Date: August 9, 2009

Title: Perseids Meteor Shower

Podcaster: Christopher Crockett

Organization: Lowell Observatory
http://www.lowell.edu/users/crockett/

Description: On August 12, the annual Perseids meteor shower will light up the night sky with up to 60 meteors per hour. But before you head out to experience this display, you might want to listen to this podcast! I will talk about what a meteor shower is and why they occur with such regularity. We will discuss how best to plan for your night with the Perseids, when and where to look, and what you can expect to see.

Bio: Christopher Crockett is a University of California, Los Angeles graduate student currently working as a predoctoral fellow at Lowell Observatory. His research involves searching for planets and brown dwarfs around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Mr. & Mrs. James Crockett, proud parents of Christopher.

Transcript:

Hello. This is Christopher Crockett from Lowell Observatory.

As the Earth swings around the Sun on its orbit, it does not move unencumbered. The solar system is filled with the debris from asteroid collisions and evaporating comets. Several times a year, our planet plows through the dust streams of ancient comets that have ventured close to the Sun. When this happens, the sky lights up with a celestial fireworks display known as a meteor shower. As you listen to this podcast, the Earth is entering the debris trail of one such comet bringing with it a perennial favorite amongst stargazers. My friends, the Perseids have returned!

Meteor showers have been observed for thousands of years, characterized by a large number of meteors, or “shooting stars”, appearing over several days and all appearing to radiate from a single point in the sky. The name “shooting star” is a bit of a misnomer, so called because the tiny points of light moving swiftly across the sky appeared star-like. We now know that meteors have little to do with the much more distant stars they resemble. Long thought to be some sort of atmospheric phenomenon, it was Edmund Halley, discoverer of his famous eponymous comet, who first deduced that the points of light streaking through the night were actually visitors from interplanetary space.

Many people are familiar with the larger bodies of our solar system: the Sun, the planets, the asteroids, and the comets. But the solar system is also teeming with much smaller debris. The plane of our solar system is filled with a dusty haze, the remnants of countless asteroid collisions. On a very dark night, you can see this lane of dust as a column of light rising out of the west after sunset or out of the east in the hours before sunrise. Astronomers call it the zodiacal light; it’s the sunlight reflecting off the billions of dust grains blown off of asteroids and planets that have now settled into quiet orbits around our Sun.

The Earth careens through this dust lane on its annual trek around the Sun. Those particles unfortunate enough to get in our planet’s way slam into the upper atmosphere at speeds approaching tens of thousands of miles per hour. At such high speeds, these tiny particles compress the air in front of them causing it to glow. To the unsuspecting person who looks up that moment, he or she sees a point of light go racing across the sky. They’ve seen a shooting star; the fiery demise of a visitor from beyond our world.

A meteor storm is characterized by a flurry of meteor of activity. The sky blazes with light from up to one or two meteors per minute. What’s also remarkable during these storms is that all of the meteors appear to originate from the same place in the sky. If you watch several of these meteors and trace their paths backwards, you’ll notice that they all intersect at a single point. Astronomers call this the radiant of the storm: the point from which all the participating meteors arrive.

A storm arises not from the day to day encounters with the zodiacal dust but rather with a much more concentrated source of extraterrestrial debris: comets. Comets are icy bodies originating in the outer solar system. Shortly after the solar system formed, a mass migration of the giant planets shoved much of the raw material that formed these worlds into deep freeze at the outer edges of the planets. Most of these icy worlds found their way into orbits just beyond the orbit of Neptune and now compose the so-called Kuiper Belt of which Pluto is a very prominent member. Beyond that, nearly a third of the way to the nearest star, is a second source of icy material: the Oort Cloud, a spherical shell of cold primordial material enveloping our Solar System. Every so often, one of these cold, distant worlds is perturbed and sent careening into the inner Solar System. As it approaches the Sun, its outer layers begin to boil off producing a long tail stretching millions of miles across space that gives rise to the prototypical image of a comet most people have in their minds.

