Satgen579  Asteroid,Comet,Meteor by GM4IHJ  (BID SGEN579) 2000-04-29

First there were Comets. Harbingers of the death of kings. Then there
were Meteors. Brilliant showers of the fireworks of the Gods. All
eventually removed of most of their mystery, as the science of
astronomy replaced the non science of astrology and myth.

Recently , space craft have probed passing comets , revealing them
as mixtures of ice and dust wrapped around a core of more solid
material. At the same time. Mathematicians have made use of the fact ,
that if you can measure a comets orbital eccentricity and its
perhelion distance at closest approach to the Sun. You can use the
simple formulae - perihelion distance = semimajor axis *(1-ecc), and
follow it with Aphelion distance furthest from Sun = semimajor axis* 
(1+ecc), to work out roughly where it came from. Far out in the solar
system. Possibly a third of the way to the stars, in what is called
the Oort Cloud. 

Only recently has it become obvious that meteors are the product of
comets. Almost all meteor showers can now be connected with parent
comets , because of the close similarity of their orbits. While other
studies have shown that many of the Asteroids orbit beyond the orbit
of the planet Mars, but inside the orbit of the planet Jupiter.

However, that is far from the whole story. As Donald Yeomans reports
in a recent paper in Nature Vol401 20April 2000, page 829. Apparently
there are some asteroids that have the same orbits as meteor showers,
and some comets which switch of their gaseous tails at times and then
look like asteroids. In fact , it appears that we may not be looking
at 3 different sets of objects, but one broad set of objects at
different times in their life cycles.

The Geminid meteor shower of the month of December, and the asteroid
Phaeton, have nearly identical orbits. Orbits which are somewhat
elliptical ie comet like, with marked eccentricity. Possibly an old
comet which has lost most of its ice and dust after countless orbits
through the heat zone near the Sun. Thereby providing a comet core of
relatively solid material or conglomerate = Phaeton. Plus a meteor
stream , from material which left the original comet thousands of
years ago, but still follows the old comet orbit around the Sun.
Other Comet/Asteroid bimodal objects are asteroid 1979VA which as
recently as 1949 had been releasing gas , and was clasified as a comet
by discoverers Wilson and Harrington. In reverse mode, asteroid Chiron
discovered in 1977, brightened 12 years later and within a year was
ejecting gas like a comet.

As recent space probes have shown however . Many of the asteroids
orbiting between Mars and Jupiter are clearly large , solid lumps of
battered cratered rock or collections of rubble. Mostly , but as
recent evidence shows , not completely devoid of water or ice. Now and
again collisions occur between asteroids . Directing them or their
pieces, into earth orbit and eventually down to earth. Surviving the
fiery passage through the atmosphere . Unlike the comet debris which
burn up completely, providing only ionised trails as they flash across
the sky reflecting radio signals.