroid” is composed of F. Marchis (SETI Institute, Carl Sagan Center, Mountain View, USA and Aix Marseille University, CNRS, Laboratoire d’Astrophysique de Marseille, France [LAM]), L. Jorda (LAM), P. Vernazza (LAM), M. Brož (Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic [CU]), J. Hanuš (CU), M. Ferrais (LAM), F. Vachier (Institut de mécanique céleste et de calcul des éphémérides, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC University Paris 06 and Université de Lille, France [IMCCE]), N. Rambaux (IMCCE), M. Marsset (Department of Earth, Atmospheric and Planetary Sciences, (function(d, s, id){
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