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showers” on schedule every year are 
a wholly different phenomenon.) The 
single exception is the August 10, 
1972, meteoroid, the only one whose 
future whereabouts can be calculated. 
The reports of sonic booms are what 
suggested to Jacchia that the mete- 
oroid had continued on into space. 
Near the middle of the observed flight 
path, Montana viewers heard the sonic 
boom, although those who saw the 
fireball farther to the north over Can- 
ada heard nothing. If the meteoroid 
had continued to fall to lower alti- 
tudes, it would have “lowered the 
boom,” producing louder sounds than 
those heard in Montana. But a sonic 
boom generated at heights above 
thirty-seven miles will reflect upward 
from the atmosphere below the gen- 
erating source, thereby preventing any 
sound from being detected at ground 
level. For this reason, Jacchia con- 
cluded that the meteoroid must have 
been lower than thirty-seven miles 
while over Montana, but higher than 
that over Alberta, where it must have 
been on the way up, not down. This 
interpretation was confirmed when 
precise measurements of the meteor- 
oid’s altitude became available from 
an unexpected source, making it pos- 
sible to determine the orbit of the 
space visitor. 
The unexpected source of fireball- 
tracking data was an artificial satellite 
orbiting the earth, which actually 
viewed the fireball from above. Details 
of the satellite findings were disclosed 
in 1974 by a group of physicists from 
the Aerospace Corporation and the 
Aerojet ElectroSystems Corporation, 
two military-contractor organizations 
in southern California. According to 
their report in the February 15, 1974, 
issue of the periodical Nature, infra- 
red radiation was recorded by the sat- 
ellite when “the meteor first became 
sufficiently hot to be detected over 
Utah,” and the measurements contin- 
ued for about 100 seconds until the 
fireball “finally cooled below the de- 
tection threshold,” 930 miles to the 
north. During this time, the meteoroid 
traveled along a shallow, curved tra- 
jectory at speeds in excess of 31,000 
miles per hour. By the physicists’ reck- 
oning, it carried an energy of “ap- 
proximately the yield of the nuclear 
weapons [that] destroyed Hiroshima 
and Nagasaki.” Analysis of the sat- 
ellite measurements also showed that 
the low point in the meteoroid’s flight 
was 35.9 miles above the ground in 
east-central Idaho, a few miles south 
of the Montana border, confirming 
Jacchia’s deduction. 
The meteoroid energy referred to 
above is not the chemical or nuclear 
energy of a bomb but rather what 
is called the kinetic energy of a rapidly 
moving, massive object. When this ki- 
netic energy is released by the object 
on impact, however, it can have an 
explosive effect. The best illustration 
of this phenomenon is Meteor Crater 
(which should, of course, be called 
Meteorite Crater), a 3 /4-mile-wide scar 
in the northern Arizona plateau. Me- 
teor Crater is at least 30,000 years 
old, and most of the large impact 
craters on the earth are even older, 
indicating that major meteoroid im- 
pacts are now very rare. 
There has been much controversy 
over the size and mass of the August 
1972 meteoroid. One estimate is that 
it was a stone of about a million tons, 
with a diameter of 260 feet. At the 
other extreme is a calculation sug- 
gesting that the object may actually 
have been smaller than the stone me- 
teorite that fell harmlessly in pieces 
near Jiling (Kirin) in the People’s Re- 
public of China in March 1976. The 
largest fragment of the Jiling mete- 
orite weighs 3,900 pounds. 
The disagreement over the August 
1972 meteoroid has several roots: mis- 
prints in tracking data, which may 
have affected some scientists’ calcu- 
lations; differences in computing 
methods; and uncertainties about the 
brightness of the fireball and the com- 
position of the meteoroid. Nature gave 
the results of the satellite measure- 
ments, in terms of the speed and al- 
titude of the meteoroid, for only three 
locations along the fireball path, but 
it turns out that the longitude of one 
location and the latitude of two were 
listed incorrectly. 
The methods that various research- 
ers used to compute the meteoroid’s 
mass involved deductions from its 
infrared radiation, from its apparent 
brightness, and from the rate at which 
the flying object was decelerated by 
air resistance. ,The conclusions 
reached about the mass and the size 
of the meteoroid, regardless of the 
computation methods, depend on 
whether the meteoroid was made of 
stone, of iron with a high niekel con- 
tent, or of a combination of stone and 
iron. These substances have very dif- 
ferent densities. The actual density 
of the August 1972 meteoroid will 
affect the results of the computations 
made by any method, but no one 
100 
