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The immense destructive power of atomic weapons derives from a sudden release of energy produced by splitting the nuclei of the fissile elements making up the bombs' core. The U.S. developed two types of atomic bombs during the Second World War. The first, Little Boy, was a gun-type weapon with a uranium core. Little Boy was dropped on Hiroshima. The second weapon, dropped on Nagasaki, was called Fat Man and was an implosion-type device with a plutonium core.

Fission

The isotopes uranium-235 and plutonium-239 were selected by the atomic scientists because they readily undergo fission. Fission occurs when a neutron strikes the nucleus of either isotope, splitting the nucleus into fragments and releasing a tremendous amount of energy. The fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. This is known as a chain reaction and is what causes an atomic explosion.

When a uranium-235 atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium-238 atom. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, and so on. This causes a nuclear chain reaction. For more on this topic, see Nuclear Fission.

Criticality

In order to detonate an atomic weapon, you need a critical mass of fissionable material. This means you need enough U-235 or Pu-239 to ensure that neutrons released by fission will strike another nucleus, thus producing a chain reaction. The more fissionable material you have, the greater the odds that such an event will occur. Critical mass is defined as the amount of material at which a neutron produced by a fission process will, on average, create another fission event.

The Difference Between the Bombs

Little Boy and Fat Man utilized different elements and completely separate methods of construction in order to function as nuclear weapons. Little Boy detonated due to a fission chain reaction involving the isotope U-235 of uranium, while Fat Man used plutonium's Pu-239 form.

Little Boy

Little Boy was powered by the uranium isotope U-235 in a process that didn't come easily to the many Manhattan Project scientists working on the uranium extraction and enrichment process. Most uranium found naturally in the world exists as uranium-238, leaving only 0.7% of naturally existing uranium as the U-235 isotope. When a neutron bombards U-238, the isotope often captures the neutron to become U-239, failing to fission, and thus failing to instigate a chain reaction that would detonate a bomb. The first challenge of the project was thus to determine the most efficient way to separate and purify uranium-235 from the overly-abundant uranium-238 - standard methods of separation could not be used due to the strong chemical similarity between the two isotopes. In order to avoid wasting time on one new method that could later prove insufficient to produce enough U-235 to allow the atomic bomb to reach critical mass, General Leslie Groves consulted with lead scientists of the project and agreed to investigate simultaneously four separate methods of separating and purifying the uranium-235: gaseous diffusion, centrifuge, electromagnetic separation and liquid thermal diffusion.

Once enough U-235 was obtained to power the bomb, Little Boy was constructed using a gun-type design that fired one amount of U-235 at another to combine the two masses. This combination created a critical mass that set off a fission chain reaction to eventually detonate the bomb. The two masses of U-235 had to combine with one another quickly enough to avoid the spontaneous fission of the atoms, which would cause the bomb to fizzle, and thus fail to explode.

Fat Man

Powered by plutonium, Fat Man could not use the same gun-type design that allowed Little Boy to explode effectively - the form of plutonium collected from the nuclear reactors at Hanford, WA for the bomb would not allow for this strategy. The Hanford plutonium emerged from the reactors less pure than the initial plutonium extracted from Ernest O. Lawrence's Berkeley Lab, instead containing traces of isotope plutonium-240, as opposed to the desired plutonium-239. Plutonium-240's higher fission rate would cause the atoms to undergo spontaneous fission before the gun-type design could bring two masses of plutonium together, which would lower the energy involved in the actual detonation of the bomb.

Thus, a new design was required. Physicist Seth Neddermeyer at Los Alamos constructed a design for the plutonium bomb that used conventional explosives around a central plutonium mass to quickly squeeze and consolidate the plutonium, increasing the pressure and density of the substance. An increased density allowed the plutonium to reach its critical mass, firing neutrons and allowing the fission chain reaction to proceed. To detonate the bomb, the explosives were ignited, releasing a shock wave that compressed the inner plutonium and led to its explosion.

Atom: building blocks of matter; made up of a small, dense nucleus surrounded by a cloud of electrons (negatively-charged particles)

Nucleus: makes up the center of the atom; consists of a number of positively-charged protons and a neutral (no charge) neutrons. An atom is classified by the number of protons and neutrons in its nucleus. The number of protons determines which chemical element the atom is (e.g. uranium), and the number of neutrons determines which isotope of that element the atom is (e.g. uranium-235).

