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Brian Marsden's "Catalogue of Cometary Orbits"..as of 1997..added to since that time...lists roughly 1,548 comets for which sufficient data exist to compute orbits...91 from the extremely scanty historical data that has come down to us from the period before the 17th Century..the rest from cometary passages recorded during the last 3 centuries.
What science really knows about comets..in other words...derives from data on an incredibly narrow sample of cometary behavior as observed from our tiny corner of the universe...in 3 centuries.
As per the article...planetary gravametrics do influnce the behavior of comets and the fragmented break up of supers which now circle in ellipses around the sun..or in ellipses around the outer solar system.
Recent study has found out that comets outgas at an unpredictable rate...most noted when heading away from the Sun.
The outgassing..like your Appollo or Space Shuttle's thrusters used for postional adjustment...in the case of the comets...they begin to barrel roll and shift postion like rocket boosters were being fired....the outcome...their postional ellipse changes....the movement changing thier angle..and time.
studying return dates of comets reveals this gravametric/thruster shifting..which equals time values...meaning that a miss by 500,000 klms and a certain number of hrs ..maybe come a close call later..and possibly an impact.
Comet Hale Bopp showed up out of nowhere...having not been seen for an estimated 4,000 yrs.
some comets with orbits of 12,000-90,000 yrs could theoretically appear out of the night sky...at any time..without warning.
The "Cape effect"..the violent outgassing of the comets reveals they can shift their orbits..making mapping them a guess.
Since the first optical confirmation of the existence of giant comets in the Kuiper belt in 1992 no such object has yet been seen to fragment.
Ordinary comets,which are intimately related to the giants in every respect are frequently observed to break apart..relasing swarms of debrie...like "MIRVed" intercontinental missile warheads.
One example was comet Biela..which had a computed orbit that came within 20,000 miles of the Earth.
Discovered in 1826....its return was calculated..retarding influence of Jupiter and Saturn incorporated.
In 1832 it missed by one month.
In 1846 something happened...it appeared in 2 seperate pieces...like 2 race horses 16,000 miles apart.
Now it has dissapeared...its fragments hanging around out their somewhere.
From 1885 to 1892 spectacular meteor showers were seen...in some locals 160,000 shooting stars...
Today the debrie of comet Biela returns annually as the Andromeda meteor shower.