Several days after the annual Perseid meteor shower last month the earth was visited by a somewhat threatening asteroid. On the night of Saturday August 17-18 a rock estimated to be the length of two football fields in size, glided past our unassuming planet beyond the moon's orbit at a distance of some 327,000 miles.

It was quite a show as I observed it from my back yard near New Windsor using a telescope. Its motion against the starry background was readily apparent. Unfortunately, the asteroid was too faint to be visible in binoculars as claimed by "experts" quoted by many news sources so few people probably actually got to see it.

This asteroid was given the preliminary designation 2002 NY40 after its discovery July 14 by astronomers using the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research (LINEAR) project telescope in Socorro, N.M. The object immediately leaped to international fame after a preliminary orbit was determined and it was discovered that there was a small chance that 2002 NY40 may collide with the earth 20 years from now.

As its orbit was refined the chances of impact were reduced. The Near-Earth Object Program Office at NASA currently estimates there is a just a one in 500,000 chance that 2002 NY40 will collide with the earth on August 18, 2022. Therefore, an actual collision appears very unlikely at that time. Still, the earth has a better chance of getting hit by the asteroid than the buyer of a lottery ticket has of winning the big jackpot.

Most asteroids are located in a debris band between the orbits of Mars and Jupiter known as the main asteroid belt. Due to gravitational perturbations from the major planets some asteroids have been jostled away from the main belt causing them to careen across the orbits of the inner planets like leaping deer across a highway at night.

Asteroid 2002 NY40 is a member of a class of asteroids called the Apollos. Apollos have orbits that cross the orbits of both Mars and Earth. Another class of Earth crossers are the Atens whose orbits are completely inside of the orbit of Mars.

Scientists are particularly interested in the Apollos and Atens because of their potential threat of impact with Earth. Asteroids have been linked with extinction events such as those which led to the demise of the dinosaurs. Although 2002 NY40 is believed not to be large enough to cause global extinction, scientists will agree that it is capable of causing massive destruction on a regional level and could kill millions of people should an impact occur.

So how rare are these rocky ships passing ours in the night? There was an even closer approach in June by the asteroid 2002 MN. That asteroid was smaller than 2002 NY40 and was not discovered until three days past its closest approach. According to Donald Yeomans at the Jet Propulsion Laboratory (JPL), "Things of this size come by every few weeks. It's just a matter of finding them."

So, is anyone trying to find them? There are several groups dedicated to detecting, cataloging, and monitoring near-earth bodies. In the United States there is the Spacewatch Project at the University of Arizona's Lunar and Planetary Laboratory. Internationally there is the Spaceguard Foundation, with headquarters in Rome.

Even though there is a lot of sky to cover and resources are limited data is being collected. A table of close encounters with asteroids is maintained on the Harvard-Smithsonian Center for Astrophysics web site at http://cfa- www.harvard.edu/iau/lists/Closest.html.

It is startling at first to realize that 40 out of the 42 listed closest encounters of all time happened only in the past 14 years! Does this mean that asteroids have suddenly declared war against Earth? No, it means that before 1990 or so few were bothering to look.

In case you were wondering, 2002 NY40 ranks 14th on the list. It is also much brighter, and therefore presumably larger, than the 13 asteroids that have passed closer to us.

Comets can also have close encounters with the Earth. In May of 1983 Comet IRAS-Araki-Alcock (1983 H1) passed within three million miles of Earth. I recall it being easily visible to the naked eye as a large gauzy blue patch whose motion was easily detectible in a telescope.

A closer comet had not passed by the earth since Comet Lexell in July of 1770, six years before the signing of the Declaration of Independence. Comet IRAS-Araki-Alcock ranks #3 on the historical list of closest comets. For the full list, check the web at http://cfa-www.harvard.edu/iau/lists/ClosestComets.html.

What about right now? Which asteroids are closest to the earth at this instant? If you are interested in keeping tabs I suggest you bookmark the "Currently Closest Asteroids" page maintained by James R. Settle at http://www.goldcoaststudios.com/jsettle/Astronomy/CurrentAsteroids.html.

Maybe you're concerned about future close encounters. According to the Sormano Astronomical Observatory, the next known approach by an asteroid closer than 2002 NY40 shall be when asteroid 1999 AN10 swings by on August 7, 2027. You can find the list on the web at http://www.brera.mi.astro.it/sormano/teca.html.

Of course, there are bound to be surprises between now and 2027 as newly discovered asteroids "come out of the woodwork." In order to gauge the possible destruction that may be wrought by asteroids and comets threatening future generations, scientists delve into the geological record looking for clues.

New extinction events from Earth's past are still being reported. In August a team from Stanford and LSU presented a study which concluded that a giant asteroid 12 to 30 miles (20 to 50 kilometers) struck earth 3.47 billion years ago causing a set back of hundreds of millions of years in the development of life and may have entirely changed its course.

Asteroids and astronomy may seem like leisurely pursuits having little or nothing to do with our everyday lives. Other than reading a horoscope (which quite frankly is superstition - astrology, not astronomy) a person may hardly ever think about what is going on in the universe around them.

Still, You should ask yourself this question. If a large asteroid capable of causing the end of the world as we know it was headed for a show down with Earth, would you want to know about it in order to make preparations, or would you rather not know since there is nothing you could do to change it?