Editing Little Boy (section)

==Physical effects==
[[File: General Effects of Atomic Bomb on Hiroshima and Nagasaki.ogv|thumb|left|''The General Effects of the Atomic Bombs on Hiroshima and Nagasaki'', a U.S. Air Force film]]

After being selected in April 1945, Hiroshima was spared conventional bombing to serve as a pristine target, where the effects of a nuclear bomb on an undamaged city could be observed.{{sfn|Groves|1962|page=267, "To enable us to assess accurately the effects of the [nuclear] bomb, the targets should not have been previously damaged by air raids." Four cities were chosen, including Hiroshima and Kyoto. War Secretary Stimson vetoed Kyoto, and Nagasaki was substituted. p. 275, "When our target cities were first selected, an order was sent to the Army Air Force in Guam not to bomb them without special authority from the War Department."}} While damage could be studied later, the energy yield of the untested Little Boy design could be determined only at the moment of detonation, using instruments dropped by parachute from a plane flying in formation with the one that dropped the bomb. Radio-transmitted data from these instruments indicated a yield of about 15 kilotons.{{sfn|Malik|1985|page=1}}

Comparing this yield to the observed damage produced a rule of thumb called the {{convert|5|psi|lk=on}} lethal area rule. Approximately all the people inside the area where the shock wave carried such an overpressure or greater would be killed.{{sfn|Glasstone|1962|page=629}} At Hiroshima, that area was {{convert|2.2|mi|km|sp=us}} in diameter.{{sfn|Glasstone|Dolan|1977|page=Nuclear Bomb Effects Computer}}

The damage came from three main effects: blast, fire, and radiation.{{sfn|Glasstone|Dolan|1977|page=1}}

===Blast===
The blast from a nuclear bomb is the result of [[X-ray]]-heated air (the fireball) sending a shock wave or pressure wave in all directions, initially at a velocity greater than the speed of sound,{{sfn|Diacon|1984|page=18}} analogous to thunder generated by lightning. Knowledge about urban blast destruction is based largely on studies of Little Boy at Hiroshima. Nagasaki buildings suffered similar damage at similar distances, but the Nagasaki bomb detonated {{convert|2.0|mi|km|sp=us}} from the city center over hilly terrain that was partially bare of buildings.{{sfn|Glasstone|Dolan|1977|pages=300, 301}}

[[File:House 1953 Nevada Nuclear Test 5 psi.jpg|thumb|Frame house in 1953 nuclear test, 5 [[Pound per square inch|psi]] overpressure]]

In Hiroshima, almost everything within {{convert|1.0|mi|km|sp=us}} of the point directly under the explosion was completely destroyed, except for about 50 heavily reinforced, earthquake-resistant concrete buildings, only the shells of which remained standing. Most were completely gutted, with their windows, doors, sashes, and frames ripped out.{{sfn|The Atomic Bombings of Hiroshima and Nagasaki, 1946|page=14}} The perimeter of severe blast damage approximately followed the {{convert|5|psi}} contour at {{convert|1.1|mi|km|sp=us}}.

Later test explosions of nuclear weapons with houses and other test structures nearby confirmed the 5&nbsp;psi overpressure threshold. Ordinary urban buildings experiencing it were crushed, toppled, or gutted by the force of air pressure. The picture at right shows the effects of a nuclear bomb-generated 5&nbsp;psi pressure wave on a test structure in Nevada in 1953.{{sfn|Glasstone|Dolan|1977|page=179}}

A major effect of this kind of structural damage was that it created fuel for fires that were started simultaneously throughout the severe destruction region.

===Fire===

The first effect of the explosion was blinding light, accompanied by radiant heat from the fireball. The Hiroshima fireball was {{convert|1200|ft|m|sp=us}} in diameter, with a surface temperature of {{convert|6000|C|F|sigfig=1}}, about the same temperature as at the surface of the sun.{{sfn|Nuclear Weapon Thermal Effects|1998}} Near ground zero, everything flammable burst into flame. One famous, anonymous Hiroshima victim, sitting on stone steps {{convert|260|m|ft|order=flip}} from the hypocenter, left a [[Human Shadow Etched in Stone|permanent shadow]], having absorbed the fireball heat that permanently bleached the surrounding stone.{{sfn|Human Shadow Etched in Stone}} Simultaneous fires were started throughout the blast-damaged area by fireball heat and by overturned stoves and furnaces, electrical shorts, etc. Twenty minutes after the detonation, these fires had merged into a [[firestorm]], pulling in surface air from all directions to feed an inferno which consumed everything flammable.{{sfn|Glasstone|Dolan|1977|pages=300–304}}

