Using an array of VLT telescopes from the European Southern Observatory, a group of astronomers have captured the sharpest and most detailed images of the asteroid Cleopatra to date. Based on the observations, the research team was also able to more accurately determine the mass and shape of a particular asteroid that resembles dog bones than ever before. Based on their results, we can also learn more about how the asteroid and its two orbiting moons formed.
“Cleopatra is really a special object in the solar system,” Francis Marchis, an astronomer at the SETI Institute in Mountain View, California, and the Astrophysical Institute in Marseille, France, tells Astronomy and Astrophysics. Study of foreign bodies. I think Cleopatra is one of them, and by understanding the complex collecting asteroid system, we can learn more about our solar system. “
Cleopatra revolves around the sun in the asteroid belt between Mars and Jupiter. Astronomers call it a “dog-bone asteroid”, because about 20 years ago, radar measurements revealed that it consisted of two lobes connected by a “neck”. In 2008, Marchis and his colleagues also discovered that two moons orbiting Cleopatra were named AlexHelios and CleoSelene after the sons of the Queen of Egypt.
To learn more about Cleopatra, Marches and his research team retrieved images from the archives that were recorded at various times between 2017 and 2019 by the SPHERE (Spectro-Polarimetric High-Extrasolar Contrast) instrument in ESO’s VLT telescopes. Using the rotation of the asteroid around its axis, they saw it from different angles and, accordingly, were able to create the most accurate 3D model of its shape to date. Illustration of shape and size showed that one lobe is larger than the other. The length of the asteroid was estimated to be about 270 kilometers, which is half the length of the English Channel.
In another study, also published in the Journal of Astronomy and Astrophysics, led by Miroslav Broch of Charles University in Prague (Czech Republic), researchers report how they can use Sphere observations to improve the orbital elements of Cleopatra’s moons. Previous work has already addressed this, but new observations by ESO VLT indicate that the satellites are not where they should be based on previous data.
“This problem had to be addressed,” Broy says. Thanks to new observations and sophisticated modeling, the group was able to accurately describe Cleopatra’s gravitational effect on the motion of its moons and determine the complex orbits of AlexHelios and CleoSelene. This, in turn, made it possible to calculate the mass of the asteroid, which was 35% smaller than previous estimates.
From the new estimates of the size and mass, a new value for the density of the asteroid can also be calculated, which is less than half the density of iron, and therefore also the value considered earlier. The low density of Cleopatra, hitherto regarded as having a mineral composition, indicates that its skeleton is porous and could rather be considered a “mound of debris”. That is, it likely formed by recombining material after a massive impact.
The structure of the debris on Cleopatra and the way it orbited may also explain the possibility of the formation of its moons. The asteroid is spinning at close to critical speed, at which time it will begin to fragment, so even small collisions can rip debris from its surface. Marches and his group believe that AlexHelios and CleoSelene, that is, Cleopatra, actually “born” their moons from these pieces of debris.
Combined with the information they provided, the creation of the new images of Cleopatra would not have been possible without the advanced adaptive optical system of the ESO VLT telescope complex in the Atacama Desert in Chile. Adaptive optics help compensate for distortions caused by Earth’s atmosphere that cause objects to appear smudged. The same thing causes starlight to flicker when viewed from the surface of our planet. Thanks to corrections, SPHERE was able to image Cleopatra, which is at least 200 million km from Earth, although its apparent diameter in the sky only corresponds to a golf ball seen from 40 km.
ESO’s ELT (Extremely Large Telescope) telescope with its advanced adaptive optical system will be an ideal tool for imaging distant asteroids such as Cleopatra. “I can’t wait for the ELT to target Cleopatra and see if there are additional moons or to improve the moons’ orbits to detect small changes,” Marches adds.
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