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(meteorobs) Here Come The Perseids!


Here come the Perseids!
Marshall Space Flight Center
http://science.nasadot gov/newhome/headlines/ast09aug99_1.htm

The 1999 Perseid meteor shower peaks on August 12th and 13th under the dark
skies of a nearly new moon.

August 9, 1999: The attention of the world will be riveted on the heavens
this week as the last total solar eclipse of the 20th century takes place on
August 11. However, for most skywatchers outside the path of totality, the
best sky show won't occur until a day and a half later, on Thursday, August
12, and Friday, August 13. That's when the annual Perseid meteor shower
reaches its peak with 50 to 150 shooting stars per hour.

In recent years the spectacular Leonid meteors have attracted considerable
attention, but historically the Perseids are the best known of all meteor
showers. It rarely fails to provide a pleasing display and, because of its
summertime appearance, it tends to attract many astronomy novices.

This year should be a good one for viewing Perseids. The shower's maximum
takes place on August 12, 1999, under dark skies just one day after the new
moon. Although the Perseids officially begin in late July, the shower builds
rather slowly in intensity until there is a series of sharp peaks in
activity near mid-August.

The figure below shows the expected activity profile for 1999 based on
observations in 1997. The shower's maximum is centered around 21 UT (2 p.m.
PDT) on Thursday, August 12, 1999. The level of activity remains relatively
high for nearly 24 hours afterward, so the early morning hours before dawn
on Thursday, August 12 and on Friday, August 13 should be good times to
observe in most places [click for more observing tips]. The other two times
indicated in the figure (0700 UT on August 12 and August 13) are launch
windows for the Science@NASA Perseids Live! high altitude balloon flight
which will transmit a live webcast of the meteor shower from the
stratosphere.

Like most meteor showers, the Perseids are caused by comet debris. As comets
enter the inner solar system, they are warmed by the sun and peppered by the
solar wind, which produces the familar tails that stretch across the night
sky when a bright comet is close to Earth. Comet tails are made of tiny
pieces of ice, dust, and rock which are spewed into interplanetary space as
they bubble off the comet's nucleus. When Earth encounters these particles
on its journey around the Sun, they strike the atmosphere speeds exceeding
100,000 mph. (The average speed of Perseid meteoroids is 130,000 mph!) 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 (popularly called 'shooting stars') are caused
by meteoroids about the size of a grain of sand, but much less dense.
Although they travel at high speeds, these tiny meteoroids pose no threat to
people or objects on the ground.

The Perseids were the first meteors ever associated with a particular comet.
From 1861 to 1863, observers noted a great increase in the number of August
Perseids. As many as 215 per hour were seen in 1983. The Italian astronomer
Giovanni Virginio Schiaparelli (better known for giving the name "canali,"
or "channels," to the dark linear markings on Mars) calculated the orbits of
some Perseid meteoroids and discovered that they closely matched that of
periodic comet Swift-Tuttle, which had been discovered in 1862 during its
close approach to Earth. Swift-Tuttle orbits the Sun once every 135 years.
The last time it passed near Earth was in December 1992. The proximity of
the comet once again caused an increase in Perseid activity and, in August
1993, observers in Central Europe were treated to 200 to 500 meteors per
hour. Swift-Tuttle won't make another swing through the inner solar system
until 2126, but when it does the comet itself is expected to be an
impressive sight as seen from Earth, rivalling Comet Hyakutake in 1996 or
Comet Hale-Bopp in 1997.

Perseid observing tips

Perseid meteors can be seen anytime after the sun has set and the
constellation Perseus is above the horizon (which is nearly all the time
from observing sites in the northern hemisphere). In practice, watching for
meteors between sunset and midnight is rarely profitable. The best time to
look is between about 2 a.m. and dawn. That's when the local sky is pointing
directly into the meteoroid debris stream (see the diagram below). The early
morning hours of August 12 and August 13 should be good times to watch if
you live in the northern hemisphere.

For northern hemisphere observers at latitudes higher than about 35 degrees
Perseus is circumpolar -- it is always above the horizon. Unfortunately, due
to the high declination of the radiant, the Perseids are not a good shower
for skywatchers south of the equator.

The sky map below represents a view of the sky looking northeast from a
mid-latitude viewing site at 3:00 a.m. (local time wherever you live). The
radiant, in the constellation Perseus, is located almost midway between
Jupiter in the east and Polaris in the north. Jupiter (magnitude -2.6) and
Saturn (magnitude +0.3) will be very bright and easy to spot.

Experienced observers suggest the following viewing stategy: Bring a
reclining chair, or spread a thick blanket over a flat spot of ground. Lie
down and look up somewhat toward the north. You don't need to stare directly
at the radiant -- the meteors can appear anywhere in the sky. Their trails
will tend to point back toward the radiant, pictured as a red dot in the sky
map below. Binoculars and telescopes are not essential. The naked eye is
usually best for seeing meteors which often streak more than 45 degrees
across the sky. The field of view of most binoculars and telescopes is
simply too narrow for good meteor observations.
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