Isotope: Isotopes of an element possess the same number of protons in their nuclei but have different numbers of neutrons.

Fission: the process by which an atom's nucleus is split into smaller particles; results in the release of neutrons and lots of energy.

E=mc²: Equation made famous by Albert Einstein. Explains how a tiny amount of matter contains a tremendous amount of energy.

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The immense destructive power of atomic weapons derives from a sudden release of energy produced by splitting the nuclei of the fissile elements making up the bombs' core. The U.S. developed two types of atomic bombs during the Second World War. The first, Little Boy, was a gun-type weapon with a uranium core.

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In 1939, the U.S. discovered that the Nazis were developing a weapon of unprecedented destructive power and began a top-secret project led by Robert Oppenheimer to beat them to it: The Manhatten Project.

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The Ivy King and the Fat Man were different types of atomic bombs. The Ivy King was the largest pure-fission nuclear bomb ever tested by the United States. It was part of operation Ivy.

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The truth about the atom bombs and the defense against them

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The truth about the atom bombs and the defense against them

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Aerial view of an atomic bomb explosion.

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Aerial view of an atomic bomb explosion.

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‪First Milliseconds of Nuclear Bomb Test Fireball‬

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‪First Milliseconds of Nuclear Bomb Test Fireball‬

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‪First Milliseconds of Nuclear Bomb Test Fireball‬

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I had uploaded a shorter version of this video earlier and deleted it. That video resurfaced on another YouTube-account. Someone appears to have downloaded it and uploaded it by himself: http://www.youtube.com/watch?v=Rcyzb9P4KX4 The first two are the Mohawk shot from Operation Redwing, 1956 (0:05) and Operation Snapper, 1952 (0:15).

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21 Facts about the atomic bombs that were dropped on Japan

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21 Facts about the atomic bombs that were dropped on Japan

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Seventy years later, the U.S. atomic bomb attacks on Hiroshima and Nagasaki remain, thankfully, the only time nuclear weapons have been used in active warfare. Some of what happened will amaze you, including the man who survived both attacks.

20. The 1st Target Was Decided an Hour Before the Drop

The good weather conditions over Hiroshima sealed the city's fate.That was determined by a weather plane that buzzed over Hiroshima. On the ground, a yellow alert rings out for 22 minutes. Many civilians ignore it, unperturbed by the familiar sight of a single B-29 plane flying over the city. The weather plane sends a coded message to Enola Gay, advising that Hiroshima is to be the primary target. Tibbets notifies his crew over the intercom and the plane sets course.

9. This Man Survived Both Bomb Attacks

Tsutomu Yamaguchi was a 39-year-old businessman who lived in Nagasaki. Yamaguchi was in Hiroshima on business for his employer Mitsubishi Heavy Industries when the city was bombed at 8:15 am, on August 6, 1945. The explosion ruptured his eardrums, blinded him temporarily, and left him with serious burns over the left side of the top half of his body.

He returned to Nagasaki the following day, and despite his wounds, he returned to work on August 9, the day of the second atomic bombing. That morning he was telling his supervisor how one bomb had destroyed the city, to which his supervisor told him that he was crazy, and at that moment the Nagasaki bomb detonated. He was not injured in that explosion.

Yamaguchi died of stomach cancer on January 4, 2010, at the age of 93.

8. The Father of the Bomb Campaigned Against Nuclear Proliferation

J. Robert Oppenheimer, the key figure of the Manhattan Project, which developed the bomb in the New Mexico desert, said the detonation of the bombs reminded him of words from the Bhagavad Gita: "Now I am become Death, the destroyer of worlds." After the war he became a chief advisor to the newly created United States Atomic Energy Commission and used that position to lobby for international control of nuclear power to avert nuclear proliferation and an arms race with the Soviet Union. After provoking the ire of many politicians with his outspoken opinions during the Second Red Scare, he had his security clearance revoked. He continued his anti-nuclear work until his death in 1967 of throat cancer at age 62.

