It has only been in fairly recent times, since the Alvarez hypothesis was advanced and much supporting evidence found, (see "Killer Comets?") that people have again considered whether comets form a hazard to humans. This has taken the form of monitoring small bodies throughout the solar system: asteroids and meteors as well as comets, and identifying and tracking those that may come near Earth to predict any possible collisions such as the one that probably changed Earth's climate so drastically ~65 million years ago. See also, the Planetary Sciences Institute at the University of Arizona:
www.psi.edu/projects/ktimpact/ktimpact_print.html (new window)
which covers both the Alvarez impact (now often called the KT Boundary Impact) and questions about other such events. Or you may want to suggest this page to advanced middle school students for additional research.
Predicting collisions might seem like an easy task, given the state of computing today. In fact, "Near Earth objects" and other "Small Bodies" are hard to observe, since they reflect little light, and their orbits are frequently perturbed by interaction with the giant planets or the Sun, a phenomenon that shaped the original comet population and continues to change the "traffic flow" of small objects throughout the solar system. It is now believed (though this may change) that large Earth impacts occur on average once a century or so. The most recent major event was a comet strike in Siberia in 1908, possibly from a detached chunk of our frequent visitor, Comet Enke. This did no more than devastate a large tract of forest, partly because the comet vaporized rather than becoming tremendously hot, like a rocky meteor. See the Near Earth Objects (NEO) page at JPL for more:
http://neo.jpl.nasa.gov/index.html (new window). From this page you can also visit the "Impact" site at NASA Ames Center, where the threat of collision with NEO's is monitored.
We've seen in other sections what effect Jupiter, the 2nd largest body in the solar system, has on the orbits of small objects like comets. As it turns out, Jupiter diverts most wandering asteroids and comets away from the inner planets like earth, acting almost like a shield against cosmic collisions. But the solar system's supply of small bodies is constantly being renewed by disintegrating comets, and asteroids that are altered by close encounters with planets, and even each other.
There are a number of news stories on asteroids and "near misses" in the NASA Science News:
http://spacescience.com/default.htm (new window)
and NASA's "Thursday's Classroom" pages:
http://www.thursdaysclassroom.com/ (new window)
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