A curiosity of our solar system in sight: Through the most detailed images so far, astronomers have gained new knowledge about the bizarre asteroid (216) Cleopatra and its two moons. Using the data, they were able to determine the 3D shape and mass of the dog-bone-shaped celestial body more precisely than before. The results provide clues to the inner structure of Cleopatra and shed light on the history of the origins of the small three-part system, which consists of the asteroid and the moons Alexhelios and Cleoselene.
The focus is on the main asteroid belt of our solar system: In the area between the planetary orbits of Mars and Jupiter, a large part of the known asteroids orbits our central star. One of the largest specimens was named after the ancient Egyptian ruler Cleopatra after its discovery in 1980. Radar observations later showed that this celestial body has an unusual shape: two bulges are connected by a kind of neck. This has earned Cleopatra the nickname “dog bone asteroid”. In 2008, astronomers also discovered that Cleopatra was orbited by two small moons. According to the name specification, they were named after the children of the Egyptian pharaoh Alexhelios and Cleoselene.
3D model developed
Despite initial assessments and recordings, the details of the three-part system have so far remained unclear. That is why the international team of researchers led by Franck Marchis from the SETI Institute in Mountain View has set their sights on it again. The astronomers divided their results into two studies. The first focused on evaluating data from the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument (SPHERE) at the Very Large Telescope of the European Southern Observatory (ESO) in Chile. The astronomers combined numerous snapshots of the asteroid, which showed it from different angles due to its rotation.
In the end, they were able to create the most accurate 3D model of Cleopatra to date and thus document the dimensions of the asteroid and its dog-bone shape in detail. The celestial body is therefore about 270 kilometers long and one of the two thickenings is larger than the other. Based on the surface data, the volume of Cleopatra could also be narrowed down better than before, the scientists report.
This information was in turn linked to the key messages of the team’s second study. In it, the astronomers report how they used SPHERE observation data to determine the correct orbits of the moons Alexhelios and Cleoselene. It showed: They are not moving on orbits around Cleopatra, as they had predicted earlier. “That had to be clarified,” says the first author of the second study, Miroslav Brož from Charles University in Prague. “Because if the orbits of the moons were wrong, the previously suspected mass of Cleopatra must also have been wrong.” Thanks to the new observations and sophisticated modeling, the team finally succeeded in describing exactly how Cleopatra’s gravity influences the moons’ movements , and how the complex orbits of Alexhelios and Cleoselene arise. From this, the asteroid’s mass emerged, which was 35 percent below the previous estimates.
Flying “heap of rubble”
By combining the new estimates for volume and mass, the astronomers were able to calculate a new value for the asteroid’s density. It is 3.4 grams per cubic centimeter, while it was previously assumed that Cleopatra has an average density of around 4.5 grams per cubic centimeter. As the researchers explain, these values indicate that the asteroid has a more porous structure and could hardly be more than a “heap of rubble”. That means it probably formed when the material clumped back together after a gigantic impact, the astronomers explain.
Cleopatra’s debris pile structure and the rather high rate of rotation of the asteroid also provide clues as to how the two moons could have formed. The asteroid is rotating close to a critical speed at which it would fall apart. This means that even small impacts can lift material off its surface. The scientists therefore suspect that Alexhelios and Cleoselene were created from such lost material, which means that Cleopatra actually gave birth to these moons, as it were.
“Cleopatra is really a very special celestial body in our solar system,” says Marchis. “I think understanding this complex asteroid system can help us learn more about our solar system. Because science often makes progress thanks to the study of strange eccentrics, ”says the astronomer. In further investigating the Cleopatra system, he and his colleagues are now hoping for a closer look at ESO’s Extremely Large Telescope (ELT), which is scheduled to go into operation in 2027. “I can’t wait to point the ELT at Cleopatra to see if there are any more moons and to see more details,” concludes Marchis.
Source: ESO