6. Emperor Hirohito's Surrender on Radio Was the 1st Time His Voice Was Heard Publicly

That's right, in power since 1926, Hirohito allowed a recording of the his surrender speech to be broadcast over the radio on August 15, 1945 (the first time the Emperor was heard on the radio by the Japanese people). He spoke in classical Japanese, making it difficult for some citizens to fully comprehend what he is saying. Despite two devastating attacks on the country, many people are shocked - the Japanese Empire has maintained it would be more noble to endure annihilation than surrender to the enemy. Japan's war minister had attempted suicide and dies the following day. Many thought the emperor would order the mass suicide of all citizens rather than surrender. He did not.

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21 Surprising Facts About the Atomic Bomb Attacks on Japan

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Seventy years later, the U.S. atomic bomb attacks on Hiroshima and Nagasaki remain, thankfully, the only time nuclear weapons have been used in active warfare. Some of what happened will amaze you, including the man who survived both attacks. The good weather conditions over Hiroshima sealed the city's fate.That was determined by a weather plane that buzzed over Hiroshima.

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The Security and Secrecy of the Manhattan project

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The Security and Secrecy of the Manhattan project

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A key component of keeping the Manhattan Project secret was making sure Project sites were secret and secure. One obvious reason the Manhattan Engineers District selected Los Alamos, NM, Oak Ridge, TN, and Hanford, WA as project sites was their geographic isolation. Still, District officials took extraordinary measures to ensure that no one without the proper clearance was allowed access to site buildings or facilities. Each site had multiple security checkpoints that were guarded by military police twenty-four hours a day, seven days a week. In addition, tall barbed-wire fencing surrounded each site's perimeter, preventing any intruders from gaining unwanted access to key buildings and deterring any employee from clandestinely sneaking out (perhaps with classified documents).

Security

Each worker at the Manhattan Project underwent a rigorous background check conducted by the FBI to ensure that he or she had no criminal history or suspicious connections with Axis sympathizers. It could often take several weeks for prospective employees to get cleared, and sometimes the FBI would contact close relatives or previous employers to clarify any suspicions that might be raised during the process of their investigation. Every employee that worked at one of the Manhattan Project sites had a security badge that displayed their picture, their job position, and their level of clearance. An employee's job usually determined their level of clearance. Construction workers, low-level engineers, and metallurgical workers usually had low-level clearance, which meant their work was highly compartmentalized and they were informed on a "need-to-know" basis. These workers were generally given red or blue badges, depending on their level. Top-level physicists, engineers, and important scientists usually had the highest clearance, and generally knew that the overall goal of the Manhattan Project was to build an atomic bomb. These workers were given white badges, which entitled them (at least those at Los Alamos) to sit in on lectures that Oppenheimer frequently delivered to discuss the project's progress and address some of the key problems scientists faced.

In addition, mail coming in and going out of Los Alamos was meticulously censored. Security officials would inspect each and every individual piece of mail, making sure that any information regarding a site's location, work activity, or technical details was removed. In some cases, military officials would follow up with civilians whose letters contained something unusually suspicious, such as coded puzzles or words. Richard Feynman, a brilliant theoretical physicist who worked at Los Alamos in Hans Bethe's theoretical group, liked to send his wife coded puzzles in the mail. Security officials immediately began questioning him about the nature of the puzzles and requested that he discontinue using coded words in his letters home. Feynman, of course, would find other ways to annoy Los Alamos' security guards. In many cases, parents simply wanted to know what their children were working on and why they had to be so secretive in their letters. Mail censorship frustrated everyone, but many Manhattan Project veterans would later argue that it was essential for secrecy and security purposes.

Every District worker, whether scientist or construction worker, had to sign a form pledging silence about the Manhattan Project. But for General Leslie Groves, forms were not enough. The MED's security and intelligence agency accepted Groves' fear that randomly dropped phrases, bits of knowledge, and names or places might fall into the hands of some centralized enemy spy network, where it might then enable reconstruction of the project. To counter such a nebulous possibility required vast expansion of the MED's controls on the way people talked and wrote. As a result, the District would undertake a massive "propaganda" campaign to remind workers of proper attitude toward language and its most dangerous manifestation, "loose talk."

Secrecy

The overriding concern of General Leslie R. Groves in managing the Manhattan Project was secrecy. Anyone who entered the grounds of the Los Alamos laboratory or one of the other "secret cities" had to have a purpose and a pass. At all the sites, signs and billboards admonished workers to protect the project's secrets: "What you see here, what you do here, what you hear here, when you leave here, let it stay here!"

