Ask A Scientist© Astronomy Archive

Author:      derek1  and  anthony w thompson
What are comets made of?  
How fast do they move?
How many are known?  How many can be seen?
Where are they located?
What happens to them?  Do they ever die out?
What would happen if a comet hit the Earth?
How big does a comet have to be to make it through the atmosphere?
How do people calculate their speed and mass?  

Response #:  1 of 2
Author:      wordsworth
Comets may be composed of many different things depending on their origins,
but many contain large quantities of ice.  Of the other contents, there may
be metals such as iron, iridium, and other minerals.  Trapped in the ice,
moreover, there may be samples of gasses, if the comet came from some
planetary form or if it passed through a gaseous cloud.  The tail of the
comet is simply a stream of evaporating water and gasses from its surface.

Response #:  2 of 2
Author:      hawley
Comets are thought to be "dirty snowballs" composed mostly of water ice and
methane ice.  They travel at speeds around 20,000 MPH.  Some appear only
once, then leave the solar system or break up.  Some return periodically to
the inner solar system (like Halley's).  Most are thought to spend most of
their time in the Oort Cloud (check a good encyclopedia or an astronomy
text).  They can only been seen when they enter the inner solar system and
reflect the light of the Sun.  Their tails are produced by material evapo-
rating off the surface.  Comets do not travel through the atmosphere unless
they are on a trajectory to hit the Earth.  If a comet hit the Earth, the
results would be catastrophic:  scientists think that might have been
responsible for the extinction of the dinosaurs.  Their speed and mass are
calculated from their different positions in the sky over time.
(I used to work in a cometary observatory)