Sunday, August 10, 2008

As mentioned previously the Perseid meteor shower will peak this coming Tuesday, August 12th between 11:30 to 14:00 hours UT. That would be 6:30 to 9:00 here in the central time zone in Winnipeg. To get an idea of the time in your locality see the Time And Date website. This website also gives the times of Moonset and Moonrise, important this year as the light from a waxing gibbous Moon will reduce meteor viewing considerably. The good news is that the Moon will be setting shortly after midnight. The bad news for European and eastern North American viewers is the time of peak. In Europe this will occur during daylight hours, and even in North America the Sun will be rising during at least part of the peak. Still, any meteors seen in the early predawn hours may turn out to be spectacular "Earth grazers".
A "meteor shower", also known as a "meteor storm" or a "meteor outburst" is when there is an unusually high number of meteors in the sky and when these events can be traced back via an imaginary line to a point of common origin in the sky called the radiant.

Meteors happen when the Earth encounters bits of cosmic debris in its orbit around the Sun. Like litter in a campground such debris is ubiquitous in space, and there is always a background of sporadic meteors that occur randomly. Some neighbourhoods, however, are filthier than others. Throughout the year the Earth periodically passes through particularly concentrated trails of material. These have usually been traced to the residue of passing comets, and their position in the orbital plane means that they regularly appear in the same place in the sky at the same time of year.

When the cosmic debris is just hanging around orbiting the Sun (maybe playing cards, smoking cigarettes and, like a Spanish policeman, doing as little as is possible) it is known as a meteoroid . It is only when the Earth catches up to this non-event that things begin to get interesting. As the Earth and the space particle collide the debris becomes a visible meteor . These pass into the Earth's atmosphere at anywhere from 14 to 71 kilometers per second. Most meteors become visible at about 60 kilometers up. They are heated to over 1,700 degrees centigrade, begin to glow from the heat and are usually consumed by the heat as they enter the inner atmosphere. This heating is not caused by "friction" but rather by something called "ram pressure". As the meteor plunges into the atmosphere it compresses the air in front of it, thus heating it up. The heat of this compressed air simple diffuses to the meteor itself. In other words the heat comes from a push rather than a rub.

Should the meteor actually make it through the atmosphere and strike the Earth it is called a meteorite. There are two basic kinds of meteorites. Iron meteorites are made up of about 91% iron. Stony meteorites contain a higher proportion of other elements such as magnesium and especially silicon and oxygen in the form of silicon dioxide- good old sand.

(Hey, Molly could make a song out of this...
"Let's build sand castles in the sky
Let them loose to see if they can fly")

Some meteors explode before they evaporate or strike the Earth. The result is commonly called a fireball, and its sound can be heard dozens of miles away. Iron meteors are more stable than rocky ones. Slow meteors are less likely to explode than fast ones. Meteors arriving at an acute angle to the Earth are subjected to less stress than those that come in at an oblique angle. Yet...even a slow travelling iron meteor that falls from near the zenith has a chance of exploding and creating a fireball.

There is reportedly another sound produced by meteors that is poorly understood. These events are referred to as electrophonic meteors which seem to produce a sound even though it is physically impossible for any sound to travel from the height of the meteor. See 'Electrophonic Meteors' and 'Listening to the Leonids'. The theory is that this sound, variously described as "sizzling" or a "snap,crackle,pop" is produced when electromagnetic radiation in the VLF range is converted into sound energy near the ground.

When a meteor strikes the ground it produces a crater anywhere from 12 to 20 times its own size. Smaller impacts produce simple bowls. When larger objects strike terrestrial rebound creates a central peak along with a rim that is often "terraced" as the ground subsides after the initial impact. The largest impacts of all create multiple inner peaks due to the creation of several rebounds.

The largest impact in recorded history occurred on June 30th, 1908 when an object struck in Siberia. This has become known as the Tunguska event. This object flattened more than 800 square miles of trees. The cause of this explosion has been disputed, but the present consensus is that it was an exploding meteor. An Italian research team has recently obtained results that are suggestive of a remnant of part of this meteorite at the bottom of Lake Cheko about 5 miles northwest of the epicentre of Tunguska. See for the article 'A Possible Impact Crater for the 1908 Tunguska Event'.

Every year many meteorite strikes are reported across the planet. The first structure to be proven as being due to a meteorite impact is the Meteor Crater of Arizona. This crater is 600 feet deep, about a mile across, and its rim rises 150 feet above the surrounding ground. The meteorite that produced this crater impacted sometime between 20,000 and 50,000 years ago. The largest meteorite ever recovered in the USA hit ground in southern Nebraska in 1848. Observers reported that the fireball, which occurred in the afternoon, was "brighter than the Sun" This item, weighing 2,360 pounds was found buried 10 feet deep in a wheat field. Of course the Russians always did it first and better, as anyone who has listened to Chekov in old Star Trek shows can tell you. In 1947 the Russian Sikhote-Alin meteorite created more than 100 small craters some 20 meters across. Eat them apples American imperialists.

There is even a recent controversial theory that the die-off of mega fauna in North America at the end of the last ice age was not due to over hunting by recently arrived paleo-indians-the accepted theory- but rather due to localized climatic change because of an impact event. Maybe yes, maybe no.

For those interested in learning more about meteors in general look to the following sites:
The International Meteor Organization
The American Meteor Society
Meteor Showers Online

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