[[File: Hiroshima Damage Map.png|thumb|Hiroshima blast and fire damage, U.S. Strategic Bombing Survey map]]

The Hiroshima firestorm was roughly {{convert|2.0|mi|km|sp=us}} in diameter, corresponding closely to the severe blast-damage zone. (See the USSBS{{sfn |D'Olier|1946|pp=22–25}} map, right.) Blast-damaged buildings provided fuel for the fire. Structural lumber and furniture were splintered and scattered about. Debris-choked roads obstructed firefighters. Broken gas pipes fueled the fire, and broken water pipes rendered hydrants useless.{{sfn|Glasstone|Dolan|1977|pages=300–304}} At Nagasaki, the fires failed to merge into a single firestorm, and the fire-damaged area was only one-quarter as great as at Hiroshima, due in part to a southwest wind that pushed the fires away from the city.{{sfn|Glasstone|Dolan|1977|page=304}}

As the map shows, the Hiroshima firestorm jumped natural firebreaks (river channels), as well as prepared firebreaks. The spread of fire stopped only when it reached the edge of the blast-damaged area, encountering less available fuel.{{sfn|The Atomic Bombings of Hiroshima and Nagasaki, 1946|pages=21–23}} The Manhattan Project report on Hiroshima estimated that 60% of immediate deaths were caused by fire, but with the caveat that "many persons near the center of explosion suffered fatal injuries from more than one of the bomb effects."{{sfn|The Atomic Bombings of Hiroshima and Nagasaki, 1946|page=21}}

===Radiation===
[[Nuclear fallout|Local fallout]] is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire. With an [[air burst]], the fission products rise into the [[stratosphere]], where they dissipate and become part of the global environment. Because Little Boy was an air burst {{convert|1900|ft|m|order=flip|sp=us}} above the ground, there was no bomb crater and no local radioactive fallout.{{sfn|Glasstone|Dolan|1977|page=409 "An air burst, by definition, is one taking place at such a height above the earth that no appreciable quantities of surface material are taken up into the fireball. ... the deposition of early fallout from an air burst will generally not be significant. An air burst, however, may produce some induced radioactive contamination in the general vicinity of ground zero as a result of neutron capture by elements in the soil." p. 36, "at Hiroshima ... injuries due to fallout were completely absent."}}

However, a burst of intense [[neutron radiation|neutron]] and [[gamma radiation]] came directly from the fission of the uranium. Its lethal radius was approximately {{convert|0.8|mi|km|order=flip|sp=us}},{{sfn|Glasstone|Dolan|1977|pages=Chapter VIII and the 'Nuclear Bomb Effects Computer'}}<ref>{{cite web |url=https://nuclearsecrecy.com/nukemap/?&kt=15&lat=34.39468&lng=132.45462&hob_opt=2&hob_psi=5&hob_ft=1968&psi=20,5,1&zm=13 |title=NUKEMAP |last=Wellerstein |first=Alex |website=nuclearsecrecy.com |publisher=[[Alex Wellerstein]] |access-date=2021-07-28}}</ref> covering about half of the firestorm area. An estimated 30% of immediate fatalities were people who received lethal doses of this direct radiation, but died in the firestorm before their radiation injuries would have become apparent. Over 6,000 people survived the blast and fire, but died of radiation injuries.{{sfn|The Atomic Bombings of Hiroshima and Nagasaki, 1946|page=21}} Among injured survivors, 30% had radiation injuries{{sfn|Glasstone|Dolan|1977|pages=545, 546}} from which they recovered, but with a lifelong increase in [[Radiation-induced cancer|cancer]] risk.{{sfn|Richardson RR 2009}}<ref>{{cite news |url=http://www.radionetherlandsarchives.org/50th-anniversary-of-the-a-bomb-attacks-the-ongoing-research-into-the-effects-of-radiation/ |title=The ongoing research into the effects of radiation |newspaper=Radio Netherlands Archives |date=31 July 2005 |access-date=16 December 2018}}</ref> To date, no radiation-related evidence of heritable diseases has been observed among the survivors' children.{{sfn|Genetic Effects}}{{sfn|Izumi BJC 2003}}{{sfn|Izumi IJC 2003}}

