Asteroids
An asteroid can be only a few hundred feet wide or it may be several hundred miles wide. They are considered to be debris left over from the formation of the solar system. Many asteroids orbit the Sun in a region between Mars and Jupiter. This “belt” of asteroids follows a slightly elliptical path as it orbits the Sun in the same direction as the planets. It takes anywhere from three to six Earth years for a complete revolution around the Sun. An asteroid may be pulled out of its orbit by the gravitational pull of a larger object such as a planet. Once an asteroid is captured by the gravitational pull of a planet, it may become a satellite of that planet. Astronomers theorize that is how the two satellites of Mars, Phobos and Deimos, came to orbit that planet. An asteroid is also capable of colliding with a planet resulting in the formation of an impact crater. Some scientists believe that just such an impact in the area of the Yucatan Peninsula in Mexico started the chain of events which led to the extinction of the dinosaurs here on Earth. Astronomers think that if it were not for the giant planet Jupiter exerting its gravitational force on the asteroids in the belt, the inner planets would be constantly bombarded by large asteroids. The presence of Jupiter actually protects Mercury, Venus, Earth, and Mars from repeated asteroid collisions.
Meteroids
Meteoroids burn up in the atmosphere and fall to the Earth as dust. Every day, approximately 3000 metric tons of dusty space material falls to Earth.
A meteoroid is a piece of stony or metallic debris which travels in outer space. Meteoroids travel around the Sun in a variety of orbits and at various speeds. The fastest meteoroids move at about 42 kilometers per second. Most meteoroids are about the size of a pebble. When one of these pieces of debris enters the Earth’s atmosphere, friction between the debris and atmospheric gases heats it to the point that it glows and becomes visible to our eyes. This streak of light in the sky is known as a meteor. Most meteors glow for only a few seconds prior to burning up before hitting the Earth’s surface. On most dark nights, meteors can be seen. The chance of seeing a meteor with the unaided eye increases after midnight. People often refer to meteors as “falling” or “shooting” stars. The brightest of the meteors are called fireballs. Sonic booms often follow the appearance of a fireball just as thunder often follows lightning. At certain times of the year, more meteors than normal can be seen. When the Earth passes through an orbiting stream of debris from a comet that has broken up, what’s known as a meteor shower occurs. Meteor showers take place on about the same dates each year.
If the meteor does not burn up completely, the remaining portion hits the Earth and is then called a meteorite. Over 100 meteorites hit the Earth each year. Fortunately, most of them are very small. There has only been one report of a “HBM” (hit by meteorite), and that occurred in 1954. Ann Hodges of Sylacauga, Alabama was slightly injured when a 19.84 kilogram meteorite crashed through the roof of her home. The larger meteorites are believed to have originated in the asteroid belt. Some of the smaller meteorites have been identified as moon rock, while still others have been identified as pieces of Mars. Large meteorites that crashed onto the Earth long ago made craters like those found on the Moon. The Barringer Meteorite Crater near Winslow, Arizona is believed to have been formed about 49,000 years ago by the impact of a 300,000 ton meteorite. The Hoba iron meteorite is the largest single meteorite known. Its present weight is estimated at 66 tons. Part of the Hoba meteorite has rusted away, therefore it’s original weight may have been as much as 100 tons! It has never been removed from its landing sight in Namibia. The largest single meteorite found in the United States is the fifteen ton Willamette (Oregon) iron meteorite found in 1902.
Comets
The name comet comes from the Latin word cometa which means “long-haired”. The earliest known record of a comet sighting was made by an astrologer of the Chinese court in 1059 B.C.
Scientists believe that comets are the debris left from the solar nebula which condensed to form the Sun and planets in our solar system. Most comets are thought to originate in a huge cloud called the Oort Cloud. The Oort Cloud is believed to surround our solar system and reach over halfway to the nearest star, Alpha Centauri, which is 150,000 astronomical units away. Scientists think that about 100 million comets orbit the Sun. A comet has a distinct center called a nucleus. Most astronomers think the nucleus is made of frozen water and gases mixed with dust and rocky material. Comet nuclei are described as dirty snowballs. A hazy cloud called a coma surrounds the nucleus. The coma and the nucleus together form the comet’s head.
Comets follow a regular orbit around the Sun. If the comet nucleus is pulled into an orbit which carries it close to the Sun, the solar heat will cause the outer layers of the icy nucleus to evaporate. During this process, dust and gases which form the coma around the nucleus are released. As the comet gets closer to the Sun, the coma grows. The solar winds push the dust and gas away from the coma causing them to stream off into space to form the comet’s tail. The solar winds cause the comet’s tail to point away from the Sun. The tails of comets can reach 150 million kilometers in length! Each time the comet passes close to the Sun, it loses some of its material. Over time, it will break up and disappear completely.
Many comets enter an elliptical orbit and repeatedly return to the inner solar system where they can be viewed from Earth at specific times. Short period comets, of which Halley’s Comet is the most famous, reappear within a 200 year time frame. Halley’s makes an appearance once every 76 years. The comet was named after Sir Edmond Halley.
A comet has no light of its own. We are able to see a comet because of the reflection of the Sun’s light off of the comet and because of the gas molecules in the coma releasing energy absorbed from the Sun’s rays.
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