This strange bone-shaped asteroid may have given birth to its own moons

Not all asteroids are built the same. Some, however, are so far removed from each other that they have officially crossed the line to become wonderfully weird.

This is the case with an asteroid named Cleopatra, which also trails quite normally in the asteroid belt between Mars and Jupiter. It consists of two lobes, connected by a long neck – a morphology that has earned it the nickname “dog bone asteroid”.

This unusual-looking space rock even has two small moons – AlexHelios and CleoSelene, named after the children of the famous pharaoh of ancient Egypt, Cleopatra.

We’ve known about this incredible spatial oddity for about two decades, but scientists have now got the most detailed images of it we’ve ever seen. This helps us understand how Kleopatra formed, and the results suggest that the moons arose from Kleopatra’s own material.

“Cleopatra is truly a unique body in our solar system,” said astronomer Franck Marchis from the SETI Institute and the Astrophysical Laboratory of Marseille in France.

“Science is making a lot of progress through the study of weird outliers. I think Cleopatra is one of them and understanding this complex and multiple asteroid system can help us learn more about our solar system.”

In two studies published in Astronomy & Astrophysics, astronomers used new images of Cleopatra to obtain a more precise set of measurement constraints for the asteroid, developing a new 3D model and more precisely defining the orbits of AlexHelios and CleoSelene.

The work was carried out from observations obtained with the powerful SPHERE instrument attached to the Very Large Telescope of the European Southern Observatory in Chile. As Cleopatra tumbled through space, researchers were able to get images from different angles.

From there, they were able to determine that Cleopatra is around 270 kilometers (168 miles) long, with one of her dumbbell lobes larger than the other, and that the two are connected by a relatively thick neck. . The newly described dimensions then allowed researchers to calculate Cleopatra’s volume.

(ESO / Vernazza, Marchis et al./MISTRAL algorithm [ONERA/CNRS])

Above: Cleopatra with AlexHelios and CleoSelene.

A second team, meanwhile, was working to constrain the orbits of AlexHelios and CleoSelene. This is important, because the orbits are constrained by the gravitational field in which they move, which in turn correlates with the masses of the system.

“This had to be resolved, because if the orbits of the moons were wrong, everything was wrong, including Cleopatra’s mass,” said astronomer Miroslav Brož of Charles University in the Czech Republic.

Using the new observations combined with mathematical modeling, the team was able to describe the orbits of the moons with greater precision than ever before. This allowed a new calculation of the mass of Cleopatra: 2.97 x 10¹⁸ kilograms, significantly lower than previous calculations, which yielded 4.64 x 10¹⁸ kilograms.

Once you have the mass and volume of an object, you can calculate its density. Using the results of Brož and his team, Marchis and his colleagues then recalculated Cleopatra’s density. Assuming that Cleopatra was rich in metals, the asteroid’s density turned out to be very low.

This can tell us something about the formation of Cleopatra. Low density suggests that the asteroid is rather porous – a loose “rubble heap” of chunks of rock that barely cling to each other. Such piles of rubble are believed to have formed when material is thrown from a parent body in a giant impact, gradually reassembling over time.

If it is porous, Cleopatra barely stands. The asteroid has a faster than average spin period of about 5.4 hours. This period is right on this side of stability; if it accelerated, the centripetal force would tear it apart.

This critical state of rotation means that the effective gravity at the equator is low and that material in this region could move away from the asteroid.

If this is true, it gives us a clue about the formation of AlexHelios and CleoSelene. If material is ejected from Cleopatra, it could have merged in orbit, forming the moons – making them, in effect, the children of the asteroid.

Both articles were published in Astronomy & Astrophysics. They can be found here and here.