The Perseid meteor shower, one of the strongest and most reliable of the "annual" meteor showers, should reach its peak display this year on Monday and Tuesday mornings, August 12 and 13. Sky-watchers, especially those observing from dark rural sites, may see as many as 60 to 100 meteors per hour emanating from the northeastern section of the sky on those two mornings, and lesser rates on the mornings preceding and following those. The moon was in its new phase this past Thursday, thus the skies for meteor-watching should be dark and moon-free.

The Perseids are caused by dust grains ejected off the nucleus of Comet Swift-Tuttle, which has an approximate orbital period of 130 years and which last returned in 1992. At the time of the comet's last visit ten years ago considerable excitement was generated when preliminary orbital calculations indicated a small possibility of a collision with Earth at the next return in 2126. Although more recent and definitive calculations have shown that the comet's "miss distance" at that time will be 14.2 million miles, this incident has nevertheless been one of several during the recent past that have increased our awareness of the threat posed by the "small" bodies of the solar system.

That awareness was reinforced a few weeks ago by the discovery of the asteroid 2002 MN by the LIncoln Near-Earth Asteroid Research (LINEAR) program based at White Sands Missile Range in central New Mexico. This object, perhaps 50 to 100 meters in diameter, had passed only 75,000 miles from Earth (one-third of the moon's distance) a few days before its discovery. This is the closest known approach for a "large" asteroid, although a small number of very tiny asteroids (i.e., 5 to 10 meters across) have been known to pass closer, the closest such approach being one of about 65,000 miles that took place in late 1994.

Strictly speaking, of course, there have been even closer approaches by much smaller asteroids, a statement that makes reference to all the meteorites that have landed upon the Earth's surface. On a somewhat larger scale, the object that exploded a few miles above a remote section of central Siberia near the Stony Tunguska River on June 30, 1908 – now considered as most likely being a piece of rock perhaps 80 meters in diameter (i.e., roughly the size of 2002 MN) -- could be considered as having made the closest "approach" by an "asteroid," at least during recent history.

The closest encounter we've had with an object that didn't hit took place exactly thirty years ago, on August 10, 1972. Early that afternoon a brilliant daytime meteor was widely observed traversing along a path over 900 miles long that extended from central Utah to southern Alberta, with some observers -- primarily near the mid-point of this path – reporting sonic booms. In addition to all the ground-based observations, a spaceborne infrared detector operated by the U.S. Air Force detected the heat that was generated by the object's passage through the atmosphere.

An analysis performed on all the available data indicates that the object, traveling in an orbit with an approximate period of 2.1 years, entered the atmosphere at such a shallow angle that it missed the Earth entirely. The closest approach was about 36 miles, which took place over Montana, after which the object headed back out into space again. A determination of the object's size has proven to be quite problematical, and is dependent upon what assumptions are made concerning its composition, but estimates range from perhaps five to ten meters in diameter up to perhaps eighty meters.

Had this object approached just a tiny bit closer, and either struck the Earth's surface or, more likely, exploded in the lower atmosphere, the effects would probably have included some significant but localized surface damage, somewhat akin to that produced by the Tunguska event. However, given the cold war political climate that was prevailing at that time, the indirect collateral effects may well have proved catastrophic for our society.

We've had no similar extremely close encounters since that time, but larger objects (such as 2002 MN) have made close approaches to Earth on numerous occasions. We have, in fact, a very close approach taking place later this month, by the asteroid 2002 NY40, discovered by LINEAR on July 14, and which is perhaps a quarter-mile to half-mile in diameter. Early on Sunday morning, August 18, 2002 NY40 will pass slightly within 330,000 miles of Earth, a bit less than 1 1/2 times the moon's distance.

At present 2002 NY40 is located in the constellation Aquarius, and requires a fairly large telescope in order to be seen. It will brighten dramatically over the next week as it approaches Earth, and on Saturday evening, August 17, it should be bright enough to be visible in small backyard telescopes, traveling northwest through the constellations of Lyra, Hercules, and Bootes at the rate of six degrees (twelve times the moon's apparent diameter) per hour. By the following night it will have passed between the Earth and the sun and will no longer be visible.

And as if this, along with the Perseids, wasn't enough to keep us busy, we also have a comet passing pretty close by right now. This object was discovered late last month by Japanese researcher M. Suzuki while examining images taken with an ultraviolet telescope aboard the SOlar and Heliospheric Observatory (SOHO) spacecraft located one million miles sunward of Earth. The comet, already a fairly bright object in the morning sky, is currently passing 24 million miles from Earth, and over the course of this next week may be faintly visible to the unaided eye as it travels rapidly through the constellations of Gemini and Lynx. It should thus give us an interesting but brief show before it pulls away from our planet and fades.