Comets and asteroids are both leftovers from the formation of the solar system about 4.6 billion years ago. The main difference is what they are made of, which determines how they behave near the Sun.
Asteroids: rock and metal
Composition: rock, metal, or both. Some contain clay minerals and carbon compounds.
Appearance: no tail. They look like faint points of light through telescopes.
Where: most orbit in the main asteroid belt between Mars and Jupiter. Others share Jupiter’s orbit (Trojans) or cross Earth’s path (near-Earth asteroids).
Size: from small boulders to Ceres, about 940 km across, which is also classified as a dwarf planet, see planets and dwarf planets.
Credit : whatcanu.com
Comets: dirty snowballs
Composition: ice (water, carbon dioxide, carbon monoxide) mixed with dust and rock.
Appearance: far from the Sun, a comet is just a dark nucleus a few kilometers across. As it approaches, the ice turns into gas, forming a glowing coma and often two tails: a curved dust tail and a straight, bluish ion tail. Tails always point away from the Sun, pushed by sunlight and the solar wind.
Where: short-period comets (orbits under 200 years) come mostly from the Kuiper Belt beyond Neptune. Long-period comets come from the distant Oort Cloud, a hypothetical shell far beyond the planets.
Famous comet: Halley’s Comet returns about every 76 years and is next expected in 2061.
The line is blurry
Some asteroids eject dust like comets (“active asteroids”), and some comets have lost their ice and look like asteroids. Missions such as Rosetta (comet 67P), OSIRIS-REx (asteroid Bennu) and Hayabusa2 (Ryugu) have brought detailed data and even samples back to Earth.
Meteoroids, meteors and meteorites
A meteoroid is a small piece of rock or dust in space.
A meteor (shooting star) is the streak of light when it burns up in our atmosphere.
A meteorite is a piece that survives and reaches the ground. Large impacts shatter rock into angular fragments that geologists call impact breccia, see conglomerate vs breccia.
Large impacts are rare but happen over geological time: an asteroid about 10 km wide contributed to the extinction of the dinosaurs 66 million years ago. NASA and other agencies track near-Earth objects (see NASA’s Center for Near Earth Object Studies), and no known large asteroid poses a significant risk for at least the next century. In 2022, NASA’s DART mission deliberately crashed into the small asteroid Dimorphos and changed its orbit, the first test of planetary defense.
