By Richard Lloyd
and J. Tillman Kennon

Events have a way of making sci-fi adventure scenarios into reality. It happened one day when a meteorite -- a small fragment of a meteor -- crashed through the roof of a home and injured a woman in Sylacauga, Ala., in 1954. (Reportedly, her home was across the street from the Comet Drive-in Theater.)

Even Mark Twain, America's most famous literary satirist, has a connection to the cosmos. He was born during the arrival of Halley's Comet in 1835 and died on its return in 1910. Halley visited Earth again in 1986. (Sadly, Twain was not resurrected during its flyby.)

Scientists have long known that comets and asteroids can strike the Earth and other planets. We actually believe that collisions among these bodies in the early history of our solar system formed the planets in a cosmic pinball billions of years ago. Although these bombardments occur far less often now, the experience of Mrs. E. Hulitt Hodges from Alabama reminds us those impacts can potentially affect our lives.

Before we elaborate, we should define a few terms. Asteroids typically orbit in a rock-strewn belt between Mars and Jupiter. Comets have been known since antiquity and are recognized by their long, wispy tails when they loop in close to the sun. When the orbit of a comet or asteroid brings them close to the Earth, they are collectively called Near Earth Objects (NEOs); when a NEO (or its fragments) enters Earth's atmosphere, it is called a meteor and it blasts out a crater when it strikes the Earth.

Today more than 100 terrestrial impact craters have been identified. Barringer Meteor Crater in Arizona was formed less than 50,000 years ago by a meteorite roughly 130 feet in diameter that gouged a crater about 1,300 feet wide and almost 600 feet deep. Chicxulub Impact Crater, for example, is 112 miles wide and lies partially under water off the Yucatan Peninsula of Mexico. The Chicxulub impact is widely credited with killing off the dinosaurs 65 million years ago.

Recent observations show that the danger posed by comets in modern times remains real. NASA recently announced that an approaching comet, designated as 73P, has fractured into several pieces although most of the fragments should pass by Earth at a distance farther than the moon. Comet breakups likely provide much of the material that makes for meteor showers in the first place; the difference here is that we caught the comet in the act of breaking up.

In 1994 comet Shoemaker-Levy 9 splintered into pieces and collided with Jupiter. The comet left vast disturbances in Jupiter's atmosphere that lingered for months and were large enough to easily swallow the entire Earth. Each impact left its mark, and the largest released 600 times more energy than the combined nuclear arsenal of every nation on Earth.

NASA's Near Earth Object Program Office has also projected that a newly found asteroid will pass between the Earth and Moon on April 13, 2029. It will pass so close that Earth's gravity will strongly deflect its path. The asteroid is about 1300 feet wide, 10 times wider and about 1,000 times heavier than the object that formed Meteor Crater in Arizona although much smaller than the Chicxulub asteroid that polished off the dinosaurs. Even so, this event warns us that impact events may occur more commonly than previously thought (for details see http://neo.jpl.nasa.gov/neo/.).

NEO impacts that actually threaten all human life on Earth are very rare. Understanding that the same impacts that formed Earth from the void and allowed life to gain foothold may also threaten our continued existence offers food for thought…and irony. Mark Twain, America's greatest literary satirist, surely would have appreciated that.

The Chemistry and Physics Department at Arkansas State University offers courses in space science, astronomy and astronomy research. Students in these classes as well as those involved in research have an opportunity to observe and track NEOs. Faculty and students provide a variety of outreach programs for area schools and the general public.

For more information contact the ASU Department of Biological Sciences at biology@astate.edu.

Dr. Kennon is an assistant professor of science education, and Dr. Lloyd is a temporary assistant professor of physics in the ASU Department of Chemistry and Physics.