After the surrender of Japan was finalized, Manhattan Project scientists began to immediately survey the city of Hiroshima to better understand the damage, and to communicate with Japanese physicians about radiation effects in particular. The collaboration became the [[Atomic Bomb Casualty Commission]] in 1946, a joint U.S.–Japanese project to track radiation injuries among survivors. In 1975 its work was superseded by the [[Radiation Effects Research Foundation]].<ref>{{cite journal |last=Putnam |first=F. W. |title=The Atomic Bomb Casualty Commission in retrospect|journal=Proceedings of the National Academy of Sciences of the United States of America |date=12 May 1998 |volume=95 |issue=10 |pages=5426–5431 |doi=10.1073/pnas.95.10.5426|pmid=9576898 |pmc=33857 |bibcode=1998PNAS...95.5426P |doi-access=free }}</ref>

In 1962, scientists at Los Alamos created a mockup of Little Boy known as "Project Ichiban" in order to answer some of the unanswered questions about the exact [[dosimetry|radiation output]] of the bomb, which would be useful for setting benchmarks for interpreting the relationship between radiation exposure and later health outcomes. But it failed to clear up all the issues. In 1982, Los Alamos created a replica Little Boy from the original drawings and specifications. This was then tested with enriched uranium but in a safe configuration that would not cause a nuclear explosion. A hydraulic lift was used to move the projectile, and experiments were run to assess neutron emission.{{sfn|Coster-Mullen|2012|pp=86–87}}

===Conventional weapon equivalent===

After hostilities ended, a survey team from the Manhattan Project that included [[William Penney]], Robert Serber, and [[George T. Reynolds]] was sent to Hiroshima to evaluate the effects of the blast. From evaluating the effects on objects and structures, Penney concluded that the yield was 12 ± 1 kilotons.{{sfn|Malik|1985|pp=18–20}} Later calculations based on charring pointed to a yield of 13 to 14 kilotons.{{sfn|Malik|1985|p=21}} In 1953, [[Frederick Reines]] calculated the yield as {{convert|15|ktonTNT}}.{{sfn|Hoddeson|Henriksen|Meade|Westfall|1993|p=393}} Based on the Project Ichiban data, and the pressure-wave data from ''The Great Artiste'', the yield was estimated in the 1960s at 16.6 ± 0.3 kilotons.{{sfn|Malik|1985|p=16}} A review conducted by a scientist at Los Alamos in 1985 concluded, on the basis of existing blast, thermal, and radiological data, and then-current models of weapons effects, that the best estimate of the yield was {{convert|15|ktonTNT}} with an uncertainty of 20% (±3 kt). By comparison, the best value for the Nagasaki bomb was evaluated as {{convert|21|ktonTNT}} with an uncertainty of 10% (±2 kt), the difference in uncertainty owing to having better data on the latter.{{sfn|Malik|1985|p=1}}

To put these numerical differences into context, it is necessary to know that the acute effects of nuclear detonations, especially the blast and thermal effects, do not scale linearly, but generally as a [[cubic root]]. Specifically, the distance of these effects scale as a function of the yield raised to an exponential power of {{frac|1|3}}.{{sfn|Glasstone|Dolan|1977|page=101}} So the range of the {{convert|5|psi}} overpressure damage expected from a detonated 12 kiloton weapon with a height of burst at {{convert|1968|ft}} would be expected to be {{convert|0.98|mi}}, whereas a 20 kiloton weapon would have the same range extend to {{convert|1.12|mi}}, a difference of only {{convert|0.14|mi}}. The areas affected for each would be {{convert|3.02|sqmi}} and {{convert|3.91|sqmi}}, respectively. As such, the practical differences in effects at these yield ranges are smaller than may at first appear, if one assumes that there is a linear relationship between yield and damage.<ref>These calculated numbers come from the [[NUKEMAP]] website, which uses the data and calculations from {{harvnb|Glasstone|Dolan|1977|pages=80-122}}.</ref>

Although Little Boy exploded with the energy equivalent of around 15 kilotons of TNT, in 1946 the [[Strategic Bombing Survey]] estimated that the same blast and fire effect could have been caused by 2.1 kilotons of [[conventional bomb]]s distributed evenly over the same target area: "220 B-29s carrying 1.2 kilotons of [[incendiary bombs]], 400 tons of [[high-explosive]] bombs, and 500 tons of [[anti-personnel weapon|anti-personnel]] [[fragmentation bombs]]."{{sfn|D'Olier|1946|p=24}} Since the target was spread across a two-dimensional plane, the vertical component of a single spherical nuclear explosion was largely wasted. A [[cluster bomb]] pattern of smaller explosions would have been a more energy-efficient match to the target.{{sfn|D'Olier|1946|p=24}}