We're interested in the changes made to our atmosphere as a result of comet or asteroid impacts. Even moderate ones, such as the famous 1908 Tunguska (Russia, Siberian region) airburst can have a measurable effect on atmospheric chemistry. We have simulated the event and produced results that agree with ice core data. We then consider a million-times larger event which may have happened over the North American ice sheet near the end of the last ice age. The results are interesting--at this point I believe I will just link a PowerPoint presentation, which should be self-explanatory. Or pdf in case you prefer that. For a more formal presentation, our paper about this has been published on Geology. An account in Science News explains the main points.
More recently, Drew Overholt led an investigation into the direct deposition of cosmogenic isotopes from asteroids and comets. It appears that interesting amounts of Carbon-14 and Aluminum-26 can be deposited by larger events. This may be a strong test of such impact hypotheses.