Though too small to earn the distinction of "planet", asteroids and comets loom large in literature and folklore. The reason is clear: one of the chunky rocks or icy mud balls will eventually slam into Earth and change the planet irreversibly. Such an impact 65 million years ago is widely believed to have killed off the dinosaurs.
Asteroids and comets are considered remnants from the giant cloud of gas and dust that condensed to create the sun, planets, and moons some 4.5 billion years ago. Today, most asteroids orbit the sun in a tightly packed belt located between Mars and Jupiter. Comets are relegated to either a cloud or belt on the solar system fringe. Gravitational tugs, orbital collisions, and interstellar jostles occasionally perturb an asteroid or comet onto a wayward path.
The distinction between asteroids and comets is fuzzy—comets tend to have more chemical compounds that vaporize when heated, such as water, and more elliptical (egg-shaped) orbits than asteroids do. And when observed through a telescope, comets appear fuzzier.
Asteroids are essentially chunks of rock that measure in size from a few feet to several miles in diameter. (Small asteroids are called meteoroids.) The largest asteroid, Ceres, is about 590 miles (950 kilometers) wide. Like most asteroids, it lies in the asteroid belt between Mars and Jupiter. Many astronomers believe the belt is primordial material that never glommed into a planet because of Jupiter's gravitational pull. Other astronomers say the belt is a planet that was broken apart during a collision.
Comets are balls of rock and ice that grow tails as they approach the sun in the course of their highly elliptical orbits. As comets heat up, gas and dust are expelled and trail behind them. The sun illuminates this trail, causing it to glow. The glowing trails are visible in the night sky.
While there are perhaps trillions of comets ringing the outer fringes of the solar system, bright comets appear in Earth's visible night sky about once per decade. Short-period comets such as Halley's are perturbed from the so-called Kuiper belt out beyond the orbit of Neptune and pass through the inner solar system once or twice in a human lifetime. Long-period comets come from the Oort Cloud, which rings the outer reaches of the solar system, and pass near the sun once every hundreds or thousands of years.
Occasional collisions and gravitational tugs send asteroids and comets careering toward the sun on highly elliptical orbits, some close enough to Earth to pose a risk of impact. Astronomers are constantly on the lookout for bodies on such a catastrophic trajectory. Most asteroids, fortunately, are too small to cause any damage. Instead, they burn up in the atmosphere and appear to us as a shooting star.