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Asteroid 216 Kleopatra

November 1, 2021

Wheat and peanuts are crops that are easily cultivated and harvested. For those reasons, these are inexpensive food ingredients that are used in many supermarket items. That's bad news for the many people with peanut allergy or gluten intolerance, who need to carefully read product ingredient labels. These ubiquitous crops are the subject of many cultural memes, as in the adage, "Man does not live by bread alone." Economists assert that some people are willing to "work for peanuts."

There's also the Charles M.Schulz (1922-2000) comic strip, Peanuts, and the foam peanut packaging material, now mostly displaced by the bubble wrap successor, polyethylene bags filled with air. The peanut is personified in the character, Mr. Peanut, used as the advertising logo and mascot of Planters for more than a century. Mr. Peanut reportedly is named, Bartholomew Richard Fitzgerald-Smythe, which sounds like the name of a character on one of the many British television series on PBS.

Arachis hypogaea (the peanut)

The peanut plant, Arachis hypogaea.

The peanut is an Angiosperm (lowering plant). The name derives from the Greek, angeion (αγγείον, vessel) and sperma (σπέρμα, seed) In the peanut case, the vessel is its shell.

The peanut is the most commercially important member of the seventy member genus, Arachis. As befits its name, the peanut is in the pea family (Fabaceae).

(An 1887 image by Franz Eugen Köhler from Köhler's Medizinal-Pflanzen, via Wikimedia Commons. Click for larger image.)

A peanut's shape is a consequence of its being a dicotyledon, the name given to plants having seeds with two elements. Since this shape is generally recognized, some items are designated as peanut-shaped. One of these is the minor planet, (486958) 2014 MU69, also known as Ultima Thule, discovered in 2014 by the Hubble Space Telescope. I wrote about Ultima Thule in an earlier article (Ultima Thule, January 7, 2019).

Color image of Ultima Thule, (486958) 2014 MU<sub>69</sub>

Image of Ultima Thule, (486958) 2014 MU69, by the New Horizons spacecraft, colorized with data from earlier images.

Ultima Thule has been called snowman-shaped and peanut-shaped.

(Ultima Thule image from NASA/Johns Hopkins University Applied Physics Laboratory/ Southwest Research Institute, colorized by Renerpho, via Wikimedia Commons.)

The asteroid, 216 Kleopatra, named after Egyptian queen, Cleopatra, was discovered in 1880 by Austrian astronomer, Johann Palisa. Kleopatra is much closer to Earth than Ultima Thule, orbiting in the center of the asteroid belt between Mars and Jupiter at a distance of 2.1-3.5 AU (315-525 million kilometers). However, because of its small size, just 75 miles (120 kilometers) in extent, it was just just a point of light to terrestrial telescopes. Radar observations at the turn of the century revealed its peanut shape, although some astronomers prefer to use the equivalent shape designation, dog-bone. Recent optical telescope observations have produced more detailed images of this asteroid.[1-3]

Kleopatra has this shape because it's a contact binary having two elements joined to each other by a neck of debris. If its rotation period of 5.4 hours was much higher, its lobes would separate to form a binary system. A huge international team of astronomers used the European Southern Observatory’s Very Large Telescope (VLT) to capture detailed images of the asteroid Kleopatra. The asteroid was imaged using the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument on the VLT between 2017 and 2019.

Since Kleopatra rotates, the images were from different angles, so they allowed creation of an accurate 3D model of its shape. The model showed that one of the lobes is larger than the other, and that the extent of the asteroid is about 270 kilometers. The density, previously thought to be 4.5 grams per cubic centimeter, was refined to 3.4.[1,3] Says Franck Marchis, an astronomer at the SETI Institute (Mountain View, California) and the Laboratoire d'Astrophysique de Marseille, France, and leader of the study,
"Kleopatra is truly a unique body in our Solar System... 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."[3]

Asteroid Kleopatra from different angles

Asteroid, 216 Kleopatra from different angles. These images were taken at different times between 2017 and 2019 with the SPHERE instrument on ESO's Very Large Telescope. (ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS) image, licensed under a Creative Commons Attribution 4.0 International License. Click for larger image.)

