The asteroid (216) Kleopatra is as distinctive as the queen it was named after, although perhaps less glamorous. One of the largest telescopes in the world has captured images confirming it is shaped like the stereotypical bone dogs gnaw on, making it one of the most oddly shaped objects in the solar system. The observations could improve our understanding of asteroid formation.
Much as flat-earthers may resist the idea, above a certain size, gravity makes astronomical objects spherical. Below this threshold, diversity flourishes – but not everything goes. Finding something as long and thin as Kleopatra, with large lobes on each end, was enough of a surprise to justify time on the European Space Observatory's Very Large Telescope (VLT), the results of which have been published in Astronomy and Astrophysics.
“Kleopatra is truly a unique body in our Solar System," said Dr Frank Marchis of the SETI Institute in a statement. “Science makes a lot of progress thanks to the study of weird outliers. I think Kleopatra is one of those and understanding this complex, multiple asteroid system can help us learn more about our Solar System.”
Marchis calls it a system because Kleopatra comes with two moons. Astronomers passed up the chance to call these Julius Caesar and Mark Antony, instead using the names of two of the original Kleopatra's children, AlexHelios and CleoSelene.
As a resident of the main asteroid belt, Kleopatra never gets close enough to us for a really good look, but its unusual shape was first observed using radar 20 years ago, and the moons found in 2008. The VLT's enormous resolving power allowed Marchis and co-authors to watch Kleopatra as it rotates and create a 3D model of its shape. Five occultations, when Kleopatra blocked out the light of a star, helped improve matters further.
To grasp the scale of the challenge, the team point out Kleopatra from Earth is the size of a golf ball 40 kilometers (24 miles) away. Marchis is not satisfied, however, saying he, “Can't wait to point the [Extremely Large Telescope] at Kleopatra,” referring to the astronomical behemoth scheduled to start work in 2027.
Kleopatra is relatively large by asteroid standards, the team determined, at around 270 kilometers (160 miles) long, but the neck is so narrow it contains just 13 percent of the volume. The lobes are almost equal in size, just 16 percent difference.
An accompanying paper provides precise orbits for the two moons, noting that the VLT showed previous estimates had been wrong, rediscovering them in places different from those predicted.
“If the moons' orbits were wrong, everything was wrong, including the mass of Kleopatra," said Charles University, Prague's Dr Miroslav Brož, lead author of the second paper. Correcting the two moons' orbits led to a 56 percent downward revision in estimates of Kleopatra's mass. This is because the asteroid, once thought to be the densest small object in the solar system, is actually of quite ordinary density.
Since Kleopatra appears to have a high metal composition, this density indicates a porous structure, thought to be a legacy of a collision that destroyed a predecessor. Gravity then drew the material back together, but more loosely than before, creating what is known as a “rubble pile” object.
Kleopatra rotates so rapidly its low density is barely enough to hold it together. Marchis suspects the moons AlexHelios and CleoSelene were formed out of components spun off Kleopatra, possibly after very small impacts.