The Fall of a Meteorite
the vacuum of space, meteorites can travel at speeds of 10 to 70 kilometers per
second. However, the situation changes rapidly when a incoming meteorite reaches the
Earth's atmosphere as air friction exerts tremendous forces on the
meteorite. The transition from vacuum to atmosphere is analogous to doing a
belly flop in a swimming pool!
The survivability of an
incoming meteorite depends on the physical strength of the meteorite, the speed
at which it enters Earth's atmosphere, and on the angle of entry. The chances of
meteorite survival increase as entry speed and entry angle
decrease thereby minimizing the destructive effects of atmospheric drag.
At 50-100 km of altitude,
atmospheric drag brings the meteorite and surrounding air to luminosity. The
resulting "fireball" can be brighter than the moon and momentarily
into day. This frictional heating initiates melting of the meteorite's outer surface.
In contrast the interior of the meteorite remains cold because meteorites have low thermal conductivities.
The thickness of Earth's atmosphere increases toward the
ground, and at 5-30 km the higher drag causes meteorites to decelerate to 3 - 8 km/s. At these speeds, the meteoric light extinguishes and
the falling meteorite enters into a phase known as "dark flight". The meteorite
continues to decelerate and ultimately assumes the behavior of a free-falling
object. At this point, the meteorite has lost much of its forward speed and
falls nearly vertically to the ground at speeds of 125-250 m/s.
The visual phenomena
of a bright fireball is often followed by sonic booms, similar the arrival of
thunder after a lightening strike. The booms are the result of shock waves that
build up in front of the fast-moving meteorite. Observers near the impact site
often report a whistle or buzzing sound produced during the free-falling
meteorite's final descent.
FIREBALL AT DRAKE CREEK, TENNESSEE (MAY
"On Wednesday the 9th about 4 o'clock P.M. the day being clear
as usual, my son and servants were planting corn in the field, they heard a
report similar to that of a cannon, which was continued in the air resembling
the firing of cannon or muskets by platoons, and the beating of drums as in a
battle. Some small clouds with a trail of black smoke, made a terrific
appearance, and from them, without doubt, came a number of stones with a loud
whizzing noise, which struck the earth with a sound like that of a ponderous
One of these stones my son heard fall about fifty yards from where he was.
In its decent to the ground it struck a paupau tree of the size of a small hand
spike, and tore it to pieces as lightening would have done. Guided by the tree,
he immediately found the spot, and there he found the stone about eight or ten
inches under the ground; this stone weighted five pounds and a quarter. The
stone was cold but had the scent of sulphur. These stones are glazed in a think
black crust and bear marks of having been through a body of fire and black
often break into smaller fragments during atmospheric flight. After
fragmentation, the smallest meteorite pieces are rapidly slowed by wind
resistance and quickly lose their forward speed. In contrast, the inertia of
larger pieces propels them father downstream. The fragments ultimately hit
ground in an elliptical pattern known as a strewnfield. This natural sorting of stones
has been documented with numerous meteorite falls.
Spots: Perceptions vs. Reality.
falls are spectacular events, but they can also be
deceiving. It impossible for a single observer to accurately determine the
size, distance, angle of trajectory, and impact location of a fireball. It
is not unusual for people to insist that the fireball was a "big as a car", or landed "just over the
hill". After a major fireball, reports from different observers often conflict
with one another.
a fireball that seemed to strike ground "just over the hill" has
disappeared from an observer's view by passing over the horizon. The resulting meteorites,
if any survived, may be hundreds of miles away.
On the other
hand, sometimes fireballs seem to "burn up" or "extinguish" directly overhead. In reality, the meteor may have entered into the "dark
flight" stage of its decent. There is a chance that meteorites may
have landed nearby!