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The 1998 Perseid Meteor Shower


       Weak Impact: The 1998 Perseid Meteor Shower

       NASA/Marshall Space Flight Center Space Science News
       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 Aug. 11-12, with as many as 80 meteors per hour
       visible from dark-sky locations throughout the Northern
       Hemisphere.

       As in the popular movie "Deep Impact", the action of the
       Perseid meteor shower is caused by a comet, in this case
       periodic comet Swift-Tuttle. Fortunately there's no danger of
       Swift-Tuttle hitting the Earth. It's about 6 miles wide and a
       collision would be catastrophic. Instead, the stars of this
       show are tiny grains of dust and debris, most smaller than a
       grain of sand. They are the rubble left behind when
       Swift-Tuttle occasionally visits the inner solar system.

       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. 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.

       A possible impact hazard?

       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.

       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.

       Viewing the Perseids

       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 Aug. 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 11:30 pm EDT 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-echo meteor obervations

       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 (http://www.imo.net/radio/index.html)

       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 shows the approximate
       location of the Perseid's radiant in equatorial coordinates
       every two days from 27 July to 16 Aug. Data are courtesy of
       the Meteor Showers Web site at Washington University. The
       dates of most intense activity are indicated in CAPS.

       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

       Moon phases

       The bright moon in mid-August will make observations of the
       Perseids more difficult after moonrise.

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