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. 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. 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.

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. The glowing trails are visible in the night sky. The sun illuminates this trail, causing it to glow.

As comets heat up, gas and dust are expelled and trail behind them. Other astronomers say the belt is a planet that was broken apart during a collision.Ĭomets are balls of rock and ice that grow tails as they approach the sun in the course of their highly elliptical orbits. Many astronomers believe the belt is primordial material that never glommed into a planet because of Jupiter's gravitational pull. Like most asteroids, it lies in the asteroid belt between Mars and Jupiter. (Small asteroids are called meteoroids.) The largest asteroid, Ceres, is about 590 miles (950 kilometers) wide. Asteroid BeltĪsteroids are essentially chunks of rock that measure in size from a few feet to several miles in diameter.

And when observed through a telescope, comets appear fuzzier. 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. Find out how these dazzling displays come about. Meteor showers bring interplanetary debris, ranging from pebbles to boulders, into Earth's atmosphere.

Comet vs asteroid