The Atomic Bombings of Hiroshima and Nagasaki | Page 6

and (2) the prevalence of "flash" burns caused by the
instantaneous heat radiation.
B. Mechanical injuries, also remarkable for the wide area in which
suffered.
C. Effects resulting from penetrating gamma radiation. The effects
from radiation were due to instantaneous discharge of radiation at the
moment of explosion and not to persistent radioactivity (of either
fission products or other substances whose radioactivity might have
been induced by proximity to the explosions).
The effects of the atomic bombs on structures and installations were of
two types:
A. Destruction caused by the great pressure from the blast; and
B. Destruction caused by the fires, either started directly by the great
heat radiation, or indirectly through the collapse of buildings, wiring,
etc.
4. The actual tonnage of T.N.T. which would have caused the same
blast damage was approximately of the order of 20,000 tons.
5. In respect to their height of burst, the bombs performed exactly

according to design.
6. The bombs were placed in such positions that they could not have
done more damage from any alternative bursting point in either city.
7. The heights of burst were correctly chosen having regard to the type
of destruction it was desired to cause.
8. The information collected would enable a reasonably accurate
prediction to be made of the blast damage likely to be caused in any
city where an atomic explosion could be effected.

THE SELECTION OF THE TARGET
Some of the most frequent queries concerning the atomic bombs are
those dealing with the selection of the targets and the decision as to
when the bombs would be used.
The approximate date for the first use of the bomb was set in the fall of
1942 after the Army had taken over the direction of and responsibility
for the atomic bomb project. At that time, under the scientific
assumptions which turned out to be correct, the summer of 1945 was
named as the most likely date when sufficient production would have
been achieved to make it possible actually to construct and utilize an
atomic bomb. It was essential before this time to develop the technique
of constructing and detonating the bomb and to make an almost infinite
number of scientific and engineering developments and tests. Between
the fall of 1942 and June 1945, the estimated probabilities of success
had risen from about 60% to above 90%; however, not until July 16,
1945, when the first full-scale test took place in New Mexico, was it
conclusively proven that the theories, calculations, and engineering
were correct and that the bomb would be successful.
The test in New Mexico was held 6 days after sufficient material had
become available for the first bomb. The Hiroshima bomb was ready
awaiting suitable weather on July 31st, and the Nagasaki bomb was
used as soon after the Hiroshima bomb as it was practicable to operate
the second mission.
The work on the actual selection of targets for the atomic bomb was
begun in the spring of 1945. This was done in close cooperation with
the Commanding General, Army Air Forces, and his Headquarters. A
number of experts in various fields assisted in the study. These
included mathematicians, theoretical physicists, experts on the blast

effects of bombs, weather consultants, and various other specialists.
Some of the important considerations were:
A. The range of the aircraft which would carry the bomb.
B. The desirability of visual bombing in order to insure the most
effective use of the bomb.
C. Probable weather conditions in the target areas.
D. Importance of having one primary and two secondary targets for
each mission, so that if weather conditions prohibited bombing the
target there would be at least two alternates.
E. Selection of targets to produce the greatest military effect on the
Japanese people and thereby most effectively shorten the war.
F. The morale effect upon the enemy.
These led in turn to the following:
A. Since the atomic bomb was expected to produce its greatest amount
of damage by primary blast effect, and next greatest by fires, the targets
should contain a large percentage of closely-built frame buildings and
other construction that would be most susceptible to damage by blast
and fire.
B. The maximum blast effect of the bomb was calculated to extend
over an area of approximately 1 mile in radius; therefore the selected
targets should contain a densely built-up area of at least this size.
C. The selected targets should have a high military strategic value.
D. The first target should be relatively untouched by previous bombing,
in order that the effect of a single atomic bomb could be determined.
The weather records showed that for five years there had never been
two successive good visual bombing days over Tokyo, indicating what
might be expected over other targets in the home islands. The