A celestial body orbiting the sun that displays  some diffuseness or a "tail"  of debris that generally points away from the sun.  Both the diffuseness and tail  (coma) are composed of gas and dust of  various atomic or molecular compositions.  The coma material  comes from a small nucleus that is usually invisible. The head  of the comet seems to be made of solid particles, some of which are frozen gases, while its tail is made of tiny particles so far apart that  stars can be seen through it.  A comet is visible because of the reflection  of sunlight on its head and gaseous tail.   Comets travel around the sun with its tail always pointing away from the sun.  Some comets  reappear every few years.  Others pass by the sun once and never return.

The head has been observed to be as much as 30,000 to 1,000,000 miles in diameter and the tail (when near the sun) has sometimes appeared to be 200million miles in length

Almost 1000 different comets have been reported http://cfa-www.harvard.edu/iau/Ephemerides/Comets/index.html

One of the more famous comets is Halley's Comet.  It has been recorded  since 204 B.C.  It seems to reappear every 75 years.

People used to be very superstitious and afraid of comets because they thought it brought disaster and death.  They even thought the earth could be destroyed by a comet.  It was thought in recent times that comets were quite harmless bodies in the solar system.

With the impact of Shoemaker-Levy into Jupiter  that is no longer  held with certainty.

What are Asteroids?                                      

They are the thousands of little planetoids between Mars and  Jupiter.  The asteroids that have been charted    range in size from  20 to 8o miles in diameter.   Asteroids may have been  formed  from a large  planet   that  broke into smaller pieces as a result of some cosmic collision,   Each asteroid  has its own individual orbit around the sun.

In  our solar system many pieces of rock and metallic substances travel in various orbits.   Some weigh only  a few grams, while others  weigh many  tons        Occasionally  the Earth pulls these substances  from their orbits and into  Earth's  atmosphere.  Friction caused by  their high speed through  the atmosphere  sets the meteoroids on fire.  As it burns , it gives off a bright light.

Ordinarily, a meteoroid burns up completely.  However,  occasionally a meteor strikes the earth before being complete ly burned up.   One example of  such an event  was the  impact  that  caused the enormous  Arizona  crater   4/5 of a mile wide and 570 ft.  deep into solid rock.    Tunguska, the cosmic mystery of the century may be an example of one that did not.  " The Manson event was an unimaginable catastrophe.  The impact  of the enormous meteorite caused destruction equivalent to approximately 10 trillion tons of TNT.  Everything within 130 miles of the point of impact was instantly  engulfed  in flames.  Trees  were toppled and standing vegetation was devastated  within 300 miles of the impact  site.  It is also believed that most animals as far as 650 miles away did  not survive ...,and dinosaurs 800 miles away would have been knocked off their feet.   
The  Western Interior  Seaway  was home to many forms of  marine vertebrates, which would  have greatly disturbed  or killed  by giant shock waves and possible   tsunami-like waves.  The seaway's habitats would have been so disrupted as to cause the death of thousands of marine creatures.  To support this theory , scientists have found evidence that certain lineages of fish ,mosasaurs and marine turtles...mysteriously vanished from the Western Interior Seaway during the approximate time of the Manson Impact event."

  The quote and photo from http://www.uhaul.com/supergraphics/crater/effects.html

Photo courtesy of Joe Tucciarone

Potentially hazardous asteroids can be accessed at http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html

The possibility of impact with a piece of comet or a piece of an asteroid with our planet needs to make tracking of these objects imperative.