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In two studies published in Astronomy & 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 accurately defining the orbits of AlexHelios and CleoSelene.
The work was conducted using observations obtained with the powerful SPHERE instrument attached to the European Southern Observatory's Very Large Telescope in Chile. As Kleopatra tumbled through space, the researchers were able to obtain images from different angles.
From this, they were able to determine that Kleopatra is roughly 270 kilometers (168 miles) long, with one of its dumbbell lobes larger than the other, and that the two are joined by a relatively thick neck. The newly described dimensions then allowed the researchers to calculate Kleopatra's volume.
Once you have the mass and the volume of an object, you can calculate its density. Using the results of Brož and his team, Marchis and his colleagues then recalculated the density of Cleopatra. Assuming that Kleopatra is metal-rich, the asteroid's density turned out to be very low.
This can tell us something about how Kleopatra formed. Low density suggests that the asteroid is rather porous – a loose "rubble pile" of bits of rock barely hanging together. Such rubble piles are thought to have formed when material is flung out from a parent body during a giant impact, gradually reassembling over time.
If it is porous, Kleopatra is barely holding itself together. The asteroid has a faster-than-average rotation period of around 5.4 hours. That period is just on this side of stability; were it to speed up, the centripetal force would tear it apart.
This state of critical rotation means that effective gravity at the equator is low, and material in this region could be lifting away from the asteroid.