An asteroid is a relatively small rocky, metallic or mixed composition body that orbits the Sun and is generally too small to become rounded by its own gravity. Most asteroids are irregularly shaped objects that formed from leftover material during the development of the Solar System. They range from tiny fragments to bodies hundreds of kilometres across, with the largest objects approaching dwarf planet dimensions.
The term asteroid historically referred to the star like appearance of these objects through telescopes, but modern astronomy defines and studies them according to their physical and orbital properties rather than appearance. Most known asteroids are concentrated in the main asteroid belt between the orbits of Mars and Jupiter, although substantial populations exist elsewhere, including near Earth, in Jupiter's Trojan populations, and in other dynamically distinct groups. Asteroids are important because they preserve evidence about the materials and processes involved in planet formation. Unlike the major planets, many asteroids have experienced limited geological alteration, allowing scientists to study primitive Solar System matter.
Asteroids formed from planetesimals and fragments left over from the accretion of planets. In the early Solar System, countless solid bodies collided and gravitationally interacted. In regions where planetary growth was efficient, many planetesimals eventually became incorporated into planets. Between Mars and Jupiter, however, the gravitational influence of Jupiter strongly disrupted the process of forming a single large planet. Collisions among bodies in this region therefore tended to produce fragments rather than allowing complete accretion into a terrestrial planet.
The modern asteroid belt is the remnant of this complex history, although its original mass was substantially greater than its present mass. Asteroids can also originate through later collisions that break larger parent bodies apart. Families of asteroids can share similar orbital properties and spectral characteristics because they formed from a common parent body. Some asteroid populations have subsequently been transported into different orbital regions through gravitational resonances, collisions and interactions with planets.
Asteroids exhibit remarkable diversity in composition and appearance. Traditional spectral classifications include carbon rich C type asteroids, silicate rich S type asteroids and metallic M type asteroids, although modern classification systems contain many more categories and reflect increasingly detailed observations. C type objects are generally dark and carbon rich and may contain hydrated minerals and primitive organic compounds. S type asteroids contain substantial silicate minerals and are generally brighter than C types.
M type asteroids are associated with metallic compositions, although the relationship between spectral appearance and actual composition is not always straightforward. Some asteroids contain substantial water bearing minerals, while others are comparatively dry. Their surfaces can be heavily cratered, covered in regolith, fractured or reshaped by impacts. Some rotate rapidly, some slowly, and many have irregular shapes caused by collisions. Their densities vary considerably because internal porosity can leave large amounts of empty space between fragments.
Asteroids are not all confined to the main asteroid belt. Near Earth asteroids are objects whose orbits bring them relatively close to Earth's orbital path. Some belong to orbital groups classified according to their relationship with Earth's orbit, including Aten, Apollo and Amor populations. Their trajectories are monitored because gravitational interactions can alter asteroid orbits over time. Jupiter Trojans occupy gravitationally stable regions associated with the planet's orbit, especially around the Sun Jupiter Lagrange points.
Other asteroid populations occur in the inner and outer Solar System. Some objects classified historically as asteroids have comet like behaviour, releasing dust or gas when heated by sunlight. The distinction between asteroids and comets is therefore not always perfectly sharp because physical composition and activity form a continuum. Modern observations increasingly emphasise an object's actual properties rather than assuming that every rocky body must be completely inactive.
Asteroids have been studied using telescopes, radar, spacecraft flybys, orbital missions and sample return missions. Spacecraft observations have revealed detailed surfaces, craters, boulder fields, grooves, ridges, dust environments and evidence of internal structure. Missions such as NEAR Shoemaker, Hayabusa, Hayabusa2 and OSIRIS REx demonstrated that asteroids can be scientifically investigated at close range and that material can be collected and returned to Earth.
Samples from asteroid Itokawa, Ryugu and Bennu have provided direct evidence about mineralogy, organic compounds, water related alteration and the early Solar System. Bennu and Ryugu in particular contain primitive materials that can preserve chemical information from the Solar System's formative period. Such missions also improve knowledge of asteroid physical properties relevant to planetary defence, including density, strength, surface structure, rotation and orbital behaviour.
Asteroids are important in planetary defence because impacts can produce severe regional or global consequences depending on an object's size, composition, impact velocity and location. Scientists therefore conduct surveys to identify near Earth objects and calculate their orbits. Determining an asteroid's future trajectory requires observations over time and gravitational modelling, and predictions can be refined as additional measurements become available.
The kinetic impact technique has been experimentally demonstrated as a method of altering the trajectory of a small asteroid moonlet, showing that spacecraft can deliberately modify the orbit of a celestial body under controlled conditions. Planetary defence does not imply that every asteroid is dangerous. Most asteroids pose no meaningful impact threat to Earth, and the vast majority remain far from Earth's orbit. The scientific objective is instead to identify potentially hazardous objects early enough that their trajectories and physical characteristics can be studied accurately.
Asteroids also have potential economic significance because some contain metals, silicates, carbon compounds or water bearing materials. Concepts for asteroid resource utilisation have proposed extracting water for use as spacecraft consumables or as a source of hydrogen and oxygen propellant, while metals could theoretically be used for industrial purposes. However, practical asteroid mining remains technologically, economically and legally challenging.
Extracting material requires spacecraft capable of rendezvous, anchoring, excavation or processing under extremely low gravity, while returning material to useful locations presents additional difficulties. The value of an asteroid cannot be judged simply by estimating the market value of its raw materials because transportation and processing costs can dominate. Scientific missions are currently much more mature than commercial large scale resource extraction. Nevertheless, asteroid resources are relevant to long term discussions of space infrastructure and the development of an economy operating beyond Earth.
Asteroids also help scientists understand the relationship between small bodies, planets and meteorites. Many meteorites found on Earth originate from asteroids, and laboratory analysis of meteorites can be combined with telescopic measurements to connect specific classes of rocks with asteroid populations. Asteroid surfaces can be modified by space weathering, micrometeorite impacts and solar radiation, complicating attempts to infer composition from reflected light.
Some asteroids have moons or binary companions, while others are rubble piles held together partly by gravity and friction rather than behaving as solid monolithic rocks. Their rotation can be changed by thermal effects such as the Yarkovsky and YORP effects, which arise from the absorption and re emission of sunlight. These processes can gradually move asteroids between orbital regions and alter their spin states. Asteroids are therefore active participants in Solar System dynamics rather than merely inert rocks floating through space.
