The dog bone-shaped asteroid orbiting Jupiter and Mars is actually a unique object in our solar system. Named after the famous Egyptian Queen Cleopatra, it travels by two small moons. It’s definitely a strange sight compared to all the boring spherical bodies that surround our Sun. This week, the European Southern Observatory has just published new images that tell us more about this strange object.
Cleopatra was discovered on April 10, 1880, by Austrian astronomer Johann Balesa at the Pula Naval Observatory in what is now Pula, Croatia. For the record, astronomer Palisa was a prolific asteroid finder who found 122 during his lifetime and did so without using any images (as other astronomers did), discovering them all with the naked eye. As an expression of his love, he named an asteroid after his wife, Penda, in 1912.
You will remember that most of the asteroids in our solar system are in what astronomers call the asteroid belt and this is where Cleopatra is located, about 2.7 times the distance between the Earth and the Sun. There are many asteroids in this region. At this point in 2021, hundreds of thousands of those bodies are now known and named. Some astronomers speculate that the total number of asteroids could exceed several million, depending on their size, ranging from golf balls to dwarf planets. More than 200 asteroids are known to be more than 70 miles in diameter, and some are so large and bright, like Vesta, that they can be seen with the naked eye. Although Cleopatra is about 80 miles long, it cannot be seen without the aid of binoculars, having a brightness of only 7.5 (6 being the limit visible to the eye and higher numbers being darker).
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Before we talk about Cleopatra, let’s see what the asteroid does. Scientists have grouped its composition into three different basic types. The most common are carbonaceous asteroids. They are rich in carbon and correspond to the primitive formation of the early solar system. The second most common asteroids are asteroids made of silicates and represent about 17% of the total population. They are usually found in the center of the belt, equidistant between Jupiter and Mars. Then there are the rare metal asteroids made of iron and nickel. Less than 10% of metallic asteroids. Some astronomers believe it comes from the mineral core of a primordial planet that somehow broke apart, possibly collided with another object or split in two by Jupiter’s intense gravitational field. Metallic asteroids are generally very bright and are believed to be the source of iron meteorites found on Earth. Kleopatra is this type of asteroid and is therefore very rare. Because it is metallic, it can be easily visualized by ground-based radar, which was manufactured in 2000 by the Arecibo Observatory in Puerto Rico. They definitely showed that the body was actually bone shaped.
As with some larger asteroids, in 2008, Cleopatra was discovered to have two small moons. Frank Marches found them using the Keck Large Telescope on Mauna Kea, Hawaii. The outer surface was named Alexhelios and the inner Cleoselene, in honor of Cleopatra’s sons, Alexander Helios and Cleopatra Selene. Both have a diameter of less than six miles and orbit with periods of 2.3 and 1.2 days, respectively. The moons could be smaller asteroids that were somehow caught en route or, some believe, broken fragments from the main body that fell.
Using the moons’ orbit times and Newton’s law of gravity, Cleopatra’s mass was determined to be about 5.4 tons. Knowing this and body size, the average density of Cleopatra is 4.6 grams per cubic centimeter, which is smaller than 5.5 on planet Earth. Accordingly, Cleopatra must be quite porous if we assume that the metal that makes up the core of the Earth and Cleopatra have the same composition. For this reason, scientists suggest that poor Kleo is not as solid as he seems and may be little more than a “pile of rubble.” Although Cleopatra has an oblong shape, it rotates very quickly. It only takes 5.4 hours for one cycle, and if it were to spin faster, the lobes would be separated from each other.
According to the ESO website, the new Kleopatra images and the information they provide are only possible thanks to the advanced adaptive optics system used on the Very Large Telescope located in Chile’s Atacama Desert. These computer-controlled optics help correct distortions caused by Earth’s atmosphere. From a distance of 320 million kilometers, looking at Cleopatra is like seeing a golf ball 40 kilometers away.
Astronomer Marchis, who discovered the moons, is now at ESO and has taken the latest images. Soon, ESO’s upcoming Very Large Telescope will be operational and ideal for imaging asteroids. “I can’t wait to point the ELT at Cleopatra to see if there are more moons and improve their orbits to detect small changes,” Marches said.