It was Purnell who had first advanced the belief that two bombs would end the war, so I knew that with him and Farrell on the ground at Tinian there would be no unnecessary delay in exploiting our first success.
Good weather was predicted for the ninth, with bad weather to follow for the next five days. This increased the urgency of having the first Fat Man ready still another day earlier. When the decision to do so was reached, the scientific staff made it clear that in their opinion the advancement of the date by two full days, from the eleventh to the ninth, would introduce a considerable measure of uncertainty. I decided, however, that we should take the chance; fortunately all went well with the assembly, and the bomb was loaded and fully checked by the evening of August 8.
Six Pumpkin-carrying planes were assigned various targets in Japan for the eighth, but because of weather only two of them reached their primary targets; three of them reached secondary targets, and one aborted and returned to Tinian. In the field order for the second atomic mission there was nothing to indicate the extraordinary nature of the bomb, although anyone reading it would realize that this was by no means a routine assignment.
There were only two targets designated this time: Kokura, primary; and Nagasaki, secondary. Niigata was not made a third target because of its great distance from the other two cities.
[…]
The Kokura arsenal was one of the largest war plants in Japan. It produced many different weapons and pieces of war equipment. It extended over almost two hundred acres and was supported by numerous machine shops, parts factories, electric power plants and the usual utilities.
Nagasaki was one of Japan’s largest shipbuilding and repair centers. It was important also for its production of naval ordnance. It was a major military port. The aiming point was in the city, east of the harbor.
[…]
It was not possible to “safe” the Fat Man by leaving the assembly incomplete prior to take-off, as had been done in the case of the Little Boy. There was considerable discussion among the technical staff about what would happen if the plane crashed, and possibly caught fire, while it was taking off. They realized that there would be a serious chance that a wide area of Tinian would be contaminated if the plutonium were scattered by a minor explosion; some thought that there was even a risk of a high-order nuclear explosion which could do heavy damage throughout the island’s installations. Of course, we had gone into all this at length during our preliminary planning, and on the basis of my own opinion, as well as that of Oppenheimer and my other senior advisers, that the risk was negligible I had decided that the risk would be taken.
As happens so often, however, there was constant interference by various people in matters that lay outside their spheres of responsibility. Throughout the life of the project, vital decisions were reached only after the most careful consideration and discussion with the men I thought were able to offer the soundest advice. Generally, for this operation, they were Oppenheimer, von Neumann, Penney, Parsons and Ramsey. I had also gone over the problems at considerable length with the various groups of senior men at Los Alamos, and had discussed them thoroughly with Conant and Tolman and with Purnell and Farrell and to a lesser degree with Bush. Yet in spite of this, some of the people on Tinian again raised the question of safety at take-off at the last moment. Their fears reached a senior air officer, who asked for a written statement to the effect that it would be entirely safe for the plane to take off with a fully armed bomb. Parsons and Ramsey signed such a statement promptly though with some trepidation, possibly with the thought that if they were proven wrong they would not be there to answer. Ramsey then advised Oppenheimer at once of the various design changes that must be made to ensure that future bombs would in fact be surely safe.
One very serious problem came up just before take-off, which placed Farrell in the difficult position of having to make a decision of vital importance without the benefit of time for thought or consultation. Despite all the care that had been taken with the planes, the carrying plane was found at the last moment to have a defective fuel pump, so that some eight hundred gallons of gasoline could not be pumped to the engines from a bomb bay tank. This meant that not only would the plane have to take off with a short supply of fuel, but it would have to carry the extra weight of those eight hundred gallons all the way from Tinian to Japan and back. The weather was not good, in fact it was far from satisfactory; but it was good enough in LeMay’s opinion, and in view of the importance of dropping the second bomb as quickly as possible, and the prediction that the weather would worsen, Farrell decided that the flight should not be held up. Just before take-off Purnell said to Sweeney, “Young man, do you know how much that bomb cost?” Sweeney replied, “About $ 25 million.” Purnell then cautioned, “See that we get our money’s worth.”
