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.

Sn-rich particles, Ni-rich particles, and cosmic spherules are found together at four discrete stratigraphic levels within the 362–360 m depth interval of the Greenland Ice Sheet Project 2 (GISP2) ice core (72.6°N, 38.5°W, elevation: 3203 m). Using a previously derived calendar-year time scale, these particles span a time of increased dust loading of Earth's atmosphere between A.D. 533 and 540. The Sn-rich and Ni-rich particles contain an average of 10–11 wt% C. Their high C contents coupled with local enrichments in the volatile elements I, Zn, Cu, and Xe suggest a cometary source for the dust. The late spring timing of extraterrestrial input best matches the Eta Aquarid meteor shower associated with comet 1P/Halley. An increased flux of cometary dust might explain a modest climate downturn in A.D. 533. Both cometary dust and volcanic sulfate probably contributed to the profound global dimming during A.D. 536 and 537 but may be insufficient sources of fine aerosols. We found tropical marine microfossils and aerosol-sized CaCO 3 particles at the end A.D. 535–start A.D. 536 level that we attribute to a low-latitude explosion in the ocean. This additional source of dust is probably needed to explain the solar dimming during A.D. 536 and 537. Although there has been no extinction documented at A.D. 536, our results are relevant because mass extinctions may also have multiple drivers. Detailed examinations of fine particles at and near extinction horizons can help to determine the relative contributions of cosmic and volcanic drivers to mass extinctions.