Asteroids, Comets, and Meteorites
Asteroids are leftovers from planetary formation. Many of them reside in the asteroid belt that lies between the inner and outer planets. Though thousands of asteroids reside in the main belt, this region is not densely populated -- all of the asteroids together only make about 5% the mass of the Moon.
Asteroids come in many sizes and shapes. They range in width from 620 miles (1000 kilometers) down to dozens of meters. Most asteroids are irregularly shaped and all have craters from impacts with other asteroids. However, the largest asteroid, Ceres, has sufficient gravity to become nearly spherical, so it is also classified as a dwarf planet! Vesta, another large asteroid, has evidence of ancient lava flows on its surface.
Asteroids are classified by their composition. Most of the known asteroids (over 75%) are C-type (carbon-rich) asteroids, located in the outer region of the main asteroid belt. These asteroids are usually composed of organic compounds and hydrated minerals. Silicon-rich (S-type) asteroids dominate the inner part of the asteroid belt, closest to the Sun. These asteroids are composed of rocky materials and small amounts of metallic iron. M-type (metallic) asteroids are predominantly metallic iron and nickel.
Although it is difficult to return asteroid samples with spacecraft, nature routinely sends samples to Earth. While large asteroid impacts with Earth are rare, small asteroids (falling as meteorites) impact the Earth every day! Scientists have collected more than 50,000 meteoritic samples of asteroids on Earth.
Beyond the orbit of Neptune, there is a collection of small, icy planetary bodies that were left over from the formation of our early solar system. This region is called the Kuiper belt. Occasionally a Kuiper belt object may have a close encounter with Neptune or another body that flings the object out of the solar system or pushes it into a closer orbit where we may observe it periodically as a comet. Most short-period comets, those with orbits less than 200 years, such as Comet Halley, originate in the Kuiper belt.
Beyond the Kuiper belt is the Oort cloud, which also contains icy remnants of our solar system's formation. The Oort cloud is a sphere that envelops our solar system and may extend 30 trillion kilometers (about 20 trillion miles) away from its center. Objects in the Oort cloud are too small and far to be seen. Long-period comets, those that take more than 200 years to orbit our Sun, such as Comet Hale-Bopp or Comet Hyakutake, may come from the Oort cloud.
As ice bodies from the Kuiper belt or Oort cloud near the Sun, the ices begin to sublimate from a solid into a gas, and take the familiar shape of a comet. The center (nucleus) of a comet ranges from 0.1 to 40 km (300 ft to 24 miles) wide. As the frozen gases sublimate, they form a thin atmosphere around the nucleus (the coma) that can make the comet appear as a large glowing cloud as big as a planet. As comets move close to the Sun, they develop tails of dust and ionized gas millions of kilometers long.
Additional information about comets is available at Ice in the Solar System
Meteors, Meteoroids, and Meteorites
Meteoroids are small particles — often no bigger than a grain of sand — that orbit our Sun. When meteoroids enter Earth’s atmosphere, they produce brilliant streaks of light that can be seen in our sky. These brief streaks of light (often called “shooting stars”) are meteors. Meteorites are rocks from space that actually have landed on Earth’s — or another planet’s — surface.
Meteor showers occur when Earth passes through the trail of dust and gas left by a comet along its elliptical orbit. The particles enter Earth's atmosphere and most burn up. Some meteor showers, such as the Perseids in August and the Geminids in December, occur annually when Earth's orbit takes it through the debris path left along the comet's orbit. Comet Halley's trails are responsible for the Orionids meteor shower.
More information about meteors, meteoroids, and meteorites is at SkyTellers About Meteors.