General Groves was the architect of an intelligence revolution that took security measures to unprecedented heights. Congressional leaders agreed to secret budget processes with no legislative oversight. Groves created separate organizations to carry out intelligence, counterintelligence, and surveillance programs both domestically and overseas. These operated outside of regular military channels, kept separate records, and reported directly to him.

As the only person knowledgeable about the entire project, Groves stood at the pinnacle of power. He controlled the project's pace, priorities, and direction through his decisions. No one could travel from one site to another without the general's permission. Knowledge was compartmentalized. Workers were told only what they needed to know and were forbidden to discuss their jobs with anyone other than designated supervisors.

Scientists, used to the free exchange of ideas, rebelled against the compartmentalization. At Los Alamos, Oppenheimer insisted that weekly scientific colloquia and other exchanges were essential to solve difficult problems. But this openness among the top echelon of scientists at Los Alamos was an exception and was contained "inside the fence." For everyone else, it was "Stick to your knitting!"

Secrecy Unveiled: The Manhattan Project Goes Public

News of the detonation of an atomic bomb over Hiroshima on August 6th, 1945 stunned the world. Never before had a weapon of such destructive proportions been used upon an enemy population with such devastating effect. The idea that a single, nuclear bomb could put an end to the greatest conflict the world has ever known ushered in a new era of modern warfare and completely changed the course of history forever.

Naturally, many ordinary Americans were curious. Newspapers across the United States and the world were filled with stories that speculated on the potential of this new, "atomic" technology. Many wondered how the United States had come to develop the world's first nuclear weapon. How long did it take the government to develop this technology? Why was it kept secret? Who worked on the project? Where were the bombs built?

Gradually, reports of a $2 billion U.S. government war project, codenamed "Manhattan Project", were released. In anticipation of the bombings of Japan, General Leslie Groves had Henry DeWolf Smyth prepare a report that was to be the official U.S. government history and statement about the development of the atomic bombs. The report, titled A General Account of the Development of Methods of Using Atomic Energy for Military Purposes was released to the public on August 12, 1945. The report also outlined the development of the then-secret laboratories and production sites at Los Alamos, NM, Oak Ridge, TN, and Hanford, WA, and the basic physical processes responsible for the functioning of nuclear weapons, in particular nuclear fission and the nuclear chain reaction.

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A key component of keeping the Manhattan Project secret was making sure Project sites were secret and secure. One obvious reason the Manhattan Engineers District selected Los Alamos, NM, Oak Ridge, TN, and Hanford, WA as project sites was their geographic isolation.

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The bombs that were dropped on Hiroshima and Nagasaki

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The bombs that were dropped on Hiroshima and Nagasaki

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Real footage of bombing of Hiroshima and Nagasaki

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Real footage of bombing of Hiroshima and Nagasaki

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Real Footage - Atomic Bomb- Hiroshima and Nagasaki

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After the Interim Committee decided to drop the bomb, the Target Committee determined the locations to be hit, and President Truman issued the Potsdam Proclamation as Japan's final warning, the world soon learned the meaning of "complete and utter destruction." The first two atomic bombs ever used were dropped on Japan in early August, 1945.

For a detailed timeline of the bombings, please see Hiroshima and Nagasaki Bombing Timeline.

Hiroshima

On August 6, 1945, the United States dropped an atomic bomb on the city of Hiroshima. The bomb was known as "Little Boy", a uranium gun-type bomb that exploded with about thirteen kilotons of force. At the time of the bombing, Hiroshima was home to 280,000-290,000 civilians as well as 43,000 soldiers. Between 90,000 and 166,000 people are believed to have died from the bomb in the four-month period following the explosion. The U.S. Department of Energy has estimated that after five years there were perhaps 200,000 or more fatalities as a result of the bombing, while the city of Hiroshima has estimated that 237,000 people were killed directly or indirectly by the bomb's effects, including burns, radiation sickness, and cancer.

