Fu-Go balloons over Hanford

On 10 March 1945, the day Tokyo was still smouldering after the largest incendiary raid of the war, a paper balloon ten metres across was drifting down over the Yakima River valley in Washington State. It had come from Japan, crossing more than 8,000 kilometres of the Pacific on winds blowing at an altitude of about ten kilometres. Near the small town of Toppenish its cables snagged on a high-voltage line carrying power from the great hydroelectric plants on the Columbia River, among them the Grand Coulee Dam. The result was a short circuit, and the power surge reached a site whose existence was known to almost no one outside a narrow circle of insiders: the Hanford Engineer Works, where three nuclear reactors were producing plutonium for the American atomic bomb. Safety devices automatically shut the reactors down.

This was the best-known consequence of the Japanese balloon offensive, and the only occasion on which the enemy – purely by chance – reached a facility of the Manhattan Project. The claim that the balloons “damaged” Hanford needs qualifying, however. The balloon did not come down on the site itself but on a transmission line several dozen kilometres away; the bomb it carried did not explode, and the line itself was not seriously damaged. The works suffered no material losses: the episode amounted to an interruption in the reactors’ operation, with no lasting consequences.

A weapon from the Noborito laboratory

The idea of a balloon bomb had been around in the Japanese army since the 1930s, but it was the Doolittle Raid of April 1942 that gave it real impetus. Imperial General Headquarters then ordered that a way be found to strike back at the territory of the United States. The navy tried a seaplane launched from a submarine: in September 1942 Nobuo Fujita dropped incendiary bombs on the forests of Oregon, to little effect. The army put its faith in balloons. The work was carried out by its Ninth Military Technical Research Institute at Noborito near Tokyo, where the project was revived by Generalmajor Sueki Kusaba.

The key turned out to be a discovery the Japanese had made earlier than anyone else. In the 1920s the meteorologist Wasaburō Ōishi, launching observation balloons, had found that at high altitude over Japan a steady and very strong wind blew from west to east – what is now called the jet stream. On the basis of his data, Hidetoshi Arakawa of the Central Meteorological Observatory calculated that a balloon holding an altitude of nine to eleven kilometres in winter would reach North America in 30–100 hours. The winds were strongest from November to March.

The balloon envelope was glued together from four or five layers of thin, tough washi paper, bonded with a paste made from konjac tubers. Thousands of secondary-school girls were put to work producing them, and the finished envelopes were tested in large halls, including Tokyo’s Ryōgoku Kokugikan sumo arena. Filled with hydrogen, the balloon measured ten metres in diameter. The hardest problem was keeping it at altitude: by day the sun warmed the gas and the balloon rose, while at night it sank. The engineers therefore built an ingenious automatic mechanism. Whenever the balloon dropped too low, barometers released in turn one of 32 sandbags hung around an aluminium ring, and a valve vented gas when it climbed too high. Once the last ballast had gone, the device released the payload – usually four incendiary bombs of about five kilograms each and one 15-kilogram fragmentation bomb – after which an explosive charge was meant to destroy the balloon itself.

The balloon offensive

Three battalions, together numbering about 2,800 men, were prepared for the attack, along with a dozen or so launch sites on the east coast of Honshu: at Ōtsu in Ibaraki Prefecture, at Ichinomiya in Chiba Prefecture and at Nakoso in Fukushima Prefecture. The first balloons were launched on 3 November 1944. According to American findings, about 9,300 balloons were released by April 1945, the largest numbers – about 2,500 each – in February and March. The army itself assumed that at most one in ten would make it across the Pacific. Some 300 were found or sighted in the United States, Canada and Mexico, from Alaska and Hawaii to Texas and Michigan. The largest number of incidents, 114, was recorded in March 1945.

According to post-war interrogations of Japanese officers, the offensive served above all a propaganda purpose: it was meant to show the public that Japan was hitting back after the bombing of its own cities. From mid-February 1945 Japanese radio announced huge forest fires, panic and hundreds of dead in America, although in reality Tokyo knew almost nothing about what had become of the balloons.

That ignorance was largely the work of American censorship. On 4 January 1945 the Office of Censorship asked newspapers and broadcasters not to report on the balloons, and the overwhelming majority of the press complied. The only balloon the Japanese learned about was one found in December 1944 near Kalispell, Montana – and they learned of it from an article in a Chinese newspaper. Without information on the results they could neither improve their accuracy nor judge whether the whole undertaking made sense.

