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compiled by James Butler from articles
listed in references
Each year thousands of tonnes of
extraterrestrial material rains down on Earth. Most is in the form of
dust or sand-sized particles which burn up harmlessly in the atmosphere.
You may see evidence of this destruction as brief streaks of light in
the night sky. Most people know them as shooting stars. These streaks
are actually meteoric dust particles shooting through the upper
atmosphere and glowing with the heat created by this rapid passage. Most
of these meteors are no brighter than a dim star.
Slightly larger
fist-size pieces of material cause a much brighter and spectacular
shooting star. These are known as fireballs and can be startling, not
only because of their brilliance but also because of their vivid colour.
Fireballs can be green, yellow, orange, blue, red, as well as brilliant
white. The light of a fireball comes from the glowing gas surrounding
the solid meteoroid, rather than from the meteoroid itself. Each element
glows with its characteristic set of colours. The eerie green light so
common in many fireballs comes from atmospheric oxygen. Nitrogen from
our atmosphere contributes blue, and sodium from the meteoroid can add
yellow to the light. Magnesium, iron, calcium, and dozens of other
elements add their own colours.
Rapidly moving fireballs will
tend to be white (a mixture of colours from many vaporized elements)
whereas slower movers will appear to be red. Many fireballs change
colour as they descend and slow down. A fireball that glows as bright
as Venus, the bright evening star, may come from a pebble of iron or
stone weighing barely an ounce. Brighter fireballs (about as bright as
the moon) requires an object weighing about 2 to 5 kilograms.
Astronomers estimate that a fireball must be somewhere between those two
extremes if a piece will survive to reach the surface.
An
exploding fireball is technically called a bolide, from the Greek
bolides, meaning a thrown spear. When fireballs explode near the end of
their flight, the meteoric debris from the fall can scatter in hundreds
of fragments spread over a wide area. The fragments of a fireball that
reach Earth, called meteorites, are the best sources of pristine, or
pure, interplanetary material. The meteorites that result from fireballs
are probably bits of asteroids, from the asteroid belt that orbits the
Sun between Mars and Jupiter. Freshly fallen meteorites are of great
scientific value. Scientists have found ancient meteorites in abundance
on the Antarctic ice, where they are relatively easy to spot on the
snowy-white background.
A spectacular fireball may be seen from
an area of 250,000 square kilometres or more, but locating a meteorite
is rarely easy. When it becomes visible as a fireball it is still tens
of kilometres from the surface of Earth. Cases have been reported where
airline pilots have veered their planes off course to avoid a mid-air
collision with a fireball, only to find, from research later, that the
fireball was 80 to 150 kilometres away and perhaps 30 kilometres higher
than the aircraft. A typical fireball first appears at a height of about
130 kilometres above Earth, and usually extinguishes at a height of
about 20 kilometres.
It is at the end of the visible fireball
path that you sometimes see explosions that result in the fireball
breaking up and burning out. The altitude of the fireball at this point
can vary considerably. If this altitude can be determined, it can help
indicate the mass and composition of the incoming meteorite material.
Fireballs that manage to penetrate as far down as 20 to 25 kilometres
above Earth's surface are most likely to drop meteorites on the surface.
Researchers estimate that the time for a meteorite to reach the ground
from a height of 20 kilometres is about 3 to 4 minutes. During that
time, the fragments become dark and invisible on their way to the
surface. The distance between the burn-out point and the impact area
depends on a fragment's mass, velocity, and the angle of its fall.
Scientists estimate that in order for the meteorite to survive
passage through the atmosphere, its velocity when entering the
atmosphere should not exceed 20 miles per second (80,000 kilometres per
hour). The object's material must also be strong enough to survive
without breaking up or losing a lot of its material by vaporization
during the flight through the atmosphere. Some meteoroids estimated to
have been very large have disintegrated completely. Often all that is
left of a bright fireball is a great deal of dust. Many spectacular
fireballs are very fragile, comet-like "dust-balls". On December 4,
1974, the brightest fireball ever recorded appeared in the skies over
Sumava, Czechoslovakia. Its brilliance rivalled the Sun, yet no
fragments were recovered.
Newfoundland has had its moments of
spectacular fireballs, too. On January 19, 1986, at approximately 7:40
pm on a Sunday evening, a brilliant fireball lit up the clear night sky
over central Newfoundland. It was seen from as far away as Cape Breton
Island and southern Labrador. At Buchans, in central Newfoundland, the
fireball was bright enough to shut off the automatic street lights.
There were 43 eye witnesses who came forward to report their
observations to scientists at Memorial University and the provincial
Mines and Energy branch. It appears that the fireball was visible
for about 5 seconds, crossing the sky in a south-southeast direction.
During its visible stage, it broke into at least three major fragments.
When the information from the observations was plotted on a map, it
seemed as if one fragment had fallen in the Lake Ambrose area of central
Newfoundland, a second travelled further south to the Meelpaeg Lake area.
The third fragment was seen to explode before impact.
A few days
later, reports came to Mines and Energy that two
hunters had discovered an unusual disturbed area in the vicinity of Lake
Ahwachanjeesh, north of St. Alban's, Bay d'Espoir. Investigation of this
report showed that a hole about 7 centimetres in diameter had been made
through the base of a tree and there was a trench leading from it about
3 to 4 metres long down an embankment to the edge of the pond. The
trench was about half a metre wide and a few centimetres deep. A natural
boulder barricade around the pond was displaced and mud, rocks and
vegetation had been thrown forward onto the frozen lake for a distance
of about 200 metres. No meteorite fragments were recovered.
Another phenomenon of the night sky that is a little more predictable
than fireballs is meteorite showers. Most meteorite showers occur yearly,
on a fairly regular schedule, when Earth's orbit crosses a trail of
debris left in the orbit of a comet. This fine-grained material enters
the atmosphere resulting in a streak of light in the night sky. These
showers are named for the constellation from which they seem to
originate. There are four such showers that have an average count of 50
or more meteorites per hour. The best known is the Perseids. It occurs
in August each year, with August 11/12 as the nights when observers will
see the most meteors. Listed below are the four showers and the peak
dates for each.
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Name |
Peak |
Period |
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Quadrantids |
January 3 |
January 1 - 4 |
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Perseids |
August 12 |
July 25 - August 17 |
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Phoenicids |
December 5 |
December 1 - 15 |
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Geminids |
December 13 |
December 7 - 15 |
For those with an interest in astronomy, or just a natural curiosity for
the unusual the night sky can offer some spectacular sites and
interesting adventures. Keep your eyes skyward!
References
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Christopher E. Spratt
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It Came From Outer Space, Astronomy,
volume 19, no. 2, pp 65-69, February, 1991.
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Gibbons, Rex
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Howley's Comet: The Search for a Newfoundland Fireball, Dialogue,
newsletter of the Newfoundland Association of Engineers, pp 4, June,
1986.
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Squires, G., & Gibbons, R.
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The Newfoundland Fireball of January 19, 1986, with some comments on
possible shatter cones of Late Precambrian age, St. John's, Nfld., in
Geological Association of Canada - Mineralogical Association of Canada -
Canadian Society of Petroleum Geologists, Program with Abstracts,
volume 13, page A115, 1988.
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