The bombing of Hiroshima, codenamed Operation Centerboard I, was approved by Curtis LeMay on August 4, 1945. The B-29 plane that carried Little Boy from Tinian Island in the western Pacific to Hiroshima was known as the Enola Gay, after pilot Paul Tibbets' mother. Along with Tibbets, copilot Robert Lewis, bombardier Tom Ferebee, navigator Theodore Van Kirk, and tail gunner Robert Caron were among the others on board the Enola Gay. Below are their eyewitness accounts of the first atomic bomb dropped on Japan.

Pilot Paul Tibbets: "We turned back to look at Hiroshima. The city was hidden by that awful cloud... boiling up, mushrooming, terrible and incredibly tall. No one spoke for a moment; then everyone was talking. I remember (copilot Robert) Lewis pounding my shoulder, saying 'Look at that! Look at that! Look at that!' (Bombardier) Tom Ferebee wondered about whether radioactivity would make us all sterile. Lewis said he could taste atomic fission. He said it tasted like lead."

Navigator Theodore Van Kirk recalls the shockwaves from the explosion: "(It was) very much as if you've ever sat on an ash can and had somebody hit it with a baseball bat... The plane bounced, it jumped and there was a noise like a piece of sheet metal snapping. Those of us who had flown quite a bit over Europe thought that it was anti-aircraft fire that had exploded very close to the plane." On viewing the atomic fireball: "I don't believe anyone ever expected to look at a sight quite like that. Where we had seen a clear city two minutes before, we could now no longer see the city. We could see smoke and fires creeping up the sides of the mountains."

Tail gunner Robert Caron: "The mushroom itself was a spectacular sight, a bubbling mass of purple-gray smoke and you could see it had a red core in it and everything was burning inside. As we got farther away, we could see the base of the mushroom and below we could see what looked like a few-hundred-foot layer of debris and smoke and what have you... I saw fires springing up in different places, like flames shooting up on a bed of coals."

Six miles below the crew of the Enola Gay, the people of Hiroshima were waking up and preparing for their daily routines. It was 8:16 A.M. Up to that point, the city had been largely spared by the rain of conventional air bombing that had ravaged many other Japanese cities. Rumors abounded as to why this was so, from the fact that many Hiroshima residents had emigrated to the U.S. to the supposed presence of President Truman's mother in the area. Still, many citizens, including schoolchildren, were recruited to prepare for future bombings by tearing down houses to create fire lanes, and it was at this task that many were laboring or preparing to labor on the morning of August 6. Just an hour before, air raid sirens had sounded as a single B-29, the weather plane for the Little Boy mission, approached Hiroshima. A radio broadcast announced the sighting of the Enola Gay soon after 8 A.M.

The city of Hiroshima was annihilated by the explosion. 70,000 of 76,000 buildings were damaged or destroyed, and 48,000 of those were entirely razed. Survivors recalled the indescribable and incredible experience of seeing that the city had ceased to exist.

A college history professor: "I climbed Hikiyama Hill and looked down. I saw that Hiroshima had disappeared... I was shocked by the sight... What I felt then and still feel now I just can't explain with words. Of course I saw many dreadful scenes after that-but that experience, looking down and finding nothing left of Hiroshima-was so shocking that I simply can't express what I felt... Hiroshima didn't exist-that was mainly what I saw-Hiroshima just didn't exist."

Medical doctor Michihiko Hachiya: "Nothing remained except a few buildings of reinforced concrete... For acres and acres the city was like a desert except for scattered piles of brick and roof tile. I had to revise my meaning of the word destruction or choose some other word to describe what I saw. Devastation may be a better word, but really, I know of no word or words to describe the view."

Writer Yoko Ota: "I reached a bridge and saw that the Hiroshima Castle had been completely leveled to the ground, and my heart shook like a great wave... the grief of stepping over the corpses of history pressed upon my heart."

Those who were close to the epicenter of the explosion were simply vaporized by the intensity of the heat. One man left only a dark shadow on the steps of a bank as he sat. The mother of Miyoko Osugi, a 13-year-old schoolgirl working on the fire lanes, never found her body, but she did find her geta sandal. The area covered by Miyoko's foot remained light, while the rest of it was darkened by the blast.

Many others in Hiroshima, farther from the Little Boy epicenter, survived the initial explosion but were severely wounded, including injuries from and burns across much of their body. Among these people, panic and chaos were rampant as they struggled to find food and water, medical assistance, friends and relatives and to flee the firestorms that engulfed many residential areas.

