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.

Field geological research, as traditionally practiced on Earth, is an extremely flexible science. Although field geologists plan their traverses ahead of time—nowadays with the advantage of remote-sensing data—initial plans are continually modified in response to observations, such that traverses evolve over time. This research modality differs from that utilized in extreme environments on Earth (e.g., on the ocean floor), on the Martian surface by the mobile laboratories Spirit and Opportunity , and by the Apollo astronauts during their explorations of the Moon. Harsh and alien conditions, time constraints, and resource limitations have led to the development of operational modes that provide a constrained and usually lower science return than traditional field geology. However, emerging plans for renewed human exploration of the Moon, Mars, and near-Earth asteroids serve as an opportunity to invent a new paradigm for advanced planetary field geology that embraces coordinated human and robotic research activities. This approach will introduce an operational flexibility that is more like that of traditional field geology on Earth. In addition, human and robotic collaborations, combined with the integration of new “smart” tools, should provide an augmented reality that leads to even greater science return than traditional field geology. In order to take full advantage of these opportunities when planetary field geology again becomes practical, it is imperative for field geologists on Earth to begin right now to learn how best to incorporate advanced technologies into their research. Geologic studies of analog sites on Earth that employ new technology-enabled strategies rather than traditional research methods provide ideal opportunities to test and refine emerging designs for advanced planetary field geologic studies, as well as to gain new insights into terrestrial geologic processes. These operational experiments will be most informative if they embrace the entire geologic research process—including problem definition, field observation, and laboratory analysis—and not simply field work. The results of such comprehensive research can be used to inform the design of a maximally effective training regimen for future astronaut explorers.