Asteroids and Meteoroids

The space between Mars and Jupiter is so great that early astronomers thought there might be a planet there. They did not find any, but instead they found small planet like bodies called asteroids. Asteroids are solid, rocklike masses with irregular shapes, so their brightness changes as they rotate. The two largest asteroids discovered are spherical in shape. These are Ceres and Pallas, each of which is about 100 kilometers in diameter. Other asteroids are less than one kilometer in diameter.

Asteroids move around the sun in the same direction as the planets (west to east). Some asteroids travel in elongated orbits. Some pass near Earth. Scientists believe that asteroids are leftover materials from a planet in its early stage of formation.

Also traveling in space are rock fragments called meteoroids. Some meteoroids are as large as boulders, while others are a tiny as particles of sand. Some meteoroids travel in groups, others travel alone. Meteoroids enter Earth’s atmosphere at speeds ranging from 16 to 71 kilometers per second. Friction between the meteoroids and particles in the atmosphere causes some meteoroids to light up. A meteor is a meteoroid that passes through Earth’s atmosphere and produces a bright flash in the sky.

On a dark night, when the sky is clear, one can see around 5 to 15 meteors. Meteors are called “shooting stars” because they look like stars falling from the sky. As a comet travels near the sun, some parts of the comet break off, producing small particles called micrometeoroids. When the path of these micrometeoroids crosses Earth’s path and the particles enter Earth’s atmosphere, large numbers of meteors are seen. This phenomenon is called meteor shower. Meteor showers happen several times a year. They are named after the constellation from which they appear to come. Among the best known meteor showers and their peak dates are the following – the Lyrids (April 21/22), the Eta Aquarids (May 5/6), the Perseids (August 11/12), the Oronids (October 21/22), the Taurids (southern, November 5/6) and northern, (November 11/12), the Leonids (November 17/19) and the Geminids (December 13/14). All meteor showers originate from comets except Geminids, which originate from an asteroid.

Some meteors do not burn out as they travel through Earth’s atmosphere. They are able to reach Earth’s surface, or the ground. The objects are now called Meteorites. There are three kinds of meteorites. The first kind and the most plateful are stones that look like dark igneous rocks. These are made of silicate and iron. The largest of the known stone meteorites weighs about a metric ton. The second kind of meteorites are called irons because they consist mostly of iron and nickel. Irons are heavier than stones. One of the largest known iron meteorites weighs 34 metric tones. The third kinds of meteorites are stony-irons, which are quite rare. They are mixture of stone and iron. The Antarctic ice cap is the most abundant source of meteorites.