Most are transient visitors to the vicinity of earth their orbits are so long that many of those arriving now (such as Hale-Bopp) must have made any previous visit to the inner solar system before the beginning of recorded history.
Or so says David Eichler, lead author of a forthcoming Astrophysical Journal Letters paper positing that a sun-grazing comet roughly the size of Hale-Bopp (with a nucleus some 30 kms in diameter), could trigger cosmic ray-generating shockwaves large enough to initiate a global electromagnetic Armageddon.