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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. |
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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. |
In this story:
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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
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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. |
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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
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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 |
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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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Author: Tony
Phillips, John Horack
Curator: Linda
Porter
NASA Official: Gregory
S. Wilson |
