Asteroid

Asteroids are small rocky, metallic, or icy bodies that orbit the Sun and are classed as minor planets—objects larger than meteoroids but neither planets nor confirmed comets. Found in vast numbers throughout the Solar System, they vary dramatically in size, composition, and orbital characteristics. The majority lie within the main asteroid belt between Mars and Jupiter, though many occupy other regions, including near-Earth space and the outer Solar System.

Classification, Composition, and Physical Characteristics

Asteroids show considerable diversity in their physical make-up and structure. They are commonly categorised into three principal spectral types:

• C-type (carbonaceous): rich in carbon and among the darkest objects in the Solar System, representing the most abundant class.
• S-type (silicaceous): composed largely of stony materials such as silicate rock and nickel-iron.
• M-type (metallic): dominated by iron–nickel metal and thought to be fragments of once-differentiated parent bodies.

Sizes range from small rubble accumulations less than one kilometre across to the nearly spherical dwarf planet Ceres, roughly 1,000 kilometres in diameter. Most asteroids have irregular shapes and lack atmospheres; however, thermal activity may occasionally cause surface ices to sublimate, producing transient comet-like features. Modern studies suggest a continuum between asteroid and comet properties rather than a strict binary distinction.

Distribution and Orbital Characteristics

Asteroids are most densely concentrated in the main asteroid belt, occupying orbits two to four astronomical units from the Sun. Their paths are generally stable and mildly elliptical, with orbital periods ranging from approximately three to six years. Beyond the main belt, significant populations reside in several distinct regions:

• Jupiter Trojans, which share Jupiter’s orbit around the Sun, clustering at its leading and trailing Lagrange points.
• Near-Earth asteroids (NEAs), which have orbits that cross or closely approach Earth’s orbit.
• Outer-Solar-System minor planets, including centaurs and trans-Neptunian objects, some of which share characteristics with both asteroids and comets.

Although asteroids are numerous—more than a million have been catalogued—their combined mass is only a small fraction of that of the Moon.

Terminology and Classification Frameworks

In 2006, the International Astronomical Union adopted the term small Solar System body to describe objects that are neither planets, dwarf planets, nor natural satellites, thereby encompassing asteroids, comets, and other minor bodies. While the term minor planet remains acceptable, it is used less frequently. The word asteroid, coined in the early nineteenth century, originally emphasised their starlike appearance through small telescopes.
Historically, bodies were divided into comets, asteroids, and meteoroids (the latter being smaller than one metre across). Because cometary and asteroidal characteristics sometimes overlap—especially among objects that exhaust their volatile ices—the boundary between the two classes is increasingly recognised as fluid. Outer Solar System bodies such as Kuiper belt objects are now often termed generically “objects” rather than being placed into either the comet or asteroid categories.

Historical Observations and Early Discoveries

Despite their abundance, asteroids were first identified only in the early nineteenth century. The discovery of Ceres in 1801 by Giuseppe Piazzi marked the beginning of systematic searches, partly stimulated by the Titius–Bode law, a proposed numerical pattern predicting a missing planet between Mars and Jupiter. Subsequent discoveries revealed numerous objects occupying this region, leading astronomers to recognise that a belt of small bodies, rather than a single planet, inhabited the gap.
Throughout the nineteenth and twentieth centuries, improvements in telescopes and photographic techniques accelerated asteroid detection. Today, wide-field digital surveys supply precise orbital data to the Minor Planet Center, which catalogues hundreds of thousands of numbered objects.
Only one asteroid, Vesta, is routinely visible to the naked eye under favourable conditions, though small asteroids occasionally make close passes to Earth and can be detected visually.

Space Missions and Scientific Exploration

Spacecraft missions have transformed understanding of asteroids by providing direct imaging, compositional data, and physical samples. Key missions include:

• Galileo, the first spacecraft to capture close-up images of asteroids.
• NEAR Shoemaker, which orbited and landed on 433 Eros.
• Dawn, which studied both Vesta and Ceres, revealing their internal differentiation and geological diversity.
• Hayabusa and Hayabusa2 (JAXA), which returned samples from 25143 Itokawa and 162173 Ryugu.
• OSIRIS-REx, which collected and delivered material from Bennu in 2023.
• Lucy, launched to study multiple Jupiter Trojans.
• Psyche, investigating the metallic asteroid 16 Psyche.
• Hera, assessing the aftermath of the DART impact.
• Tianwen-2, launched to explore the near-Earth asteroid Kamooalewa and the active asteroid 311P.

These missions demonstrate the scientific value of asteroids as remnants of early Solar System formation and as potential resources for future exploration.

Near-Earth Asteroids and Planetary Defence

Near-Earth asteroids pose a potential hazard due to their proximity to Earth’s orbit. Large impacts can have severe environmental consequences, illustrated by the Chicxulub event, which is associated with the mass extinction at the end of the Cretaceous period.
To address such risks, space agencies have begun developing active defence strategies. The Double Asteroid Redirection Test (DART) in 2022 successfully altered the orbit of Dimorphos by kinetic impact, marking a milestone in planetary protection research.

Outer Solar System Minor Bodies

The discovery of Chiron in 1977 expanded the known distribution of small Solar System bodies beyond Jupiter, prompting the recognition of the centaurs. Further discoveries in the 1990s identified numerous trans-Neptunian objects, including members of the Kuiper belt and the scattered disc. These icy bodies, often inactive at great distances from the Sun, would exhibit cometary features if moved inward, highlighting the blurred distinctions between asteroid-like and comet-like categories.

Continuing Research and Scientific Importance

Asteroids are valuable for understanding Solar System history. Their compositions preserve early planetary materials, while their dynamical behaviour reveals insights into gravitational interactions and long-term orbital evolution. Ongoing missions and survey technologies promise to improve knowledge of asteroid diversity, structure, and physical processes.