You may omit the comma following a short introductory phrase: On Thursday the committee decided the dispute. In 1954 the Supreme Court desegregated the public schools. But if you omit this comma once, you should do the same throughout the paper for all short introductory phrases.
But, over time the differences seen among the ever growing number of these visitors has led to many more categories than three. At the present there are approximately fifty types of meteorites recognized. They still fall mainly into the three original families; the irons, the stony-irons and the stones.
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.