216 Kleopatra
216 Kleopatra is a unique asteroid located in the asteroid belt between Mars and Jupiter, notable for its dumbbell shape, which resembles a thigh bone. First discovered in 1880, its unusual form was confirmed in 2000 using advanced telescopic techniques. The asteroid has two moons, Alexhelios and Cleoselene, named after Cleopatra VII's children, which contribute to our understanding of its formation and structure. Kleopatra is classified as a rubble-pile asteroid, indicating it is not solid but composed of loosely bound fragments, with estimates suggesting that 30% to 50% of its volume contains empty space.
Scientists believe that Kleopatra may be the result of a collision between two metallic asteroids shortly after the solar system's formation. The moons are thought to be remnants from this collision, with Alexhelios likely breaking off around 100 million years ago and Cleoselene more recently, about 10 million years ago. Ongoing research into such asteroids is crucial not only for understanding their formation but also for assessing potential threats to Earth from near-Earth objects. Kleopatra serves as a significant case study in the broader investigation of asteroid characteristics and their role in planetary science.
216 Kleopatra
FIELDS OF STUDY: Astronomy; Astrophysics; Asteroid Impact Avoidance
ABSTRACT: Kleopatra is an asteroid that was discovered in 1880. It is a large asteroid that stretches 217 kilometers (135 miles) end to end and 65 kilometers (40 miles) across. In 2008, astronomers discovered two moons in orbit around Kleopatra, Alexhelios and Cleoselene. Astronomers once believed that all large asteroids were solid. However, to date, all asteroids that have been discovered to have moons are in fact porous rubble piles held together by gravity. Astronomers theorize that Kleopatra is the remains of two rocky, metallic asteroids that smashed together following the formation of the solar system about 4.6 billion years ago.
A Dumbbell-Shaped Asteroid
Kleopatra is an asteroid that is shaped like a dumbbell. It is long and thin, with the ends wider than the middle. An asteroid is a rocky body that orbits the sun. Kleopatra is one of billions of asteroids found between the orbits of Mars and Jupiter, an area referred to as the asteroid belt. Kleopatra’s orbital period is 4.67 years. Kleopatra is orbited by two moons, Alexhelios and Cleoselene. Alexhelios is the outer moon, and Cleoselene is the inner moon.
The asteroid was named after the Egyptian queen Cleopatra VII (70–30 BCE), and the two moons were named after her twin children: Alexander Helios and Cleopatra Selene II. Each of Kleopatra’s moons measures about 8 kilometers (5 miles) in diameter.
A Rubble-Pile Asteroid
In 1994, astronomers discovered the first moon orbiting an asteroid. In 1998, astronomers using the Canada-France-Hawaii Telescope were the first to discover a moon orbiting an asteroid using a ground-based telescope with adaptive optics. Adaptive optics improve the resolution of telescopes on the ground so that astronomers can view celestial bodies more clearly. The discovery of moons in orbit around asteroids was significant. To date, all asteroids found with moons are not solid but are rubble-pile asteroids.
Kleopatra was first discovered by Austrian astronomer Johann Palisa in 1880. Astronomers previously thought that Kleopatra had a tubular shape similar to a cigar. It was not until 2000 that astronomers determined the asteroid’s shape is more like a thigh bone or a dumbbell. In 2008, a team of astronomers focused on Kleopatra. Franck Marchis, a research astronomer at the University of California, Berkeley, and his collaborators sought to confirm Kleopatra’s thigh-bone shape. They used an adaptive-optics system on the Keck II, an enormous telescope on the top of Mauna Kea in Hawaii. They confirmed the asteroid’s unusual shape—and also discovered that the asteroid is orbited by two small moons and that the asteroid is not solid.
The discovery that Kleopatra is a rubble pile surprised Marchis and the team, because most asteroids of this type are much smaller. With a length of 217 kilometers (135 miles), Kleopatra is one of the largest rubble-pile asteroids; it is second only to Sylvia, which is 280 kilometers (174 miles) long. By charting the orbits of the moons and calculating the total mass of the system, astronomers were able to determine Kleopatra’s density and estimated that about 30 to 50 percent of the asteroid is empty space.
In 2021, Marchis and his team further studied Kleopatra's moons to determine their correct orbits. Doing this enabled them to calculate Kleopatra's mass, which is 35 percent lower than their previous estimates. With this, they were able to calculate the asteroid's density, which was also lower than their first estimate. This suggests that the astroid might be more a bit more than a pile of rubble although it is still considered a rubble-pile asteroid.
Formation Theories and Research Efforts
Scientists believe Kleopatra is actually the remains of two rocky, metallic asteroids that smashed together sometime after the formation of the solar system about 4.6 billion years ago. They theorize that the two moons are pieces of rubble that broke off from the asteroid, possibly when another collision caused Kleopatra to spin. Multiple collisions may also be responsible for Kleopatra’s odd shape. While Alexhelios probably broke off one hundred million years ago, Cleoselene is thought to have formed more recently, possibly only ten million years ago.
Marchis and the team continue to study asteroids in the hopes of discovering their moons. They believe that many other large asteroids may also be rubble piles. They would like to figure out what percentage of all asteroids fit this description. They also hope that by studying rubble-pile asteroids, they will learn more about how planets form.
Astronomers also study asteroids to protect Earth from potential collision. Asteroids occasionally crash into planets, sometimes causing devastation. While it is unlikely that Kleopatra will ever be a threat, other rubble-pile asteroids might be. Near-Earth asteroid (29075) 1950 DA is on the National Aeronautics and Space Administration’s (NASA) list of potential impact threats to Earth. This asteroid has about a one in four thousand chance of crashing into Earth in the year 2880. If it does, 1950 DA, which is approximately 1.2 kilometers (0.75 miles) in diameter, has the potential to create an explosion about 3.75 million times stronger than the nuclear bomb dropped on the city of Hiroshima, Japan, during World War II. In 2014, astronomers discovered that 1950 DA, like Kleopatra, is a rubble-pile asteroid. Understanding more about this type of asteroid can help scientists determine the best way to change its course to prevent collision with Earth.
principal terms
- asteroid: a rocky body that orbits the sun.
- rubble-pile asteroid: a nonsolid asteroid that is made up of rocks of various sizes held together by gravity.
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