At first the Americans did not believe the balloons could be coming straight from Japan. Suspicion fell on submarines off the coast, and even on German prisoners in the camps or on interned Japanese Americans – people the government had removed from the West Coast under Executive Order 9066. The answer came from geology: researchers from the US Geological Survey analysed the sand in the ballast bags and identified a beach at Ichinomiya as one possible place of origin. Fighters managed to shoot down only about twenty balloons; they flew too high and were difficult to detect.

The gravest concern was that the balloons might carry biological weapons. The fear was not unfounded: Japan’s Unit 731 had for years been conducting experiments on human beings in Manchuria and had used germs against the population of China. Doctors, vets and agricultural services in the western United States were told to report any unknown disease of people, animals or plants, and balloon wreckage was examined in laboratories. After the war the Americans established that no biological or chemical payloads had been planned for the balloons.

The line to Hanford

The Hanford works were built in 1943 on the Columbia, in a wilderness in the south-eastern part of Washington State. The site was approved by Leslie Groves, who had headed the Manhattan Project since September 1942; he entrusted its construction and operation to the DuPont company. The choice was decided by, among other things, water for cooling the reactors and cheap hydroelectric power. Reactor B went critical in September 1944 – Enrico Fermi was present at its start-up – and produced its first plutonium in November. Reactors D and F started up in December 1944 and February 1945. On 5 February 1945 the first batch of plutonium was delivered to the laboratory at Los Alamos, and according to the site’s history all three reactors were running at full power by 8 March. Two days later the balloon arrived.

The reactors needed uninterrupted cooling, since even after shutdown the decay of fission products went on generating heat. The designers had therefore provided for the pumps to switch to reserve power sources if the supply failed. On 10 March the system worked as it should: the short circuit on the line caused a power surge, the automatic systems carried out an emergency shutdown of the reactors, and cooling did not stop. Accounts differ on the length of the interruption. According to some, the power was cut for only a fraction of a second and the site’s work was disrupted only briefly; according to others, it took about three days to bring the reactors back to full power. There is no indication that the incident delayed plutonium production in any lasting way.

The event had an almost ironic counterpart on the other side of the war. Less than three years earlier, in July 1942, the wire of a British balloon from Operation Outward short-circuited a high-voltage line near Leipzig and, according to post-war findings, caused a fire at the Böhlen power station. The Japanese balloon, though technically far more sophisticated, did less damage at Hanford than the primitive British balloon had done in Saxony. The Japanese, for their part, never found out – nor did they know what Hanford was.

The end of the offensive and the victims at Bly

The wet winter and spring of 1945, exceptionally rainy in the western United States, meant that the incendiary bombs started no major forest fires; at most one or two small grass fires are attributed to them. By mid-April Japan had run out of paper and hydrogen, and the army had no evidence that the balloons were reaching their targets at all. The last was launched on 20 April 1945, and the offensive was never resumed.

Its only fatal victims died later. On 5 May 1945, in the forest on Gearhart Mountain near Bly, Oregon, Elsie Mitchell, the pregnant wife of a local pastor, and five Sunday-school children came across a balloon lying on the ground during an outing. The fragmentation bomb exploded, killing all six. They were the only people killed by enemy action on the continental territory of the United States during the Second World War. On 22 May the authorities lifted the publication ban in order to warn people not to touch unknown objects. In Canada and the United States balloon wreckage, sometimes with unexploded bombs, was still being found decades after the war.

The Japanese records of the programme were destroyed on 15 August 1945, the day the surrender was announced, so what is known about it rests mainly on American investigations and post-war interrogations.

Its place in the war

The Fu-Go balloon bomb was the first weapon in history with an intercontinental range – more than a decade before the first intercontinental missile. Militarily it proved almost useless: thousands of balloons, a great industrial effort and the labour of thousands of schoolgirls produced a few small fires, one short circuit on a power line and six deaths. Its story says more about Japan’s position in the spring of 1945 – a state that could no longer reach its enemy with aircraft or ships while American bombers were burning its cities.

The incident of 10 March nonetheless carries a symbolic weight. Plutonium from the reactors that the Japanese balloon had briefly shut down was used a few months later in the Trinity test in New Mexico and in the Fat Man bomb, dropped on Nagasaki on 9 August 1945. The most successful strike of the Japanese balloon offensive had by chance hit the plant producing the weapon that was to end the war in the Pacific – and halted it for a few days at most.