Here’s Our Best View Yet of Asteroid Kleopatra

The European Southern Observatory returns intriguing views of enigmatic asteroid 216 Kleopatra.

It’s not every day we get a new look at a distant world, let alone a strange misshapen asteroid. But that just what happened last week, when the European Southern Observatory’s Very Large Telescope in Chile released new images of asteroid 216 Kleopatra.

The four 8.2-metre Unit Telescopes of the Very Large Telescope, along with the four 1.8-metre Auxiliary Telescopes and the VLT Survey Telescope (VST). Credit: ESO/G.Hüdepohl.

The images were obtained courtesy of SPHERE, the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument, attached to the 8.2 metre VLT Unit Telescope 3. Though SPHERE is designed for direct visual observations of planets around other stars, it does a pretty solid job at resolving objects in our own solar system, including asteroids.

Detailed view of the SPHERE optical bench. Credit: ESO

What you’re seeing in the image sequence is twin-lobed, dog-boned shaped asteroid 216 Kleoptra. Discovered on the night of April 10th, 1880 by astronomer Johann Polisa from the Austrian Naval Pola Observatory, the asteroid ranges on a 4.7 year orbit, from a perihelion 2.1 Astronomical Units (AU) from the Sun, to a distant aphelion of 3.5 AU. SPHERE uses an adaptive optics system which cancels out atmospheric distortions to nab fine detail.

216 Kleopatra
The orbit of 216 Kleopatra. Credit: NASA/JPL.

“Kleopatra is truly a unique body in our Solar System,” says astronomer Frank Marchis (SETI-Institute) in a recent press release. “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.”

The strange world of 216 Kleopatra is 276 kilometers along its longest axis. The asteroid is named after the famous 1st century BC queen of Egypt. To understand what an amazing feat it is to resolve detail on such a tiny distant object even in our own solar system with SPHERE, consider the viewing geometry: at 449 million kilometers (279 million miles) away, an object 276 kilometers across appears just over 0.1” (a tenth of an arcsecond) in apparent size… that’s around 250 times smaller than Mars at its very closest to Earth.

A size comparison of asteroid Kleopatra with northern Italy. Credit: ESO/M. Kornmesser/Marchis et al.

Kleopatra spins on its axis once every 5.4 hours, and the late Arecibo observatory nabbed its contact binary nature during radar observations in 2000. Observations from Keck Observatory in 2008 also revealed the presence of two tiny (7-9 kilometer-sized) moonlets in orbit around 216 Kleopatra: Alexhelios and Cleoselene, named after the children of Cleopatra.

Processed SPHERE image showing the moons of Kleopatra. Credit:
ESO/Vernazza/Marchis et al/MISTRAL/ONERA/CNRS.

Discovery of the moons is a lucky break for astronomers, as it allows them to peg the overall density and mass of 216 Kleopatra. But it also presented a mystery, as positions noted in the recent SPHERE observations of the moonlets did not match up with estimates from early observations.

“This had to be resolved,” says Miroslav Brož (Charles University, Prague) in a recent press release. “Because if the moons’ orbits were wrong, everything was wrong, including the mass of Kleopatra.”

SPHERE VLT observations of the shape and size of 216 Kleopatra along with the orbits of the two moons allowed astronomers to further characterize the main asteroid as a loose ‘rubble pile’ about half the density of iron, 35% lower than previously thought.

Astronomers also realized the fast rotational period of 216 Kleopatra could well explain the existence of the two moons, as material from impacts was flung off from the asteroid’s surface. Both moonlets orbit the asteroid once every 1.2 and 2.3 days respectively, and would possess a strange sky, dominated by the dumbbell-shaped primary.

Observing 216 Kleopatra

If you have a large telescope, you can spot 216 Kleoptra: Generally, it’s at +12th magnitude, but it can reach +9th magnitude on a favorable opposition near perihelion. This occurs next in 2027. Currently, the asteroid is located in the constellation of Ophiuchus the Serpent Bearer.

Kleopatra path
The path of asteroid 216 Kleopatra through September and October 2021.

Seeing 216 Kleoptra up close is a tour de force for the power of ground-based adaptive optics. The VLT-SPHERE configuration has also taken images of 4 Vesta that rivaled those taken by NASA’s Dawn spacecraft when it visited the dwarf planet in 2011-2012.

Far from being ‘boring space rocks’ recent observations of NEO asteroids have revealed that 15% are binary or contact binaries, and worlds like 216 Kleopatra may hold clues to early solar system history and planetary formation.

Are all asteroids this weird?

Read the original papers, Kleopatra, a Low-Density, Critically Rotating M-type Asteroid and An Advanced Multi-Pole Model for the Kleopatra Triple System.

Lead image: Asteroid Kleopatra from different angles. Credit: ESO/Vernazza, Marchis et al./MISTRAL/ONERA/CNRS.