Stones in all sizes
By far, arriving particles range from smoke dust to sand grains. Less frequently a car size boulder strikes, showering whatever happens to be below with no consequences thus far like the March 27/2003 Park Forest event which sprinkled a Chicago suburb with stones. Objects this size arrive 4 to 10 times a year, larger objects less frequently.
A significant comet or asteroid is likely to strike Earth once in a century releasing an energy equivalent to a fifth to half the Hiroshima atomic bomb though the probability of doing so over a populated area is remote. A true mass destruction event is likely to occur within a 100,000 year span. Killer impactors got attention when in 1980 a geologist group led by Luis Alvarez identified a thin worldwide iridium layer of suspiciously alien origin pointing to a meteoritic event as the probable cause for the demise of dinosaurs 65 million years ago. Hildebrand reported in 1991 geologic evidence of an ancient impact basin submerged off the coast of Yucatan and subsequently named Chicxulub for the Maya town nearby (by the way it means "tail of devil" in maya...). The crater's age (64.98 ± 0.05 million years) and diameter (170 km) closely match estimated size and energy of the purported killer object.
On the lookout
Worldwide mass destruction caused by extraterrestrial objects is now recognized as both a factor in the past evolution of life on Earth, and as a distinct menace to be reckoned with. The NEO organization (Near Earth Objects) carries an ongoing effort with the purpose of identifying and cataloging objects which by virtue of its size and orbital trajectories pose a distinct threat.
