Not all asteroids are built the same. Some, however, are so far from equal that they have officially crossed the line to become wonderfully strange.
Such is the case of an asteroid called Kleopatra, which otherwise hangs 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 “bone dog asteroid”.
This unusual-looking space rock even has two small moons of its own: Alex Helios and Cleo Selena, named after the children of the famous Egyptian pharaoh Cleopatra.
We’ve known about this incredible space rarity for about two decades, but now scientists have obtained the most detailed images we’ve seen so far. This helps us find out how Kleopatra formed and the results suggest that the moons were born from Kleopatra’s own material.
“Kleopatra is truly a unique body in our solar system,” said astronomer Franck Marchis of the Institut SETI and the Laboratoire d’Astrophysique de Marseille in France.
“Science is advancing a lot thanks to the study of strange extreme values. I think Kleopatra is one of those and understanding this multiple asteroid complex can help us learn more about our solar system.”
In two studies published in Astronomy and astrophysics, astronomers used new images of Kleopatra to obtain a more accurate 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 through observations obtained with the powerful SPHERE instrument connected to the Very Large Telescope of the Southern European Observatory in Chile. As Kleopatra fell through space, researchers were able to obtain images from different angles.
From this, they were able to determine that Kleopatra is approximately 270 kilometers long, with one of its lobes weighing more than the other, and that the two are joined by a relatively thick neck. The recently described dimensions allowed the researchers to calculate the volume of Kleopatra.
(ESO / Vernazza, Marchis et al. / MISTRAL Algorithm [ONERA/CNRS])
Above: Kleopatra with Alex Helios and Cleo Selena.
Meanwhile, a second team was working to restrict the orbits of AlexHelios and CleoSelene. This is important, as the orbits are constrained by the gravitational field that moves, 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 would go wrong, including Kleopatra’s mass,” explained 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 a greater degree of accuracy than ever before. This allowed a new calculation of the mass of Kleopatra: 2.97 x 1018 kilograms, significantly lower than the previous calculations, which obtained 4.64 x 1018 kilograms.
Once you have the mass and volume of an object, you can calculate its density. From the results of Brož and his team, Marchis and colleagues recalculated Cleopatra’s density. Assuming Kleopatra is rich in metals, the density of the asteroid turned out to be very low.
This can tell us something about how Kleopatra was formed. The low density suggests that the asteroid is quite porous: a loose “pile of debris” from pieces of rock that barely hang together. These piles of debris are believed to form when parent body material is thrown during a giant impact, which is gradually reassembled over time.
If it is porous, Kleopatra barely stays together. The asteroid has a rotation period faster than the average 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 the material in this region could move away from the asteroid.
If this is true, it gives us a clue about the training of AlexHelios and CleoSelene. If material was ejected from Kleopatra, it could have fused into orbit, forming the moons, making them, in fact, the children of the asteroid.
Both papers have been published in Astronomy and astrophysics. They can be found here and here.