Weak Impact: The Perseid Meteor Shower

August 5, 1998: This summer's science-fiction offerings were full of large-meteorite impacts with harrowing consequences.  But the science-fact of the summer skies promises to deliver just as beautiful a show, with a lot less mess to clean-up.  The Perseid Meteor Shower makes its annual return to the summer skies on August 11/12, with as many as 80 meteors per hour visible from dark-sky locations throughout the Northern Hemisphere.

A Perseid meteor photographed in August 1997. Credits.

This photo of Comet Hale-Bopp shows its spectacular tail, made up of dust, gas, and rocky debris. The straight, blue ion tail is caused by the solar wind and the curved dust tail is caused by solar radiation pressure.

As comets enter the inner solar system, they are warmed by the sun, and ablated by the solar wind, which produces the familar tails that we see, sometimes quite strikingly, as in the case of comet Hale-Bopp in 1997 (image left). This debris is left in space, and is comprised of particles of ice, dust, and rock. When the Earth encounters these particles on its journey around the Sun, they strike the atmosphere with tremendous speed. Most are observed as a bright streak across the sky that can last for several seconds, but occasionally a large fragment will explode in a multicolored fireball. Most of the streaks are caused by meteoroids about the size of a grain of sand, although meteoroids are porous and much less dense than sand.

Impact Hazards? An impact crater found on the HST WF/PC camera radiator. Crater depth: 360 microns, and diameter: 980 microns. more info..

At its peak, the Perseids produce 50 - 150 meteors per hour. Are we in any danger from falling debris? Probably not. Most of the dramatic streaks we see in the sky are caused by particles that incinerate before they hit the ground. However, satellites and spacecraft can be damaged. Meteors can poke holes in solar panels, pit surfaces, and short out electronics. The image (left) shows a meteroid impact crater in the the Hubble Space Telescope. It was discovered in 1994, after the 1993 Leonid meteor storm.

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Most meteor experts do not expect the Perseids to pose a significant hazard to the more than 2500 commercial, military and science satellites in Earth orbit. The Leonids may be a different story. Once or twice every 33 years the earth passes through a dense stream of debris from periodic comet 55P/Tempel-Tuttle. The result is a spectacular display of 1,000 to 200,000 meteors per hour. The next severe Leonid meteor storm is due this November, and satellite operators are devising stretegies to protect their hardware. Antennas, cameras, and other delicate instruments will be be turned away from the expected stream of particles to minimize damage.

Comet Swift-Tuttle, pictured above in false color, visits the inner solar system once every 135 years. Based on what we know of its orbit there is no significant danger of a collision with Earth.

How to View the Perseids

The image indicates the general region of the sky from which the Perseid Meteors appear to eminate (blue circle).  This point, called the radiant, is really an optical illusion - the meteors are moving along parallel paths, but appear to come from a single point, just as a stretch of parallel railroad tracks will appear to meet at a point on the horizon.  Click on the image for a larger view.

The Perseids are perhaps the most famous and most watched of all meteor showers. They begin in late July and are most intense during the nights of 11/12 and 12/13 August. Viewing conditions this year will not be ideal because a bright, waning gibbous moon will make the dimmer meteors difficult to see. The good news is that Perseid showers in recent years have produced a high proportion of bright meteors.  Normally the best time to view meteors is after midnight, when the earth's rotation aligns our line of sight with the direction of the Earth's travel around the Sun. Then we're heading directly into the stream of meteors. This year may be an exception. The gibbous moon rises around 10:30 pm local time in mid-August brightening the sky from then until dawn. So, the best time to look may be in the early evening before the moon comes up.

After sunset, the constellation of Perseus will be low in the northeast. Take a lawn-chair and a blanket to your favorite viewing area, and set up. Allow yourself 10 or 15 minutes for your eyes to become adjusted to the dark. Locate the constellation of Casseopeia in the northern part of the sky, recognizing its familiar `W' or `M' shape, with the top of the W pointed north.   The location in the sky from which the meteors will appear to come is located approximately 20 degrees (the width of your hand from thumb to little finger with your arm totally extended) to the west, and a few degrees to the south of Casseopeia.  Focus your eyes on the stars, in the general direction of Perseus, relax, sit back, and enjoy the show!!!

Radio Meteors  An unusual method for observing meteors is growing in popularity among amateur astronomers: radio echos. When a meteor burns up in the atmosphere it leaves behind a trail of ionized gas. The ionization rapidly dissipates, but transmissions from distant radio stations are briefly reflected from the ionized trail back down to Earth. During an intense meteor shower, a simple shortwave receiver can detect many echos per minute from stations thousands of km away. More information.

Meteor Shower Ephemeris

As the earth moves along its orbit, our position changes with respect to the debris stream of comet Swift-Tuttle. As a result, the radiant of the Perseid meteor shower changes noticably on a daily basis. The table, right, shows the approximate location of the Perseid's radiant in equatorial coordinates every 2 days from 27 July to 16 August. Data are courtesy of the Meteor Showers web site at Washington University.  Current Moon Phase   Updated every 4 hours.  The bright moon in mid-August will make observations of the Perseids more difficult after moonrise.

Radiant Ephemeris  Date RA (deg) DECL (deg) July 27 27.1 +53.2 July 29 29.3 +53.8 July 31 31.6 +54.4 Aug. 2  33.9 +55.0 Aug. 4 36.4 +55.5 Aug. 6 38.9 +56.0 Aug. 8 41.5 +56.5 Aug. 10 44.3 +56.9 Aug. 12 47.1 +57.3 Aug. 14 50.0 +57.7 Aug. 16 52.9 +58.0 The dates of most intense activity are indicated in red.

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