Definition
An asteroid is a small solid celestial body, mainly composed of rock, metal, or a combination of both. It orbits a star, usually within a specific zone like a belt or gravitational cloud. Unlike planets, asteroids do not have a stable spherical shape or an atmosphere. They do not emit light and lack the mass to initiate fusion or achieve hydrostatic equilibrium.
Structure
The internal structure of asteroids varies based on their composition, size, and origin. Some are compact and dense with metallic cores, others are porous, fractured, or made of loosely bound fragments called "rubble piles". Their surfaces display craters, fissures, and layers of regolith. Their density, reflectivity, and mineral content enable spectral classification.
Distribution
Asteroids often cluster in specific regions of a stellar system. In a planetary system, they are found in a main belt between two large planets or in peripheral areas such as trans-Neptunian regions. Others share planetary orbits (Trojans) or cross planetary paths (Earth-crossers). This distribution reflects the system’s dynamic history.
Origin and Evolution
Asteroids are solid remnants from the formation of planetary systems. They originate from regions where matter didn’t coalesce into planets, often due to gravitational disturbances. Their orbital paths can change due to planetary influences, thermal effects, or collisions. Some break apart, others merge, and some are ejected from the system.
Function in System
Asteroids affect the dynamic evolution of a stellar system. Through collisions, they create craters on planets and moons. They can also transport elements like carbon or water to other celestial bodies. Some contribute to gravitational balance or orbital clearing. They are useful indicators of early planet-forming processes.
Limits and Classification
The distinction between asteroids, minor planets, comets, and other small bodies is based on volatile activity, composition, size, and orbit. Bodies with certain activity or distance may be reclassified. Asteroids lack cometary tails and sublimation-driven surface activity. Their classification evolves based on spectral, dynamical, or geophysical data.