Having no point of reference for the bomb's absolute devastation, some survivors believed themselves to have been transported to a hellish version of the afterlife. The worlds of the living and the dead seemed to converge.

A Protestant minister: "The feeling I had was that everyone was dead. The whole city was destroyed... I thought this was the end of Hiroshima-of Japan-of humankind... This was God's judgment on man."

A six-year-old boy: "Near the bridge there were a whole lot of dead people... Sometimes there were ones who came to us asking for a drink of water. They were bleeding from their faces and from their mouths and they had glass sticking in their bodies. And the bridge itself was burning furiously... The details and the scenes were just like Hell."

A sociologist: "My immediate thought was that this was like the hell I had always read about... I had never seen anything which resembled it before, but I thought that should there be a hell, this was it-the Buddhist hell, where we were thought that people who could not attain salvation always went... And I imagined that all of these people I was seeing were in the hell I had read about."

A boy in fifth grade: "I had the feeling that all the human beings on the face of the earth had been killed off, and only the five of us (his family) were left behind in an uncanny world of the dead."

A grocer: "The appearance of people was... well, they all had skin blackened by burns... They had no hair because their hair was burned, and at a glance you couldn't tell whether you were looking at them from in front or in back... Many of them died along the road-I can still picture them in my mind-like walking ghosts... They didn't look like people of this world."

Many people traveled to central places such as hospitals, parks, and riverbeds in an attempt to find relief from their pain and misery. However, these locations soon became scenes of agony and despair as many injured and dying people arrived and were unable to receive proper care.

A sixth-grade girl: "Bloated corpses were drifting in those seven formerly beautiful rivers; smashing cruelly into bits the childish pleasure of the little girl, the peculiar odor of burning human flesh rose everywhere in the Delta City, which had changed to a waste of scorched earth."

A fourteen-year-old boy: "Night came and I could hear many voices crying and groaning with pain and begging for water. Someone cried, 'Damn it! War tortures so many people who are innocent!' Another said, 'I hurt! Give me water!' This person was so burned that we couldn't tell if it was a man or a woman. The sky was red with flames. It was burning as if scorching heaven."

Nagasaki

Three days after the United States dropped an atomic bomb on Hiroshima, a second atomic bomb was dropped on Nagasaki on August 9 - a 21-kiloton plutonium device known as "Fat Man." On the day of the bombing, an estimated 263,000 were in Nagasaki, including 240,000 Japanese residents, 9,000 Japanese soldiers, and 400 prisoners of war. Prior to August 9, Nagasaki had been the target of small scale bombing by the United States. Though the damage from these bombings was relatively small, it created considerable concern in Nagasaki and many people were evacuated to rural areas for safety, thus reducing the population in the city at the time of the nuclear attack. It is estimated that between 40,000 and 75,000 people died immediately following the atomic explosion, while another 60,000 people suffered severe injuries. Total deaths by the end of 1945 may have reached 80,000.

The decision to use the second bomb was made on August 7, 1945 on Guam. Its use was calculated to indicate that the United States had an endless supply of the new weapon for use against Japan and that the United States would continue to drop atomic bombs on Japan until the country surrendered unconditionally.

The city of Nagasaki, however, was not the primary target for the second atomic bomb. Instead, officials had selected the city of Kokura, where Japan had one of its largest munitions plants.

The B-29 "Bockscar", piloted Major Charles Sweeney, was assigned to deliver the "Fat Man" to the city of Kokura on the morning of August 9, 1945. Accompanying Sweeney on the mission were copilots Charles Donald Albury and Fred J. Olivi, weaponeer Frederick Ashworth, and bombadier Kermit Beahan. At 3:49am, "Bockscar" and five other B-29s departed the island of Tinian and headed towards Kokura.

When the plane arrived over the city nearly seven hours later, thick clouds and drifting smoke from fires started by a major firebombing raid on nearby Yawata the previous day covered most of the area over Kokura, obscuring the aiming point. Pilot Charles Sweeney made three bomb runs over the next fifty minutes, but bombardier Beahan was unable to drop the bomb because he could not see the target visually. By the time of the third bomb run, Japanese antiaircraft fire was getting close, and Second Lieutenant Jacob Beser, who was monitoring Japanese communications, reported activity on the Japanese fighter direction radio bands.

