ExtrasMeteorsA brilliant meteor, called a fireball, may weigh many kilograms, but even a meteor weighing less than a gram can produce a beautiful trail. Some of these visitors from space are large enough to survive (at least partially) their trip through the atmosphere and impact the ground as meteorites. Fireballs are sometimes followed by trails of light that persist for up to 30 minutes; some, called bolides, explode with a loud thunderous sound. Meteoroids The term meteor comes from the Greek meteor on, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the solar system falls into Earth's atmosphere creating temporary incandescence resulting from atmospheric friction.

This typically occurs at heights of 80 to 110 kilometers (50 to 68 miles) above Earth's surface. The term is also used loosely with the word meteoroid referring to the particle itself without relation to the phenomena it produces when entering the Earth's atmosphere. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micro meteoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized. Meteorites Meteorites have proven difficult to classify, but the three broadest groupings are stony, stony iron, and iron.

The most common meteorites are chondrites, which are stony meteorites. Radiometric dating of chondrites has placed them at the age of 4. 55 billion years, which is the approximate age of the solar system. They are considered pristine samples of early solar system matter, although in many cases their properties have been modified by thermal metamorphism or icy alteration. Comets Now we know that comets are lumps of ice and dust that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips.

When comets get close enough to the Sun, heat makes them start to evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long. In 1985-1986, a spacecraft called Giotto visited the most famous comet, Halley, on Halley's most recent visit to the inner solar system.

In 1994, comet Shoemaker-Levy became trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere! In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the brightest comets ever seen from Earth. Comet Linear was discovered in 1999 and made its closest approach of the Sun in July 2000. The Stardust spacecraft flew by Comet Wild 2 in January 2004, collecting samples of the comet to return to Earth. The newest comet mission is Rosetta -- it will land on a comet named Churyumov-Gerasimenko!