Cometary events can be ongoing or a thing of past. Likewise they can be
periodic,
pseudo-periodic or
a one-shot event.
For example, we know of ongoing periodic cometary cycles like the 27.9 million year cycle followed by Nemesis and its accompanying cometary swarm (see chapter 13 to 19 of
Earth Changes and The Human-Cosmic Connection) or the ongoing 3,600 year cometary cycle described in my article '
Volcanoes, Earthquakes And The 3,600 Year Comet Cycle'.
There are also ongoing pseudo-periodic cycles like Comet Halley, whose average period is 77 years, but whose single periods span from 74.33 years to 79 years.
There are one-shot events like the 12,900 BP cometary event described in my article '
Of Flash Frozen Mammoths and Cosmic Catastrophes'.
And finally there are past pseudo-periodic comets like cometary Venus from 5,200 BP to 4,600 BP with decreasing orbital period: from 160 years for pass 1 to 60 years for pass 7.