Hale-Bopps duration estimated at 4,210 yrs...with previous passage in 2210 BC.
Calculating the return of prriod comets was markedly changed when comet Swift Tuttle did not return as expected..Brian Marsden of Cape Observatory South Africa discovering Comet Swift Tuttles 'Out gassing' as it rocketed away from the Sun.
The 'Cape effect' was also detected in Halley's comet by the Giotto space probe..the gas jetting exerted a force of about 5 million pounds..or nearly as much as all the space shuttle engines volume on lift off from the pad..the jetting of the comet continuing hour after hour..day after day.
Outgassing comets..altering their trajectories..some previous crossings near earth so may days short of a month in proximity..but what of the next passing on ellipse?
In Marsdens 1997 catalogue lists 1,548 comets with sufficient data to compute orbits,-91 from fragment data prior to the 17th Century.
What science really knows about comets ,in other words ,derives from data on an incredibly narrow sample of cometary behaviour as observed from our tiny corner of the universein three insignificant centuries.
Comet Encke,with the shortest of all known periods,at 3 1/3 yrs has become increasingly erratic in keeping its appointments in our skies.
Other comets which previously candessed and outgassed..have become inert ..approaching with no reflectivity..and exiting..a giant dark rock..tens of kilometers wide.
As Hale Bopp appeared suddenly..so could other comets of long duration..with orbits of 15,20 and 90,000 yrs.
Appear suddenly..and be in magnitudes of hundreds of kilometers wide...and inert;
Jet black Missiles...mirved into multiple warheads from gravametrics.
