Narrator: Listen to part of a lecture in an astronomy class. The professor will [bad word]  [bad word] are small bodies in space that are characterized by gaseous emissions, and consist of a solid nucleus, a cloudy atmosphere which is called [bad word]  and a tail. This is an example of [bad word] 
As you can see, the nucleus is made of ice with rocks and dust particles [bad word] in it. As [bad word] 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 [bad word]  The tails [bad word] always point away from the sun because the solar wind pushes them back. [bad word] have a nucleus that is less than ten miles in diameter, but [bad word] can extend out nearly one million miles. Some tails have been known to trail 100 million miles behind [bad word] 
We [bad word] as either [bad word] or [bad word]  depending on how long they take to orbit the sun. [bad word] require fewer than 200 years, whereas [bad word] need more than 200 years. As far as we can tell, the [bad word] have their origin in a belt [bad word] that lies just beyond the orbit of Pluto. The [bad word] come from a cloud [bad word] one thousand times farther away than Pluto. [bad word] 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 [bad word] 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 [bad word] collisions. And even the craters on the Earth's moon could have been caused [bad word] 

Narrator: Now get ready to answer the questions.