Audition 4 lecture in astronomy class comets

Narrator: Listen to part of a lecture in an astronomy class. The professor will discuss comets.

Comets are small bodies in space that are characterized by gaseous emissions, and consist of a solid nucleus, a cloudy atmosphere which is called the coma, and a tail. This is an example of a comet.
As you can see, the nucleus is made of ice with rocks and dust particles encrusted in it. As a comet approaches the sun, some of the ice on the surface vaporizes, and the gas and dust particles that were embedded in the ice of the nucleus are released and blown back by the solar wind, forming a hydrogen atmosphere and the tail of the comet. The tails of comets always point away from the sun because the solar wind pushes them back.
Most comets have a nucleus that is less than ten miles in diameter, but the comas can extend out nearly one million miles. Some tails have been known to trail 100 million miles behind their comets.
We classify comets as either short-period comets or long-period comets, depending on how long they take to orbit the sun. Short-period comets require fewer than 200 years, whereas long-period comets need more than 200 years. As far as we can tell, the short-period comets have their origin in a belt of comets that lies just beyond the orbit of Pluto. The long-period comets come from a cloud of comets one thousand times farther away than Pluto. Most comets travel in elongated orbits that cross the circular orbits of the planets. Thus, the possibility of collision does exist at the points where the orbits intersect. Look at this drawing which shows the orbit of four planets with several comets intersecting them. See what I mean?
Oh yes, another interesting point. Some of the craters on the satellites of the outer planets are probably evidence of past cometary collisions. And even the craters on the Earth's moon could have been caused by comets.

Narrator: Now get ready to answer the questions.