ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)
Scientists have snapped the best images yet of an oddball asteroid called 216 Kleopatra, revealing new insights about the huge dumbbell-shaped rock and the tiny moons that orbit it.
First discovered in 1880, Kleopatra is about 75 miles long and occupies a central region of the asteroid belt that extends between the orbits of Mars and Jupiter. For years, scientists have tried to pin down the core properties of the rock, nicknamed the “dog bone asteroid,” such as its size, density, rotation speed, and the orbital parameters of its moons.
Now, a sophisticated instrument has captured unprecedented images of Kleopatra that finally resolve some of these questions. A pair of teams led by Franck Marchis, an astronomer at the SETI Institute in California and at the Laboratoire d'Astrophysique de Marseille, and Miroslav Brož, an astronomer at Charles University in Prague, reported these new findings in separate studies published on Thursday in the journal Astronomy & Astrophysics.
“We can learn a lot, by studying those outliers, about the formation of our solar system and our past history,” said Marchis, whose team studied the density and possible formation of Kleopatra, in an email. “They force us to revise our theories, build new models, and more importantly drop our prejudice, making scientists think outside the box.”
Brož and his colleagues, meanwhile, published an updated model of this three-body orbital system in their paper. Together, the two studies reveal that Kleopatra is much lighter and more porous than previously assumed, a finding that has big implications for its formation and evolution.
Though Kleopatra was first spotted more than a century ago, it wasn’t until 2008 that Marchis and his colleagues discovered its two moons with the W. M. Keck Observatory in Hawaii. Named Alexhelios and Cleoselene after the famed Egyptian queen’s children, Alexander Helios and Cleopatra Selene II, the moons are roughly five miles in diameter.
In the years since their discovery, scientists have struggled to accurately predict the orbits of the moons. The lack of clarity about these orbital parameters made it difficult to estimate the mass and density of Kleopatra, which in turn has delayed answers about its formation and the origin of the moons. For instance, it has been unclear whether Alexhelios and Cleoselene are “infants” born from dust and debris that has gradually been shed from Kleopatra, or if they formed as the result of an ancient impact with another celestial body.
Marchis, Brož, and their colleagues were finally able to unravel some of these longstanding mysteries thanks to the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument, which is attached to the European Southern Observatory’s Very Large Telescope in Chile.
SPHERE uses a technique called adaptive optics to counteract the blurring effect of the atmosphere on observations captured by ground telescopes. As a result, its images are almost as clean as pictures taken from space. The researchers studied Kleopatra with SPHERE at various points from 2017 and 2019, an effort that led to the best pictures ever taken of the bizarre system.
“Thanks to this amazing image quality, we can now see details about the shape of the asteroid, see the two lobes and the bridge connecting them,” Marchis said. “Combining these observations with others taken using other techniques (radar, occultation, lightcurve) we have built a detailed 3D shape model of the 120-km metallic asteroid.”
“From the moons’ orbits, we derived the mass and now we have a much more reliable density measurement” which is equivalent to “less than half the density of iron,” he added.
As it turns out, Kleopatra is about 35 percent less massive than implied by previous studies, which explains the unexpected orbits of Alexhelios and Cleoselene. The lower mass and density estimates also suggest that the asteroid is a “critically rotating” body, according to the studies, meaning that it is spinning just below the speed at which its two lobes would break apart into a binary system. At this clip, even small impacts on the asteroid’s surface could cause material to fall off and accumulate, over millions of years, into its two moons.
“It’s possible that this weird shape is indeed due to the fast rotation and a composition made of loosely-bound fragments,” Marchis said. “When Kleopatra formed, it may have been very close to being split into two pieces.”
As illuminating as SPHERE’S observations are for understanding Kleopatra, plenty of open questions remain about its composition, age, and evolutionary past.
“Kleopatra is a puzzling multiple system due to the unique characteristics of the primary,” Marchis and his colleagues concluded in their study. “This system certainly deserves particular attention in the future, with the Extremely Large Telescopes and possibly a dedicated space mission, to decipher its entire formation history.”