Binary asteroid

Lore & Background

The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple asteroids have been detected. The mass ratio of the two components – called the 'primary' and 'secondary' – is an important characteristic. Most binary asteroids have a large mass ratio, i.e., a relatively small satellite in orbit around the main component. Systems with one or more small moons include 87 Sylvia, 107 Camilla and 45 Eugenia (all triples), 121 Hermione, 130 Elektra (a quadruple), 22 Kalliope, 283 Emma, 379 Huenna, 243 Ida and 4337 Arecibo (in order of decreasing primary size). Some binary systems have a mass ratio near unity, i.e., two components of similar mass. They include 90 Antiope, 2006 VW139, 2017 YE5 and 69230 Hermes, with average component diameters of 86, 1.8, 0.9 and 0.8 km, respectively. In August 2024 Gaia reported 352 new binary asteroid candidates.

Several theories have been posited to explain the formation of binary-asteroid systems. Many systems have significant macro-porosity (a 'rubble-pile' interior). The satellites orbiting large main-belt asteroids such as 22 Kalliope, 45 Eugenia or 87 Sylvia may have formed by disruption of a parent body after impact or fission after an oblique impact. Trans-Neptunian binaries may have formed during the formation of the Solar System by mutual capture or three-body interaction. Near-Earth asteroids, which orbit in the inner part of the Solar System, most likely form by spin-up and mass shedding, likely as a result of the YORP effect. Numerical simulations suggest that when solar energy spins a 'rubble pile' asteroid to a sufficiently fast rate by the YORP effect, material is thrown from the asteroid's equator. This process also exposes fresh material at the poles of the asteroid.

Reader's Guide

Binary asteroids are notable for revealing the diversity of asteroid systems in the Solar System. The discovery of 243 Ida's binary nature by the Galileo spacecraft in 1993 opened a new field of study. Since then, numerous binary and triple systems have been detected, with mass ratios ranging from large (a small satellite orbiting a larger primary) to near unity (two components of similar size). Examples of near-unity systems include 90 Antiope, 2006 VW139, 2017 YE5, and 69230 Hermes, with average component diameters from 86 km down to 0.8 km. In August 2024, Gaia reported 352 new binary asteroid candidates, greatly expanding the known population. The formation of these systems varies by location: main-belt binaries likely form by disruption or fission after impact, trans-Neptunian binaries by mutual capture or three-body interaction during Solar System formation, and near-Earth binaries by spin-up and mass shedding due to the YORP effect. Numerical simulations indicate that the YORP effect spins rubble-pile asteroids until material is thrown from the equator, also exposing fresh material at the poles. These systems provide insight into asteroid interiors, impact processes, and the evolution of small bodies.

Did You Know?

• The binary nature of 243 Ida was discovered by the Galileo spacecraft in 1993.
• 130 Elektra is a quadruple asteroid system.

Physical Nature and Taxonomic Diversity

Asteroids are rocky, metallic, or icy bodies lacking any atmosphere, ranging in size from rubble piles under a kilometer across to Ceres, which measures nearly 1000 km in diameter and holds dwarf-planet status. They sit above the one-meter threshold that separates them from meteoroids, and below the scale of recognized planets. The IAU's 2006 framework places them under the broader umbrella of small Solar System bodies, a category that also encompasses comets, centaurs, trojans, and trans-Neptunian objects. Compositionally, asteroids fall into three broad taxonomic groups: carbonaceous C-types, metallic M-types, and silicaceous S-types. Their shapes are irregular, and their physical character reflects the raw building blocks from which the inner Solar System assembled. Despite their diversity in size and makeup, all share the defining trait of orbiting the Sun without developing the sustained atmospheric or cometary activity that would reclassify them.

Orbital Architecture and Distribution

The vast majority of the roughly one million known asteroids cluster in the main belt, a ring of rocky and metallic debris stretching between the orbits of Mars and Jupiter, roughly two to four astronomical units from the Sun. These bodies trace slightly elliptical paths, all revolving in the same prograde direction as Earth, and completing a full circuit in three to six years. Their combined mass is a mere three percent of the Moon's. Beyond this central population, smaller groups orbit as Jupiter trojans, sharing the giant planet's path, while near-Earth asteroids venture closer to our world. For nearly two centuries after Ceres's 1801 discovery, every known asteroid remained at or inside Jupiter's orbit. That changed in 1977 with 2060 Chiron, the first recognized centaur permanently residing beyond Jupiter. By 1992, 15760 Albion marked the first find beyond Neptune, opening the door to the vast trans-Neptunian and Kuiper-belt populations where ices remain solid and cometary activity is minimal.

The Space Age of Asteroid Exploration

Asteroids have been visible from Earth since Ceres was spotted in 1801, but the first true close-up view came from NASA's Galileo spacecraft. Since then, a steady stream of dedicated missions has transformed our understanding. NASA's NEAR Shoemaker orbited Eros, while Dawn visited both Vesta and Ceres. Japan's JAXA sent Hayabusa and Hayabusa2 to Itokawa and Ryugu, each returning physical samples to Earth. OSIRIS-REx collected material from Bennu in 2020, delivering it back in 2023. More recently, Lucy launched in 2021 to study ten asteroids—two in the main belt and eight Jupiter trojans—while Psyche, launched in October 2023, targets the metallic asteroid of the same name. ESA's Hera, launched October 2024, will examine the aftermath of DART's deliberate September 2022 impact on Dimorphos, a planetary-defense test. China's CNSA launched Tianwen-2 in May 2025 to explore Kamoʻoalewa and 311P/PanSTARRS, collecting regolith samples along the way.

Naming, Definition, and the Blurred Line with Comets

The word asteroid was coined by Sir William Herschel from the Greek asteroeidēs, meaning star-like, after early telescopes showed these objects as mere points of light indistinguishable from stars except for their apparent motion. For much of the nineteenth century, the terms asteroid and planet were used interchangeably. The IAU's 2006 resolution introduced small Solar System body as the preferred umbrella term, though minor planet remains acceptable. The boundary between asteroid and comet has always been fuzzy: a comet is defined by its coma, the glowing tail produced when solar radiation sublimates near-surface ices. Yet observations suggest a continuum—some objects first catalogued as minor planets later showed cometary activity, while many comets eventually exhaust their volatile ices and become asteroid-like. Highly eccentric asteroids are likely dormant comets. In the cold outer reaches beyond Jupiter, where ices stay solid, the distinction blurs further, and bodies are often simply called objects to sidestep the classification problem entirely.