What are comets, asteroids, and meteors?

Astronomers group this debris by composition and behavior. An asteroid is a rocky or metallic body orbiting the Sun, most of them in the asteroid belt between Mars and Jupiter, ranging from a few meters to hundreds of kilometers across. A comet is a body of ice, dust, and rock that develops a glowing head and tail when it nears the Sun. A meteoroid is a much smaller chunk, from a grain of sand to a boulder. When a meteoroid streaks through Earth's atmosphere and glows, that flash is a meteor, or shooting star. If any of it survives the plunge to reach the ground, the recovered piece is called a meteorite.

The key divide is ice. Asteroids formed in the warm inner solar system, where volatile ices could not condense, so they are essentially dry rock and metal. Comets formed far out, in the frigid Kuiper Belt beyond Neptune or the distant Oort Cloud, locking away water ice and frozen gases. When a comet swings close to the Sun, that ice sublimates directly into gas, releasing dust and forming a fuzzy cloud called a coma plus long tails. Radiation pressure and the solar wind push those tails away from the Sun, so a comet's tail points outward regardless of which way the comet itself is actually traveling.

Spacecraft now study these bodies directly. NASA's OSIRIS-REx returned samples of asteroid Bennu to Earth in 2023, and Japan's Hayabusa2 brought back pieces of asteroid Ryugu in 2020. The European Rosetta mission orbited comet 67P/Churyumov-Gerasimenko and landed a probe on its surface. In 2022, NASA's DART spacecraft deliberately crashed into a small asteroid's moon, Dimorphos, and measurably shortened its orbit, the first test of deflecting a threatening object. Meanwhile, annual meteor showers such as the Perseids occur when Earth plows through the dusty trail left behind by a comet, and NASA's planetary defense program continuously tracks near-Earth objects.

Myth: a shooting star is a star. Reality: it is a speck of rock or dust, often no bigger than a pea, burning up from friction with the air. Myth: comets and asteroids are basically the same thing. Reality: comets are icy and grow tails, while asteroids are dry rock and metal that do not. Myth: a comet's tail streams behind it like exhaust. Reality: the tail always points away from the Sun, so it can even lead the comet as it heads back outward. Myth: meteors and meteorites are the same. Reality: the meteor is the streak of light, and a meteorite is the surviving rock found on the ground.