Dynamical studies of the asteroid belt reveal it to be an inadequate source of terrestrial impactors of more than a few kilometers in diameter. A more promising source for large impactors is an unstable reservoir of comets orbiting between Jupiter and Neptune. Comets 100–300 km across leak from this reservoir into potentially hazardous orbits on relatively short time scales. With a mass typically 10 3 –10 4 times that of a Chicxulub-sized impactor, the fragmentation of a giant comet yields a highly enhanced impact hazard at all scales, with a prodigious dust influx into the stratosphere over the duration of its breakup, which could be anywhere from a few thousand to a few hundred thousand years. Repeated fireball storms of a few hours' duration, occurring while the comet is fragmenting, may destroy stratospheric ozone and enhance incident ultraviolet light. These storms, as much as large impacts, may be major contributors to biological trauma. Thus, the debris from such comets has the potential to create mass extinctions by way of prolonged stress. Large impact craters are expected to occur in episodes rather than at random, and this is seen in the record of well-dated impact craters of the past 500 m.y. There is a strong correlation between these bombardment episodes and mass extinctions of marine genera.

The discovery of many substantial objects in the outer solar system demands a reassessment of extraterrestrial factors putatively implicated in mass extinction events. These bodies, despite their formal classification as minor (or dwarf) planets, actually are physically similar to comets observed passing through the inner solar system. By dint of their sizes (typically 50–100 km and upward), these objects should be considered to be giant comets. Here, I complement an accompanying paper by Napier, who describes how giant comets should be expected to cause major perturbations of the interplanetary environment as they disintegrate, leading to fireball storms, atmospheric dustings, and bursts of impacts by Tunguska- and Chelyabinsk-class bodies into the atmosphere, along with less-frequent arrivals of large (>10 km) objects. I calculate the terrestrial impact probability for all known asteroids and discuss why the old concept of single, random asteroid impacts causing mass extinctions is deficient, in view of what we now know of the inventory of small bodies in the solar system. Also investigated is how often giant comets might be thrown directly into Earth-crossing orbits, with implications for models of terrestrial catastrophism. A theme of this paper is an emphasis on the wide disparity of ideas amongst planetary and space scientists regarding how such objects might affect the terrestrial environment, from a purely astronomical perspective. That is, geoscientists and paleontologists should be aware that there is no uniformity of thought in this regard amongst the astronomical community.