Asteroids are small, rocky, and metallic bodies that orbit the Sun, mostly concentrated in the main belt between Mars and Jupiter. While often thought of as solitary travelers, a significant number of these minor bodies possess companions. Asteroids can definitely have moons, and these multiple-body systems represent a fascinating field of study in planetary science. These paired objects provide fundamental insights into the physical properties and evolutionary history of the minor bodies from which they formed.
Confirmation of Asteroid Satellites
Astronomers refer to an asteroid with one moon as a binary asteroid system, and objects with two moons are called triple systems, such as 87 Sylvia. These satellites are also commonly termed companions or secondary components. The first confirmed discovery occurred in 1993, when the Galileo spacecraft imaged the main-belt asteroid 243 Ida and its tiny satellite, Dactyl. This finding marked a turning point, disproving the earlier notion that such systems were rare.
Since then, the number of confirmed multi-asteroid systems has grown substantially, thanks to advancements in technology. Techniques like radar observations and high-resolution imaging have rapidly expanded the catalog. For instance, an estimated 15% of near-Earth asteroids are now known to be binary or multiple systems. The primary component is typically much larger than its secondary, though systems with components of nearly equal size are also known.
Mechanisms of Moon Formation
The formation of asteroid companions is explained by several distinct physical processes, often depending on the asteroid’s location and size. For small, rapidly rotating near-Earth asteroids (NEAs), the most common mechanism is rotational fission, driven by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. This effect describes how the absorption and asymmetrical re-emission of solar radiation creates a torque, causing the asteroid’s rotation rate to increase over time. As the asteroid spins faster, centrifugal forces overcome the weak gravity, causing material to shed from the equator. Since the parent body is often a “rubble pile”—a collection of fragments held loosely by gravity—it is susceptible to this spin-up and mass shedding.
This ejected material then re-accretes into orbit around the primary to form a satellite, a process that can also lead to triple systems. Satellites of larger main-belt asteroids are typically created by major impact events. A catastrophic collision can disrupt the parent body, with fragments mutually capturing each other into orbit. Less energetic collisions can also eject material that re-accretes into a satellite around the remaining primary body. Gravitational capture of a passing asteroid is another possibility, though long-term, stable capture is less common in the main asteroid belt.
Unique Characteristics of Binary Systems
Asteroid moons differ significantly from the large, distant planetary moons in our Solar System, largely due to the differences in mass and gravity. A distinguishing feature is the size ratio between the components; asteroid satellites are often much larger in proportion to their primary body than planetary moons are to their planet. This can result in systems where the two bodies are nearly equal in size, such as 90 Antiope, which are sometimes referred to as contact binaries.
The orbits of these companions are often close and highly irregular due to the low surface gravity of the primary asteroid. The orbits in binary asteroid systems are subject to significant perturbations from solar radiation and other forces, resulting in orbits that can be highly elliptical and generally less stable than those in systems dominated by a single massive planet. Furthermore, the components of a binary system are typically made of the same material, supporting the formation theories involving fission or impact-ejecta re-accretion.
Mapping and Scientific Importance
The study of binary asteroid systems provides astronomers with a powerful tool to determine the physical makeup of these distant objects. By precisely observing the orbital period and distance of the satellite around its primary, scientists use Kepler’s laws of motion to calculate the total mass of the system. When combined with estimates of the asteroid’s size determined through imaging or radar, this mass figure directly yields the asteroid’s density.
Density measurements are invaluable because they reveal the asteroid’s internal structure, indicating whether it is solid rock or a loosely bound “rubble pile” with significant internal porosity. For example, the discovery of a satellite orbiting 45 Eugenia allowed researchers to determine that the primary has a very low density, suggesting it is a rubble pile with considerable empty space. Well-known examples like the binary near-Earth asteroid 65803 Didymos, with its moon Dimorphos, offer real-world laboratories for studying the evolution of these systems. The discovery of the moon Dactyl orbiting 243 Ida was the first step in this specialized field.