The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

Where do comets come from?
We think that comets are sort of "cosmic leftovers" from the formation of our solar system. There seems to be millions of them far, far out in our solar system, far beyond the orbit of Pluto. They are made up of ice, rock, and dust. They are much smaller than they look. The comet itself may only be a mile across but the tail can be millions of miles long (but it's just thin gas and dust).

Why does a comet have a tail behind it?
A comet is made up of frozen ice, dust, and rocks. But when it gets close enough to the sun, the sunlight starts to vaporize the ice on the surface of the comet. Gases and particles of dust are spewed out into space. There is a "solar wind" — a wind of gaseous particles — that blows out from the sun. It blows the gases away from the comet to form the gas tail. The dust gets blown back too, to form a dust tail. The gas tail tends to curve to follow the curved magnetic fields that reach out from the sun, but the dust forms a straight tail. So a comet generally has TWO tails, a curved gas tail and a straight dust tail. The tails reach out away from the sun. This is true even if the comet has passed the sun and is moving away from the sun. That means the tails are going in front of the comet as it moves away from the sun!

Can meteors hit the earth?
You may be surprised that meteors hit the earth all the time! The vast majority are very small and burn up when they hit the earth's atmosphere. A few make it through and hit the ground or fall into the ocean. I have one, about the size of a pebble, in my rock collection. It is very rare for a meteor big enough to do any damage to hit the earth. Have you ever heard of the Meteor Crater in Arizona? I have visited it. It's about a mile across and occurred about 25,000 years ago. Fortunately these days, astronomers are tracking all the larger objects and keeping track of them in orbit. Hopefully if one if them looks like it might hit the earth, we can do something about it.

What is the difference between a meteor shower and a meteor storm?
From Earth we usually talk about a "meteor shower," but the only time I can think of when we talk about a "meteor storm" is when someone is in space. They are basically the same thing. A storm is bigger than a shower, so I guess it seems more spectacular when you are in space in the middle of it!

What is a quasar?
The name quasar comes from an early name for some strange objects — Quasi-Stellar Objects (or QSO). These were objects that looked like tiny points in astronomical photographs, just like stars, but we could tell that they were not stars but an unusual kind of galaxy. Later we found that they are the very bright centers of some of the most distant galaxies in the universe. They probably have black holes in the center, with big explosions going on in the center, which gives off huge amounts of light (which is what we photograph).

What does a quasar look like?
How about some pictures of quasars taken by the Hubble Space Telescope? Go to http://antwrp.gsfc.nasa.gov/apod/ap961125.html and click on the quasars section. By the way, quasar stands for "Quasi-Stellar Object," or QSO, which means it looks like a star in a picture. So don't be surprised if the quasar looks, well, like a star in the pictures!!!

I've heard that quasars are at the center of most young galaxies. Can you explain what quasars are, how they are formed, and if the above statement is true?
Yes, we think that quasars are very likely at the centers of young galaxies. A quasar appears to be a huge outpouring of light from tremendous explosions going on at the center of a young galaxy. The light is so great that at first, astronomers saw only the quasar in their photographs, but later the fainter galaxy surrounding the quasar was also seen. We think that there is probably a large black hole at the center of the quasar, and its strong gravity provides much energy for the explosions. Also we think that in early times, galaxies often collided into each other, so this may play a part too — perhaps some of the gas and stars of one galaxy crashes into the black hole in the center of the other.

Older galaxies also seem to have black holes at their centers, even our own Milky Way! But whatever that was going on — gas and stars falling into the black hole — that powered the quasars has gone away with the passage of time.