The Falling Stars of Perseus

Aug 12, 03:34 PM John McKay

The Perseids Meteor Shower is an annual event that peaks about every August 12; however, the actual shower begins in late July and extends through mid August. It is named after the Perseus constellation which is prominent in the sky during this season.

It may be useful to define what is meant by a “meteor shower.” Meteors are the bright streaks of light that are formed as rocks and other solids fall toward the Earth and burn up in atmospheric entry. Meteors are created by all types of space debris: sometimes its just space junk left in orbit by man from various space missions, the odd nut or bolt, for instance but usually it is from naturally occurring space rocks called meteoroids. Most of these meteoroids are pea-sized rocks of metal rich ore. It is not uncommon for these meteroids to be solid iron or even contain significant amounts of iridium. (Iridium is related to platinum and is considered to be the rarest non-radioactive solid on Earth; the amount of iridium deposited on the planet by meteroids may exceed the amount of iridium that is naturally occurring in the Earth’s crust.)

As these meteoroids fall toward the Earth, they must first pass through several miles of atmosphere. They tend to fall at a fairly high speed (in excess of 30,000 miles per hour) and almost all of their kinetic energy is transferred to heat due to ram pressure (the falling object compresses the air immediately in front of its path, causing the air to superheat around it, consequently melting the surface the meteoroid) in a process that is over 100 times more efficient than any man-made explosive. A normal gravel-sized meteoroid will heat up to over 3,000 degrees Fahrenheit and the air flow around it will blow away the melted portion creating a bright, luminescent wake which will be visible up to 60 miles away. Naturally, most of these meteoroids completely burn up before they hit the ground. However, a number of factors can affect this: size and density of the meteoroid, and angle of atmospheric entry. Given the right conditions a meteoroid can survive the trip through the atmosphere and impact with the Earth’s surface. At this point, it is called a meteorite. (To be proper on the terminology, a meteoroid is a meteoroid right up until impact, after which it is a meteorite, and a meteor describes the whole glowing phenomenon of its descent. Simple, no?)

During the Perseids shower, what is actually happening is the Earth passing through a gravelly patch in space. The Earth is passing through this wall of gravel and sand and the individual particles (or meteoroids) pass through the atmosphere and light up into meteors. Some of them may fall to the ground as meteorites. And all of this debris has at its source the Swift-Tuttle comet.

In 1862 the comet was observed independently by two astronomers, Lewis Swift and Horace Tuttle. During the months this comet was visible they managed to calculate its orbit to be 120 years or thereabouts. Its orbital period is actually closer to 130 years and it made its next visible appearance near Earth in 1992, though it was too faint to see with the naked eye at that time. Now, one very interesting fact about the Swift-Tuttle comet is that its orbit actually crosses that of the Earth. Someday the two objects may collide. Its next appearance will be in 2126 but it should be about 15 million miles from Earth. In 3044 it may get as close as a million miles. That’s spittin’ distance in the cosmic scale of things.

A comet is basically a big snowball. Swift-Tuttle is about 6 miles in diameter, so that’s a really big snowball. It also has lots of little rocks and all the different kinds of debris that make up those meteoroids we talked about earlier. Every 130 years or so, when it gets near enough to the sun, it gets a little melty. That’s what comets do. Comets are the big snowballs of the solar system and as they melt a bit, they lose mass and it gets swept away from the sun. This tail doesn’t always trail the comet, it always points away from the sun. As the comet approaches the sun, you can see the comet in front of the tail; it appears the tail follows the comet. However, as the comet travels away from the sun, the tail precedes the comet. Very untail like. That’s just the way it works. What we see as the tail is actually the sun melting bits of the comet away and the solar wind pushes it out away from the comet and the light from the sun reflects off this debris and we see a long silvery tail.

As it happens this comet has been around our sun a few times over the past few millennia. Its appearance has been recorded on Earth as early as 69BC, and every time it passes through the neighborhood it leaves tons and tons of gravel and sand in its wake and every year the Earth crosses through its debris field and we have the Perseids Meteor Shower.

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