Satgen 635  Comet Asteroid Orbits  by GM4IHJ (BID SGEN635) 2001-05-26

There is growing interest in the possibility that one of the many
asteroids or Comets coming near earth , may one day get too close .
For every piece of space stuff likely to do this , there are millions
that will never harm us . However now that the ordinary media has
realised there is a subject here which can be used to provide
headlines . It is to be expected that they will publish a lot of
misinformation at frequent intervals. How does one sort out the sense
from the nonsense ?

There are in fact some excellent data sources, using several
different types of presentation:-
In the simplest example these take the form of graphic illustrations
in reputable astronomy magazines, showing the track of the asteroid
or comet against the back ground of the fixed stars.
In other cases the position of the object in the sky is given for
each day at time 0000 utc. With position reported as right ascesion
and declination in exactly the same way as the positions of stars are
reported. Both this method and the graphic illustration type reported
above are best used if you have a simple star atlas - Nortons Star
Atlas is probable the cheapest and simplest to use. More expensive
atlases tend to be far too big and bulky for every day use.
If however precise tracking over a period of days or years, ie too
long for a simple presentation, is required, there are a set of very
useful data sources for most of the known comets and asteroids. But
their orbital ephemeris data is presented in a form which is
naturally rather different from the Keplerian elements favoured by
radio amateurs for tracking objects which complete one or several
orbits per day. But not all that unfamiliar if you have software for
tracking the Sun, Moon or Planets in that it tables - 
Name of Comet: Perihelion time: Eccentricity: Perihelion distance
when nearest the Sun ( noting that these orbits are under the
influence of solar rather than earth gravity, and also note the
distance is given in AUs astronomical Units  of roughly 93 million
miles per AU): Revs per year: argument of perhelion at the given
Epoch ( usual epoch 2000 these days though some old data quotes
1950): Longitude of the ascending node : and Inclination 

Sky and Telescope magazine published in USA , available on European
newstands, is an excellent source of comet and asteroid data in the
graphic or daily right ascension and declination format .

Catalogue of Cometary Orbits by Brian G Marsden . Available from the
Minor Planet Center , Smithsonian Astrophysical Observatory, 
60 Garden St  Cambridge  MA 02138 USA  . Is an excellent source of
data on Comets old and new.    While the same author and address will
get you The Catalogue of Discoveries and Identifications of Minor
Planets, plus a second volume titled Catalogue of Orbits of
Unnumbered Minor Planets.

For most observers a good magazine like Sky and Telescope should be
adequate . But if you want to delve into whether the comet precursor
of the Perseids meteors was Swift-Tuttle or Kegler. Get the lot.