Kleopatra has two moons, named AlexHelios and CleoSelene, the names of Cleopatra's children. A companion study used the SPHERE observations to determine the orbits of these two two moons with better accuracy than previous studies.[2-3] A determination of these orbits was required to correctly determine Kleopatra's mass, which turned out to be 35% lower than previous estimates.[3] The mass, along with the volume, allowed calculation of Kleopatra's density, which is half that of iron.[3] Since the asteroid is thought to have a metallic composition, this leads to the conclusion that it's just a porous pile of rubble, formed from the remains of a giant impact.[3]

Since Kleopatra rotates at almost the speed at which it would start to fling off material, even small impacts would eject debris from its surface, and it's thought that its moons, AlexHelios and CleoSelene, were formed in this fashion.[3] adaptive optics allowed SPHERE to image Kleopatra even though its apparent size on the sky is equivalent to that of a golf ball at about 40 kilometers distance.[3] ESO has planned a successor to the VLT, appropriately called the Extremely Large Telescope (ELT).[3]

Size comparison of asteroid Kleopatra with northern Italy

Size comparison of asteroid, 216 Kleopatra with northern Italy.

Kleopatra is shown as a computer model image.

(ESO/M. Kornmesser/Marchis et al. image, licensed under a Creative Commons Attribution 4.0 International License.)


  1. F. Marchis, L. Jorda, P. Vernazza, M. Brož, J. Hanuš, M. Ferrais, F. Vachier, N. Rambaux, M. Marsset, M. Viikinkoski, E. Jehin, S. Benseguane, E. Podlewska-Gaca, B. Carry, A. Drouard, S. Fauvaud, M. Birlan, J. Berthier, P. Bartczak, C. Dumas, G. Dudzinski, J. Durech, J. Castillo-Rogez, F. Cipriani, F. Colas, R. Fetick, T. Fusco, J. Grice, A. Kryszczynska, P. Lamy, A. Marciniak, T. Michalowski, P. Michel, M. Pajuelo, T. Santana-Ros, P. Tanga, A. Vigan, O. Witasse and B. Yang, "(216) Kleopatra, a low density critically rotating M-type asteroid," Astronomy & Astrophysics, vol. 653, article no. A57 (September 9, 2021). DOI: 10.1051/0004-6361/202140874. A PDF file can be found at arXiv.
  2. M. Brož, F. Marchis, L. Jorda, J. Hanuš, P. Vernazza, M. Ferrais, F. Vachier, N. Rambaux, M. Marsset, M. Viikinkoski, E. Jehin, S. Benseguane, E. Podlewska-Gaca, B. Carry, A. Drouard, S. Fauvaud, M. Birlan, J. Berthier, P. Bartczak, C. Dumas, G. Dudzinski, J. Durech, J. Castillo-Rogez, F. Cipriani, F. Colas, R. Fetick, T. Fusco, J. Grice, A. Kryszczynska, P. Lamy, A. Marciniak, T. Michalowski, P. Michel, M. Pajuelo, T. Santana-Ros, P. Tanga, A. Vigan, D. Vokrouhlický, O. Witasse and B. Yang, "An advanced multipole model for (216) Kleopatra triple system," Astronomy & Astrophysics, vol. 653, article no. A56 (September 9, 2021). DOI: 10.1051/0004-6361/202140901. A PDF file can be found at arXiv.
  3. ESO captures best images yet of peculiar "dog-bone" asteroid, European Southern Observatory Photo Release eso2113, September 9, 2021.
  4. Location of Kleopatra in the Solar System, European Southern Observatory video, September 9, 2021. Also appears as a YouTube video.

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