Cleopatra orbits the Sun in the main asteroid belt between Mars and Jupiter. Ever since radar observations revealed its unusual shape about 20 years ago – a pair of lobes connected by a thick neck – astronomers have called it a ‘dog bone’. In 2008, Franck Marchis and his colleagues discovered a pair of small bodies orbiting Cleopatra. The moons were named after the children of the famous Egyptian queens Alex Helios and CleoSelene.
To learn even more about the asteroid Cleopatra, Franck Marchis and his team used images of the asteroid taken between 2017 and 2019 using the SPHERE instrument, which works on the ESO / VLT (Very Large Telescope) telescope. Thanks to the rotation of the asteroid, astronomers were able to observe the body from different angles. They were thus able to create the most accurate 3D model of the shape of an object so far and specify its volume. They also found that one of the lobes was slightly larger than the other, and set the length of the asteroid at 270 kilometers.
Czech scientists are studying an asteroid
In the second study, also published in the journal Astronomy & Astrophysics, a team of scientists led by Miroslav Brož from Charles University described how they used this data to refine the orbits of the two moons of the asteroid Cleopatra. Previous work has reported them, but new observations have shown that the moons are different from the predicted positions.
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“This discrepancy had to be resolved,” explains Miroslav Brož. “Because if the orbits of the moons are wrong, everything is wrong, including the weight of Cleopatra.” They were able to calculate the mass of the asteroid, which is 35 percent lower than originally estimated.
A remarkable object
In addition, combining the newly determined volume and mass allowed astronomers to calculate the average density of the asteroid. It is about half the density of iron and at the same time significantly lower than previously thought by scientists. The low density of Cleopatra suggests that the body could be highly porous and have a structure known as a ‘rubble pile’. This would mean that Cleopatra was probably formed after a major collision by regrouping the fragments.
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The incoherent structure and the way the asteroid rotates, in turn, indicate how its moons may have formed. Cleopatra rotates at almost a critical speed, the speed at which the body would begin to disintegrate. The impacts of small bodies can thus throw boulders out of the surface. Franck Marchis and his team believe that from these tiny fragments, AlexHelios and CleoSelene could have gradually formed, and Cleopatra did indeed ‘give life’ to his moons.
And so on
Although this research has yielded remarkable details, astronomers have high hopes for a new technique that could take knowledge even further. The upcoming ESO / ELT (Extremely Large Telescope) telescope with an even more advanced adaptive optics system will be ideal for a task such as imaging distant asteroids. “I can’t wait to point the ELT telescope at Cleopatra to see if there are more months, and to refine the trajectories to see minor changes,” adds Marchis.
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