The extraordinary asteroid Cleopatra is shaped like a dog’s bone and has two moons: Now researchers have discovered surprising things about the celestial body and its satellites.
The dog-bone asteroid Cleopatra is half the length of the English Channel.
Photo: ESO / M. Kornmesser / Marchis et al
Cleopatra is an outsider. The asteroid, named after the legendary Egyptian pharaoh, orbits the sun in the asteroid belt between Mars and Jupiter and, as a celestial body, is very different from the other known asteroids. Now the asteroid made an extraordinary astronomical discovery.
“Investigations of strange outsiders regularly lead to significant scientific advances. That should also be the case with Cleopatra. If we understand this complex asteroid system better, we can learn a lot about our solar system, ”says Franck Marchis, astronomer at the SETI Institute in the US Mountain View. He is the head of the study on the asteroid Cleopatra, which they also call the dog-bone asteroid.
Dog-bone asteroid Cleopatra has at least two moons of its own
Cleopatra is unusually shaped: the elongated asteroid, which is 200 million kilometers from Earth, has two thick ends and a slim connector and looks like a stylized chew bone for dogs. In addition, two moons orbit the asteroid, as Marchis and his colleagues discovered: They bear the names of Cleopatra’s children: AlexHelios and CleoSelene. The researchers have now found out how logical this designation is.
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Because the astronomers have succeeded in the most precise modeling of the three-dimensional shape of Cleopatra to date, which is now used as the basis for studies on the mass and density of the dog-bone asteroid. For this purpose, Marchis and his team used images of the asteroid that were recorded between 2017 and 2019 with the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument on the Very Large Telescope (VLT) of the European Southern Observatory ESO.
The asteroid Cleopatra is shaped like the bone of a dog. Because it rotates, the pictures of the VLT, which were taken over two years, show it from different perspectives.
Photo: That
The VLT in the Chilean Atacama Desert has adaptive optics. These help correct distortions caused by the earth’s atmosphere and make objects appear out of focus. It is precisely because of this effect that stars that we observe with the naked eye from Earth seem to twinkle. Such a sharp image of the distant asteroid was only possible with the help of these corrections. Because the dog-bone asteroid rotates, the high-resolution images show it from different angles. Cleopatra is around 270 kilometers long: that’s half the length of the English Channel. And one of the two thickenings at the ends is larger than the other.
Researchers recalculate lunar orbits
In a second study, led by Miroslav Brož from Charles University in Prague, Czech Republic, a team used SPHERE observations to study the orbits of Cleopatra’s two moons. Scientists had tried to predict the trajectories before. However, these prognoses did not coincide with the new observations that the astronomers had made with the VLT. “We had to resolve this discrepancy,” says Miroslav Brož. “Because if the orbits of the moons were wrong, everything was wrong, including the mass of Cleopatra.” With the help of the new observations and detailed 3D modeling, the researchers were now able to describe exactly how Cleopatra’s gravity influences the moons’ movements. This enabled them to recalculate the asteroid’s mass. Accordingly, this is 35% below previous estimates.
Asteroid, meteorid, comet: what are the differences?
- Basically, the celestial bodies differ in size, composition and their distance from the sun.
- A Asteroid Usually consists of solid substances and contains few or no volatile substances, which is why it has no tail
- A Comet consists largely of volatile substances such as water. Because the distance from the sun is great, the volatile substances are frozen. When comets approach warmer regions, the ice melts and the typical comet tail is created.
- Meteorites are the smallest objects. They often arise from the particles of a comet’s tail or when larger celestial bodies collide. If they enter the earth’s atmosphere, they can be perceived as a meteor or falling star.
Using the new values for volume and mass, the research team was also able to derive a new value for the density of the asteroid. With less than half the density of iron, this is lower than previously assumed. Cleopatra probably has a metallic composition and a porous structure. In fact, the asteroid is now believed to be a rather loose heap of rubble, made up of parts that clumped together again after a gigantic impact.
Asteroid Cleopatra rotates at extreme speed and almost flies apart
And that allows conclusions to be drawn about the formation of the two moons. The asteroid rotates almost at the so-called critical speed: This means the speed of rotation at which the matter that makes up the asteroid would fly apart. Say: Cleopatra is quite unstable. Therefore, even small impacts can loosen material from the surface. Probably the unusual situation that the asteroid has two moons of its own: AlexHelios and CleoSelene could have formed from the dissolved material. To stay in the picture: Cleopatra gave birth to her moons herself.
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In the future, research into asteroids should once again make significant progress: The ESO’s Extremely Large Telescope (ELT), which is currently still being built, has even more advanced adaptive optical systems. “I can hardly wait to point the ELT at Cleopatra to see whether there are more moons and to measure the lunar orbits so precisely that even small changes can be detected,” says Franck Marchis.
Asteroids often come from the asteroid belt, which is located between the planetary orbits of Mars and Jupiter. It used to be assumed that the asteroid belt must have arisen from a catastrophic collision between a planet and an asteroid: It was believed that a now-defunct planet, called Phaeton, must have given its debris after the collision as asteroid orbiting the sun. This thesis has now been refuted. Because the total mass of all asteroids is lower than that of our Earth’s moon – too little for a planet. Rather, the asteroid belt is likely to have arisen from planetesimals, i.e. from planetary components, in the early phase of our solar system. Due to the force of gravity of Jupiter, these could never completely come together to form a planet.
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Sometimes asteroids break out of the belt, for example after colliding with other debris. Then they go their own way and can theoretically get close to the earth – like the Asteroid Bennu. The 500-meter-tall celestial body repeatedly crosses the earth’s orbit and regularly comes very close to our planet. Even an impact cannot be ruled out: on September 24, 2182, this scenario could become a reality. This is the result of the latest analyzes of the asteroid that astronomers have long been targeting. Bennu is considered to be one of the most dangerous asteroids of all, although the probability of an impact is relatively low: the chances that Bennu will hit the earth by 2300 are 1: 1750. Asteroid Cleopatra, however, which was discovered in 1880, cannot endanger the earth: It moves its orbits in the distant asteroid belt.
Comet from a completely unknown world
Some of the celestial bodies that move in our solar system have nothing to do with the asteroid belt – they come from a much more distant area, such as the Comet 2I / Borisov. Most comets come from the so-called Oort cloud, a collection of astronomical objects on the edge of the solar system – at least that’s the theory, because nobody has seen this cloud yet. This area is named after the Dutch astronomer Jan Hendrik Oort, who developed his theory in 1950. According to this, the cloud, which surrounds the solar system in a spherical shape, consists of planetesimals – pieces of rock and ice lumps of different sizes that remained as planetary building blocks after the formation of the solar system. Borisov, however, comes from an even more unknown region: he is an interstellar comet from the infinite expanses of the Milky Way, which crossed our solar system, but will probably never come back.
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