There is now widespread acceptance that the dinosaurs and more than 70% of all species suffered mass extinctions about 65 million years ago as the result of an impact of a comet or asteroid. The remnants of this collision remain in the Yucatan Peninsula of Mexico. About 140 major impact craters on Earth (and countless craters throughout the solar system) prove that planets are regularly battered by huge meteorites. In July 1994 Comet Shoemaker-Levy 9 impacted the planet Jupiter. Recently scientists have found evidence in Chad (in Africa) for a similar impact on planet Earth (but with much smaller comet fragments).
The entrance speeds (11 to 74 km/s) of comets and asteroids are so high that these objects possess more kinetic energy per mass than the chemical energy per mass represented by explosives such as TNT. A 10 km diameter asteroid entering at 30 km/s would have the energy equivalent to 15 billion times that of the Hiroshima nuclear bomb. The effects from a major (about10 km in size) impact include: shock waves in the crust of the earth, tsunamis (tidal waves), firestorms over large portions of a continent, destruction of the ozone layer, atmosphere alteration (due to the huge amount of material ejected into the upper atmosphere), etc. and range in time scales from hours to thousands of years.
While we would, on average, expect one such dramatic impact every ten million years, our state of ignorance of the population of near earth objects (NEOs) is such that we have no guarantee that it will not happen during our lifetime. Also, we would expect that a 50 to 100 m sized object, still capable of generating dangerous tsunamis, would strike Earth every century or so (the last such event occurred in June, 1908 in Tunguska in Siberia). A land impact of a more modest object can also cause severe destruction, through earth shocks and firestorms. One can estimate the approximate area of devastation from such objects by using the relationship area of devastation (in square km) =3D 400 x (kinetic energy in terms of megatons of TNT)2/3. A 500 m object would be expected to devastate an area comparable to Ontario plus Quebec.
The probability of a comet impact is probably less than that posed by near earth asteroids, although it is also much more difficult to defend against (e.g. Comet Hyakutake, which passed very close to the Earth in late March this year, had only been discovered two months prior). The relative speed of impact for comets is, on average, much higher so that a smaller object can pose a greater threat.
Can our society protect itself from a catastrophic impact? The answer is probably yes, if we choose to make the effort. A relatively minor (~ 0.1 m/s) push on a typical NEO when it is nearest the sun (at perihelion) will result in its orbit being changed sufficiently to safely miss the Earth. We need to better estimate the hazard posed (and identify the specific consequences with more precision), through better models of the atmospheric effects of major impacts. It would be irresponsible to spend huge sums of money in a search for NEOs, if the effects were in fact not those currently expected. The precise effects depend on the degree of atmospheric fragmentation of the impactor, and the fragmentation of large meteoroids is being actively researched in Atlantic Canada. Next we need to better estimate the populations of dangerous objects, and determine the orbits of dangerous objects. The first earth-intersecting orbit asteroid was found only in 1932, and half of all known objects have been found since 1990. However, even at the current rate of detection it is estimated that it will require several centuries to have detected 99% of the dangerous objects. Finally, we need to develop techniques for diverting potential impactors to prevent catastrophic collisions. The most likely technique involves explosion of a nuclear weapon at (or above) the surface of the NEO (chemical explosives do not possess enough energy per mass), although other techniques (such as "solar sails") are also possible.
Duncan Steel Rogue Asteroids and Doomsday Comets: The search for the million megaton menace that threatens life on Earth (Wiley, 1995, ISBN 0-471-30824-2).
John Lewis Rain of Fire and Ice: The very real threat of comet and asteroid bombardment (Addison-Wesley, 1996, ISBN 0-201-48950-3).
Ahrens, T.J. & Harris, A.W. (1992) Nature, 360, 429-433, "Deflection and fragmentation of near-Earth asteroids" (fairly technical - in university libraries)
Alvarez, W. and F. Asaro (1990) Sci. Amer., 263 , (4), 76-84, "What caused mass extinctions? An extraterrestrial impact"
Chapman, C.R. & Morrison, D. (1994) Nature, 367 , 33-40, "Impacts on the Earth by asteroids and comets: assessing the hazard"
Chyba, C. (1993) Astronomy, 21 , (12) 38-45 "Death from the sky" An interesting account of the Tunguska 1908 impact event, and consideration of the probability of impacts of various magnitude .
Dickinson, T. (1994) Equinox May/June, 1994, 65-76 "The big hit"
Dietz, R.S. (1991) Astronomy, 19 , (7), 30-37, "Demise of the dinosaurs: A mystery solved?"
Gallant, R.A. (1994) Sky & Telescope, 87, (6), 38-43, "Journey to Tunguska"
Gehrels, T. (1996) Sci. Amer., 274 (3), 54-59, "Collisions with comets and asteroids"
Grieve, R.A.F. (1990) Sci. Amer., 262 , (4), 44-51, "Impact cratering on the Earth"
Morrison, D. (1995) Astronomy, 23 , (10), 34-41, "Target: Earth"
Sagan, C. & Ostro, S. (1994) Nature, 369, 501-502, "Dangers of asteroid deflection"