The ‘dog bone’ asteroid orbits so fast that its mane may have formed from thrown material.

In the asteroid belt, located between the planets Mars and Jupiter, we find – as the name suggests – many asteroids. Well-known are Bennu and Ryugu. But we also find asteroid Kleopatra, which because of its remarkable shape is also jokingly called the “dogbone” asteroid. In a new study, researchers have captured the most detailed images ever of this unusual asteroid. And that leads to some interesting discoveries.

Kleopatra

“Kleopatra is truly a unique object within our solar system,” said study leader Franck Marchis. And that is partly because of its unique shape. It was nicknamed “dogbone asteroid” when radar observations some twenty years ago revealed that Kleopatra consists of two lobes connected by a thick “neck”. In 2008, researchers discovered that Kleopatra is also surrounded by two moons. These have been called AlexHelios and CleoSelene; referring to the children of the famous Egyptian queen.

Eleven images show the asteroid Kleopatra, which is seen at different angles due to its rotation. The images were taken at various times between 2017 and 2019 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO’s VLT. Image: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)

To learn more about Kleopatra, researchers took new images using the Very Large Telescope (VLT). As the asteroid rotates, the team managed to view it from different angles. Ultimately, this yielded the most accurate 3D models of her shape. The researchers found that one lobe is slightly larger than the other. The total length of the asteroid is approximately 270 kilometers: roughly the distance between Maastricht and Groningen.

moons

But that might not even be the most special thing. Researchers have also taken care of Kleopatra’s moons. In doing so, they were mainly focused on establishing the correct orbits of AlexHelios and CleoSelene. Although researchers have already made an attempt to do so in previous studies, after analysis of the new images they turned out to be no longer correct. “This had to be resolved,” said researcher Miroslav Brož. “If the orbits of the moons are wrong, everything is wrong, including Kleopatra’s mass.”

This image shows the twin moons of asteroid Kleopatra AlexHelios and CleoSelene, which can be seen as the two small white spots at the top right and bottom left. Image: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)

Thanks to the new images and advanced models, the team was able to accurately re-determine the orbits of Kleopatra’s moons. This then allowed them to calculate the asteroid’s mass as well. And surprisingly, it turned out to be as much as 35 percent lower than previous studies had suggested. In addition, the researchers discovered that the asteroid also has a much lower density than previously thought. The density of Kleopatra appears to be half the density of iron. This suggests that the asteroid has a rather porous structure and may be little more than a loose clump of debris. This means that it probably formed from material that accumulated after a giant impact.

Moon formation

That discovery then leads to another remarkable discovery. The ‘mess structure’ and the way Kleopatra rotates also provide more insight into how her two moons could have formed. The asteroid rotates on its axis at lightning speed; so fast, in fact, that she is about to fall apart. As a result, even small impacts can cause pebbles from the surface to be thrown into space like ping-pong balls. The researchers therefore suspect that AlexHelios and CleoSelene formed from this escaping material. And that means Kleopatra may have given birth to her own mane herself.

The research thus provides striking new insights into how Kleopatra and the two moons orbiting her came to be. And in that way we learn more and more about our own cosmic backyard. “Science is making a lot of progress thanks to research from strange misfits like Kleopatra,” Marchis says. The researchers do not plan to close the Kleopatra chapter yet. Because with the future Extremely Large Telescope (ELT) they can take even better pictures. “I can’t wait to focus the ELT on Kleopatra,” says Marchis. “Then we’ll see if there might be more moons.”