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.

The spacecraft flybys of Comet Halley in 1986, along with other new techniques for observing long- and short-period comets, have greatly increased our knowledge of cometary nuclei. In addition, new dynamical studies have expanded our understanding of the Oort cometary cloud and suggested the existence of a massive inner Oort cloud that can be the source of intense “cometary showers” into the terrestrial planets zone. This new information is used to obtain revised estimates for the current cometary cratering rate on the Earth, both from the steady-state flux of long- and short-period comets, and from random cometary showers caused by close stellar passages and encounters with giant molecular clouds. It is found that the cometary showers account for approximately 17 percent of terrestrial craters ≥10 km in diameter, versus the steady-state flux of long- and short-period comets, which provides about 12 percent of the cratering flux. The total current calculated cratering rate for the Earth, including Earth-crossing asteroids, is in good agreement with the terrestrial cratering rate estimated from counted craters on dated surfaces. No evidence is found for an enhanced cometary flux at this time, or for periodic cometary showers every 26 to 32 m.y.

Micropaleontological study of the sedimentary record from the Montagnais impact structure, located on the shelf off Nova Scotia, has shown that the impact had neither regional nor global effects on biological diversity. This result provides new evidence for the lower threshold of extinctions due to the impacts, indicating that impacting bolides must be larger than 3 km in diameter to cause extinctions. We use this evidence to construct simplified curves to test relations between mass extinctions, bolide diameter, and impact periodicity. These data point to reoccurrence of mass extinctions of amplitude comparable to the Cretaceous/Paleogene event an average of once in every 100 to 500 m.y., whereas the probability of extinction of life on Earth triggered by an impact of a bolide larger than 60 km in diameter has an average frequency of once in 1 b.y. Data presented in this chapter do not support a direct parallel between the 26-m.y. extinction periodicity and the impact cratering record in Earth.