Running low on fuel, the crew aboard Bockscar decided to head for the secondary target, Nagasaki. When the B-29 arrived over the city twenty minutes later, the downtown area was also covered by dense clouds. Frederick Ashworth, the plane's weaponeer proposed bombing Nagasaki using radar. At that moment, a small opening in the clouds at the end of the three-minute bomb run permitted bombardier Kermit Beahan to identify target features.

At 10:58 AM local time, Bockscar visually dropped Fat Man. It exploded 43 seconds later with a blast yield equivalent to 21 kilotons of TNT at an altitude of 1,650 feet, about 1.5 miles northwest of the intended aiming point.

The radius of total destruction from the atomic blast was about one mile, followed by fires across the northern portion of the city to two miles south of where the bomb had been dropped. In contrast to many modern aspects of Hiroshima, almost all of the buildings in Nagasaki were of old-fashioned Japanese construction, consisting of wood or wood-frame buildings with wood walls and tile roofs. Many of the smaller industries and business establishments were also situated in buildings of wood or other materials not designed to withstand explosions. As a result, the atomic explosion over Nagasaki leveled nearly every structure in the blast radius.

The failure to drop Fat Man at the precise bomb aim point caused the atomic blast to be confined to the Urakami Valley. As a consequence, a major portion of the city was protected from the explosion. The Fat Man was dropped over the city's industrial valley midway between the Mitsubishi Steel and Arms Works in the south and the Mitsubishi-Urakami Ordnance Works in the north. The resulting explosion had a blast yield equivalent to 21 kilotons of TNT, roughly the same as the Trinity blast. Nearly half of the city was completely destroyed.

Bockscar's copilot Fred J. Olivi published a detailed account of the effects of the atomic explosion over Nagasaki. Here are some of his reactions:

Olivi: "Suddenly, the light of a thousand suns illuminated the cockpit. Even with my dark welder's goggles, I winced and shut my eyes for a couple of seconds. I guessed we were about seven miles from "ground zero" and headed directly away from the target, yet the light blinded me for an instant. I had never experienced such an intense bluish light, maybe three or four times brighter than the sun shining above us."

"I've never seen anything like it! Biggest explosion I've ever seen...This plume of smoke I'm seeing is hard to explain. A great white mass of flame is seething within the white mushroom shaped cloud. It has a pinkish, salmon color. The base is black and is breaking a little way down from the mushroom."

"The Mushroom cloud was coming right at us. I immediately looked up and could see that he was right, the cloud was getting close to Bockscar. We had been told not to fly through the atomic cloud because it was extremely dangerous to the crew and aircraft. Knowing this, Sweeney put Bockscar into a steep dive to the right, away from the cloud, throttles wide open. For a few moments we could not tell if we were out-running the ominous cloud or if it was gaining on us, but gradually we pulled away from the dangerous radioactive cloud before it engulfed us, much to everyone's relief."

Tatsuichiro Akizuki: "All the buildings I could see were on fire... Electricity poles were wrapped in flame like so many pieces of kindling... It seemed as if the earth itself emitted fire and smoke, flames that writhed up and erupted from underground. The sky was dark, the ground was scarlet, and in between hung clouds of yellowish smoke. Three kinds of color-black, yellow, and scarlet-loomed ominously over the people, who ran about like so many ants seeking to escape... It seemed like the end of the world."

Aftermath

On August 14, Japan surrendered. Journalist George Weller was the "first into Nagasaki" and described the mysterious "atomic illness" (the onset of radiation sickness) that was killing patients who outwardly appeared to have escaped the bomb's impact. Controversial at the time and for years later, Weller's articles were not allowed to be released until 2006.

Controversy

The debate over the bomb - whether there should have been a test demonstration, whether the Nagasaki bomb was necessary, and more - continues to this day.

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Bombings of Hiroshima and Nagasaki - 1945

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After the Interim Committee decided to drop the bomb, the Target Committee determined the locations to be hit, and President Truman issued the Potsdam Proclamation as Japan's final warning, the world soon learned the meaning of "complete and utter destruction." The first two atomic bombs ever used were dropped on Japan in early August, 1945.