Fireballs Flashing In Our Sky
+ Von Del Chamberlain +
November 25, 1998
We have been hearing much of late about meteors resulting from comets,
those cosmic vagabond litterbugs from the suburbs of our planetary city
that come speeding into downtown solar system scattering primordial sand
and gravel in their wakes. The Leonid Meteor Shower did not live up to
expectations in one way, but exceeded them in another. The density of
meteors was not as great as hoped, but the number of splendid fireballs was
a pleasant surprise. And you did not need to go to Asia to see them.
A fireball is a spectacular meteor, brighter than any of the stars
or planets. They vary in brilliance from something like a flashlight aimed
at you out of distant darkness, through the range of automobile headlights
on high beam approaching on the highway, to dazzling fireworks bursting in
the sky. Some illuminate the night-time landscape. Occasionally they can
even be seen as resplendent lights burning through clear sky on sunny days.
The meteors that are typical of most meteor showers, commonly
referred to as shooting or falling stars, are caused by particles the sizes
of sand grains and pellets with an occasional marble thrown into the batch.
They are beautiful to witness, especially if enough of them are seen to
hold you out at night with upturned eyes. Fireballs are produced by chunks
ranging from marbles, golf balls, baseballs, footballs and basketballs, on
up to giant boulders and beyond. All of these come streaking into the
blanket of air we are snuggled in at speeds of many miles each second--a
typical velocity is 40 miles per second. They impact each molecule of air
that gets in their way. The reactions are furious, a rapid exchange of
energy of motion into heat and light. It is primarily the glowing air we
see. Thus the term meteor, literally meaning "of the air," is appropriate.
The objects plunging from space into the atmosphere are called meteoroids.
A meteoroid the size of a football might produce a moving glowing plasma
of radiant air the size of a football stadium, engulfing the grandstands as
well as the playing field.
Even the fireballs we saw as part of the Leonid shower were made by
rather small clumps of matter. The fact that there were lots of pieces
large enough to produce modest fireballs tells us that we were encountering
debris recently broken away from Comet Tempel-Tuttle. Most of the comet
material was likely frozen gasses which would continue to vaporize and
subdivide into smaller pieces as time went on. This is encouraging for
meteor shower watchers, creating renewed hope that we might see a truly
great shower next November when Earth carries us once more through the
pathway the comet took as it rounded the Sun. So, if you enjoyed this
years Leonid shower--or if you missed it--you might begin to make plans now
for next November. The experts are predicting that it might best be
observed from eastern Europe. If you do plan a trip into that part of the
world for next November, be sure to include other reasons for your
destinations, remembering that we cannot accurately predict exactly when
the shower will be most intense, how intense it will be, and, of course,
the weather that you might find yourself under at the appointed time. The
1999 Leonid shower can be a good excuse to travel, if you are looking for
Fireballs are produced from two different sources. Comets are one
source. Asteroids are the other. Comets are made mostly of frozen water
and gases with bits of rocky material imbedded in the ices. Asteroids, on
the other hand, are boulder and mountain sized objects made of rock and
metal. Most comets move in great elongated orbits carrying them from the
frigid fringes of the solar system into the fierce heat of the Sun.
Asteroids move in orbits well within the zone of planets. Most, if not
all, meteoroids from comets are vaporized in their passage through the
atmosphere, some of the material eventually filtering down as fine dust.
Good-sized asteroidal meteoroids are more likely to deposit recognizable
pieces on the ground.
A truly spectacular fireball is one of Nature's grandest
spectacles. You never know when you might see one when out at night
looking around. When a large meteoroid crashes into the atmosphere it
begins to glow about 100 miles up. Penetrating deeper, it rapidly grows in
brilliance. A mass of air is pushed in front of it, building pressure that
slows the meteoroid and often shatters it into many pieces. Sonic booms
are produced by every piece moving beyond the speed of sound. Ceasing to
glow while many miles above ground, the fragments drop as dark objects to
scatter over the landscape.
An observer on the ground sees the fireball blazing across the sky,
flashing brighter at times, perhaps bursting like a roman candle, then
disappearing. Beneath the trajectory, explosive sounds are heard a few
minutes later, and in the area where pieces rain down they might be seen
and heard buzzing through the air and thumping onto the ground. When
pieces of these cosmic projectiles are found they are called meteorites,
and that is the topic I want to explore with readers in my next column.