Leonid fireballs dazzle sky-watchers

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The 1998 Leonids: a bust or a blast?

Professionals and amateurs alike were impressed by the fiery show

November 27,
1998: In Hong Kong the night of November 17, 1998 was exceptional.
A cold front had just passed through, skies were clear, and seeing was superb.
One member of the Hong Kong Astronomical Society called it
"the best night in Hong Kong this decade." It couldn't have happened at a better time, as
sky-watchers there and in other parts of the world were treated
to a spellbinding display of Leonid meteors.

Right: This stunning
-20 magnitude fireball exploded around 2025 UT on November 16th over Hong Kong.
It was captured on film by Charanis Chiu of the Hong Kong Astronomical Society.
More photos of this fireball are located at the
Leonids Live! photo gallery.

At its peak the 1998 Leonid meteor shower produced "only" 500
shooting stars per hour, well below the 10's of thousands per hour that
rained down during the 1966 Leonids Storm.

However, what this year lacked in numbers it made
up for in dazzle. An unusually high percentage of Leonid meteors
were "fireballs", shooting stars with magnitudes brighter than -3.
Some were so bright that they cast shadows, and some were seen during daylight hours.
"It's the most spectacular meteor display I have ever seen" said
Brett Gladman, an experienced observer at the Observatoire de Nice where the skies were
clear and 450 meteors/hr were observed.

Above: Many of the 1998 Leonid
meteors were so
bright that they could be
seen in daylit skies, like the one
shown here in an early morning photo
from Hong Hong. This image was
captured by Yan On Sheung at 2200 UT, 16 Nov 1998.

Fireballs are caused by meteoroids that are essentially no different than the
ones responsible for
normal meteors. They are simply bigger (usually larger than a millimeter in size)
and, contrary to intuition,
slower.
Many scientists think that meteoroids which travel very fast
break up into smaller pieces high in the atmosphere
where the air is thin. The tiny fragments burn
up in the thin air without
producing much of a luminous trail. Slower moving
meteoroids penetrate without fragmenting
to deeper, denser
levels of the atmosphere where they become bright meteors, or
even fireballs.

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Leonid meteoroids hit Earth's
atmosphere at about 72 km/s (158,000 mph), much faster than typical meteoroids. If fireballs
are caused by slow-moving bodies, then why were so many fireballs produced
by the fast-moving Leonids? Astronomer Dr. Tony Phillips speculates
that "the debris stream from comet Tempel-Tuttle -- the parent comet of the Leonids --
probably
contains a significant number of larger-than-average meteoroids. So
while the Leonids are fast-moving and probably fragment in the
upper atmosphere, the fragments themselves are large enough to produce
fireballs when they hit the lower atmosphere."

Left: An
exploding meteor photographed by Ben Wong at Sai Kung og Hong Kong on
17/11/98. The local time was approximately 3:30 am. A second meteor
is visible above the fireball.
If a fireball penetrates to the stratosphere,
below an altitude of about 50 km, and explodes,
there is a chance that sonic booms can be heard on the ground below.
Such events are rare, but
two sonic booms from Leonids were reported to
Science@NASA.
Mike Steele of Austin, TX contributed this account:

"[On Nov. 17] at 06:12am CST (partial daylight) I saw a LARGE yellow/red
fireball, which ended in a large flash. A few moments later, perhaps
30-45 seconds afterward, there was a long rumble and retort
from the same direction. The sound was similar to distant
thunder or a sonic boom."

Another reader, Paulo Mansur Raymundo in Brazil, also heard
loud noises apparently caused by a fireball:

"I was happy to witness such a tremendous spectacle.... A bluish one of magnitude -13 produced shadows and
an image of itself reflected on the Atlantic Ocean,
with a train that persisted for over 2 minutes.
Unforgettable... Four sonic booms were heard 8 minutes later
but I'm not certain they [were] related to it.

Mr. Raymundo captured part of the fireball on film, shown right lighting up the
night sky.

Leonids over Brazil

These photos were taken by Paulo Raymundo in Bahia,
Brazil at 0717 UT on November 17th. Although the pictures show blue sky,
the photos were actually taken at night. The first image shows a
-13 magnitude fireball only partially captured on the frame. It
was so bright that the sky was briefly illuminated like daytime.

A second
image 2 minutes later shows the
residual smoke train. The belt and sword of Orion are visible
in the upper right.

Where there's fire there's smoke

According to the American Meteor Society, fireballs
can develop two types of trails behind them: "trains" and "smoke trails".
A train is a glowing trail of hot, ionized gas
that can persist for many minutes, or even as long as a half-hour.
Long-lasting trains, like the ones pictured below,
often change shape over time as they are blown about by
winds 100 kilometers or so above the ground.

exploded over Hong Hong. The photo was taken at about 2030 UT on
November 16th, 1998. The same train was photographed by another
Hong Kong astrophotographer, Ms. Ruby Leung. Her picture is located at the
Leonids Live! photo gallery.

A second type of trail is called a smoke trail.
Smoke trails are composed of non-luminous particles
stripped away from a meteoroid by friction as it flys
through the atmosphere (this process of stripping
is called ablation). They
appear similar to contrails left behind by aircraft,
and are generally seen during the daytime because of the sunlight
they reflect. Although several Leonid fireballs were reportedly seen during
daylight hours, we have not yet received reports or photographs of daytime
smoke trails.

A bust or a blast?

Valentin Grigore of the Romanian Society for Meteors
and Astronomy is an experienced meteor observer who
monitored the Leonid shower from Targoviste, Romania on the night of
16/17 November.
In a 6-hour observing period under dark skies he saw
796 meteors, of which 193 were fireballs. When the shower
peaked in Romania, with 200+ meteors per hour, nearly 40%
of the shooting stars were brighter than magnitude -3. Many were
brighter than Jupiter, Venus, and the full moon.

Some have characterized the 1998 Leonids as a disappointment because
the number of meteors per hour did not approach that of
the 1966 storm, when 40 meteors per second were seen in
some places. The peak of the 1998 Leonids was hardly
greater than an outburst of Giacobinid meteors earlier this year.
Still, many of us like Valentin Grigore, who were favored with dark skies
on the nights of November 16th and 17th, recall the
shower of fireballs as one of the most dazzling sky shows ever.
The International
Meteor Organization has suggested
that the 1998 Leonid shower is similar in some respects to the 1965
shower, which preceded the Great Storm of 1966. If they are
correct, the 1998 Leonids could be the prelude to an even greater
spectacle next year. Or it might be another "bust". The only way to be
sure is to go outside and look!