But long after the comet swings away from the Sun and begins its return to the outer Solar System, pieces of the frozen world remain. All of the material that boiled off as it passed through the Earth’s orbit stays locked in the same path that the comet took during its visit. On each subsequent visit from the comet, more debris is dumped into this path creating a debris stream marking out the comet’s orbit. Some of those debris streams intersect the Earth’s orbit. When the Earth ploughs through the cometary detritus, the number of meteors in the sky jumps sharply. And since all of the meteors are following the same path, an observer on our world sees all of these meteors appearing to come from the same place in the sky. It is then that we have ourselves a meteor shower.

Because the orbits of the cometary debris are stable, the Earth always encounters the same debris trails at the same points in its orbit. This is why certain meteor showers always appear at the same time every year.

We are now entering the debris trail that drives the annual Perseids meteor shower, so named because the meteors all appear to radiate from the constellation Perseus. It is one of the most spectacular and reliable showers of the year. The Perseids have been known for at least 2000 years. The Chinese recorded the storm in the year 36 A.D. with further observations noted in the 8th through 12th centuries by Japanese and Korean astronomers. In medieval Europe, the storm came to be called the “Tears of St. Lawrence” as the annual return of the shower closely coincided with the anniversary of the martyrdom of St. Lawrence on August 10, 258 A.D.

We now know that is not the fiery tears of a martyred saint that rain down in our skies in mid-August but rather the icy remnants of an interplanetary traveler. The comet that produces the Perseids is an elusive visitor from the outer solar system known as Comet Swift-Tuttle. The comet was discovered by two independent observers in July of 1862: Lewis Swift of Marathon, NY, and Horace Tuttle of Harvard Observatory in Massachusetts. Just a few years after this discovery, Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseids meteors and noted that they closely aligned with the orbit of the recently discovered comet making the Perseids the first shower to be matched with a specific comet. It’s probably no coincidence that the Perseids were unusually spectacular in those years as the comet stream had recently been replenished by the returning comet!

In 1992, the comet returned bringing with it spectacular meteor showers in the following couple of years with European observers in the summer of 1993 reporting as many as three to eight per minute!

The comet itself returns every roughly 130 years with the next visit not occurring until sometime around 2122 (mark your calendars!) It moves about the Sun on a highly elliptical orbit that brings it from deep in the outer solar system – five times the distance to Saturn or roughly 5 trillion miles away – to a point just inside the Earth’s orbit. The orbit is also highly inclined relative to the orbits of the planets which means it spends most of its time well above the plane of the Solar System. This has actually aided in making the Perseids stream more stable over the past several thousand years as the debris does not have to contend as much with disturbances from passing planets.

If you plan to take part in this annual spectacle, and I certainly insist that you do, try to find a nice dark spot away from street lamps and headlights. The best way to view a meteor shower is to spread out a blanket and lie on your back so you can comfortably view the entire sky. Be sure to allow yourself about 20 minutes for your eyes to completely adapt to the darkness. While the meteors will seem to radiate from the constellation Perseus, they will be easily visible all across the sky. If you just stare directly overhead, you’re bound to see many, many meteors. Don’t worry about binoculars or a telescope as these will limit your field of view; you want the whole sky available to you! Bring something warm to ward off the evening chill and don’t forget the bug spray!

This year’s peak is predicted to occur on August 12 at around 15:00 Greenwich Mean Time. The best time to go out will be that night or the evening before. If you want to maximize the number of meteors you can see, you might want to wait until after midnight. In those early morning hours, you’re location on the Earth is facing in the direction of the Earth’s orbit and towards the cometary stream. Around the peak, you can expect to see roughly one a minute. You will have to contend with a moderately bright moon that will mask the dimmest meteors but don’t let that discourage you! And if you can’t make it outside on the evening of the 12th, the Perseids will be ongoing for a couple of weeks before and after the peak with the greatest amount of activity occurring between August 8 and 14.

The annual meteor showers are our connection to the primordial material from which our world was built. When you watch the Perseids, you are witnessing the fiery death of visitors from beyond the realm of the planets. The rain of materials that these interlopers bring has been going on for billions of years and is widely thought to be responsible for delivering the water and organic material to an infant Earth that we now depend on for our existence. While the infall of interplanetary debris is a mere fraction of what it was in those early days, it’s never really stopped and is a reminder that we truly do come from star dust. Reflect on that as you watch the return of the Perseids over the next week and participate in a spectacle that humans have watched for thousands of years and will continue to observe for many thousands more.

End of podcast:

365 Days of Astronomy
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