Because of the weather, instead of flying in formation, the planes flew separately. To save fuel, they did not fly over Iwo Jima but went directly to the coast of Japan. Their plan was to rendezvous over the island of Yokushima, but this did not work out. The planes were not in sight of each other during their overwater flight and only one of the observation planes arrived at the rendezvous point. The missing plane apparently circled the entire island instead of one end of it, as it was supposed to do according to Sweeney’s plans. Although Sweeney had identified the one plane that did arrive he did not tell Ashworth. Unfortunately, because it did not come close enough, Ashworth was unable to determine whether it was the instrument-carrying plane, which was essential to the full completion of the mission, or the other, which was not. Sweeney’s orders were to proceed after a short delay of fifteen minutes but he kept waiting hopefully beyond the deadline. The result was a delay of over half an hour before they decided to go on to Kokura, anyway.
At Kokura, they found that visual bombing was not possible, although the weather plane had reported that it should be. Whether this unexpected condition was due to the time lag, or to the difference between an observer looking straight down and a bombardier looking at the target on a slant, was never determined.
After making at least three runs over the city and using up about forty-five minutes, they finally headed for the secondary target, Nagasaki. On the way they computed the gasoline supply very carefully. Ashworth confirmed Sweeney’s determination that it would be possible to make only one bombing run over Nagasaki if they were to reach Okinawa, their alternate landing field. If more than one run had to be made they would have to ditch the plane—they hoped near a rescue submarine.
At Nagasaki, there was a thick overcast and conditions at first seemed no better for visual bombing than at Kokura. Considering the poor visibility and the shortage of gasoline, Ashworth and Sweeney decided that despite their positive orders to the contrary, they had no choice but to attempt radar bombing. Almost the entire bombing run was made by radar; then, at the last moment, a hole in the clouds appeared, permitting visual bombing. Beahan, the bombardier, synchronized on a race track in the valley and released the bomb. Instead of being directed at the original aiming point, however, the bomb was aimed at a point a mile and a half away to the north, up the valley of the Urakami River, where it fell between two large Mitsubishi armament plants and effectively destroyed them both as producers of war materials.
On the way to Okinawa warning ditching orders were announced; but the plane made it with almost no gas left. Sweeney reported there wasn’t enough left to taxi in off the runway.
The Nagasaki bomb was dropped from an altitude of 29,000 feet. Because of the configuration of the terrain around ground zero, the crew felt five distinct shock waves.
The missing observation plane, which fortunately was the one without the instruments, saw the smoke column from a point about a hundred miles away and flew over within observing distance after the explosion. Because of the bad weather conditions at the target, we could not get good photo reconnaissance pictures until almost a week later. They showed 44 per cent of the city destroyed. The difference between the results obtained there and at Hiroshima was due to the unfavorable terrain at Nagasaki, where the ridges and valleys limited the area of greatest destruction to 2.3 miles (north-south axis) by 1.9 miles (east-west axis). The United States Strategic Bombing Survey later estimated the casualties at 35,000 killed and 60,000 injured.
While the blast and the resulting fire inflicted heavy destruction on Nagasaki and its population, the damage was not nearly so heavy as it would have been if the correct aiming point had been used. I was considerably relieved when I got the bombing report, which indicated a smaller number of casualties than we had expected, for by that time I was certain that Japan was through and that the war could not continue for more than a few days.
To exploit the psychological effect of the bombs on the Japanese, we had belatedly arranged for leaflets to be dropped on Japan proclaiming the power of our new weapon and warning that further resistance was useless. The first delivery was made on the ninth, the day of the Nagasaki bombing. The following day General Farrell canceled the drops, when the surrender efforts of the Japanese made any further such missions seem ill-advised.
The amount of a black-brown pigment, eumelanin, in the iris is the main determinant of eye color, Morgan Worthy explains (in I Have Known the Eyes Already): 
