An asteroid is a small, rocky object that typically orbits a star. In our Solar System, they mostly orbit the Sun and are concentrated in the main asteroid belt between Mars and Jupiter. Asteroids are remnants from the early stages of planetary formation, offering valuable insight into the Solar System’s history dating back around 4.6 billion years.
Asteroids vary widely in size—from pebble-sized fragments to dwarf-planet-scale bodies like Ceres (933 km in diameter). Most have irregular shapes, lack atmospheres, and rotate unpredictably.
They are classified by composition into several types: C-type (carbonaceous), which are dark and rich in carbon; S-type (silicaceous), composed mostly of silicate rock; and M-type (metallic), which contain a high amount of nickel and iron.
Based on their orbits, asteroids are grouped into categories such as Main-Belt asteroids, Trojan asteroids, near-Earth asteroids (NEAs), and Mars-crossers. NEAs are of particular interest because some of them come close to Earth. If an NEA is larger than about 140 meters and its orbit brings it within 7.5 million kilometers of Earth, it is classified as a potentially hazardous asteroid (PHA).
Impact risk varies by size. Objects around 30 meters in diameter enter Earth’s atmosphere roughly once per decade, often burning up. Larger ones—like the 50 meter Tunguska object or kilometer-scale bodies—can cause regional to global damage, but are much rarer. Most large near-Earth asteroids have already been discovered and tracked.
Asteroids are studied not only for planetary defense but also for what they reveal about solar system formation and their potential as future resources. NASA’s OSIRIS-REx and ESA’s Hera (following NASA’s DART) are missions studying asteroid composition and testing deflection techniques.
Their orbits and properties are tracked via telescopes and spacecraft, helping assess impact risk and explore opportunities in in‑situ resource utilization (ISRU).