Asteroids

(Left: Galileo image of asteroid Ida and its companion) (Right: Galileo photo of the asteroid Gaspra) As we leave the red planet and its moons behind us, we next encounter a strange collection of small planet-like objects. This is the asteroid belt. These asteroids, also known as planetoids, range in size from several hundred miles to several hundred feet. At least 30,000 of these giant rocks are known to be floating in their own individual orbits between Mars and Jupiter. Scientists are not sure why there are asteroids here instead of a planet. Some believe that a large, rocky planet may have existed here in the past. This planet may have then been blasted to pieces by a cataclysmic impact. Others think that the asteroids are composed of the leftover material from which the solar system was originally formed. The largest of these asteroids, Ceres, is over 630 miles in diameter. It was discovered by Giuseppe Piazzi in 1801. Not all asteroids are confined to the asteroid belt. There are several hundred known asteroids that regularly cross the orbit of the Earth. They are known as Earth-crossing asteroids. They range in size from several miles to the size of a small pebble. Sometimes these objects get close enough to the Earth that they are pulled in by its gravity. Outside the Earth's atmosphere, these objects are known as meteroids. They can enter the atmosphere at speeds of up to 45,000 miles per hour. Once a meteroid enters the atmosphere, it is known as a meteor. Most meteors burn up in the atmosphere before they can reach the ground. The bright, glowing streak they leave in the sky is cause by hot ionized gases formed from the heat of friction. Occasionally, a large meteor will survive its trip through the atmosphere to reach the ground. A meteor that has fallen to the ground is called a meteorite. Meteorites can be found in most parts of the world, and are generally composed of stone, iron, or a combination of the two. Several times during the year, hundreds of meteors can be seen in a single night. This event is known as a meteor shower and happens when the Earth passes through the debris left behind by a comet. Most of this debris is the size of dust or sand. Some particles can be a little larger, about the size of a small pebble. These larger particles can cause extremely bright meteors known as fireballs.

Comets

(Left: Comet West by amateur John Laborde) (Right: NASA photo of comet Kohoutek) In our travels through the solar system, we may be lucky enough to encounter what appear to be a giant balls of ice. These are the comets. Some astronomers have referred to comets as "dirty Snowballs" or "icy mudballs" because they are composed mainly of ice with dust and fragments of rock. The ice can be both water ice and frozen gases. Astronomers believe that comets may be composed of the very material that formed the solar system. Although most of the smaller objects in our solar system represent very recent discoveries, comets have been well-known since ancient times. The Chinese have records of comets that date back to 260 B.C. This is because comets are the only small bodies in the solar system that can be seen with the naked eye. Comets that are in orbit around the Sun can become quite a breathtaking sight. Comets are actually invisible until they begin to get close to the Sun. As they begin to heat up, an amazing transformation takes place. The dust and gases frozen within the comet begin to expand and burst forth at explosive velocities. The solid part of the comet is called the nucleus, while the envelope of dust and gas around it is known as the coma. Solar winds cause the material in the coma to trail behind the comet for a much as a million miles. As the Sun illuminates this material, it begins to glow brightly. This forms the famous tail of the comet. Comets and their tails can usually be seen from Earth and can be quite bright if conditions are right. Some comets may have as many as three separate tails. One will be composed mainly of hydrogen gas, and is invisible to the eye. Another tail of dust glows bright white, while a third tail of plasma usually will take on a thin, blue glow. As the Earth passes through these dust trails left behind by comets, the dust enters the atmosphere and creates meteor showers. Some comets are in an orbit that brings them near the Sun at regular intervals. These are known as periodic comets. Periodic comets lose much of their material every time they near the Sun. Eventually, after all of this material is lost, they will cease to become active and wander the solar system as a dark ball of rock and dust. Comet Halley is probably the most famous example of a Periodic Comet. Halley makes its appearance every 76 years.