At 6:45 AM on March 1, 1954, a blinding flash brighter than the sun lit up the sky above Bikini Atoll in the south Pacific. From an observation bunker on Enyu Island 30 kilometres away, scientists watched as a monstrous, glowing fireball rose over the lagoon, expanding to more than seven kilometres in diameter. Within a minute, this fireball had grown into a towering mushroom cloud, stretching eleven kilometres into the stratosphere. Codenamed Castle Bravo, the blast marked history’s first test of a practical Hydrogen Bomb, and ushered the Cold War into a terrifying new era. Thousands of times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki, hydrogen bombs gave humanity the means to obliterate entire cities – and perhaps even end all life on earth. But what is a ‘hydrogen bomb’ anyway? How were they developed, and what makes them so apocalyptically destructive? Well, climb down into your fallout shelter and put on your radiation suit as we dive into the history and physics of the most terrifying weapon of mass destruction ever created.

While ordinary nuclear weapons are based on the process of fission – the splitting apart of heavy Uranium or Plutonium atoms – hydrogen bombs – more correctly known as thermonuclear weapons – exploit the process of fusion – combining lighter hydrogen atoms to form heavier helium atoms. As hydrogen atoms normally repel each other due to electrostatic forces, fusion requires extremely high temperatures and pressures to bring the atoms close enough for the attractive strong nuclear force to overcome this repulsion and bind the atomic nuclei together. Such conditions are found in the cores of stars, and indeed it is the fusion of hydrogen which keeps them giving off their extreme energy. The process of stellar nucleosynthesis was first worked out in 1937 by German physicist Hans Bethe, who would later play a key role in the Manhattan Project, the wartime effort to develop the first atomic bomb. However, it was one of Bethe’s colleagues, Italian physicist Enrico Fermi, who first proposed in September 1941 that a nuclear fission reaction could be used to trigger nuclear fusion and release even greater amounts of energy. This reaction, however, would be different from the light hydrogen fusion found in stars, which is far too slow and requires too much energy to make an effective weapon. Instead, Fermi proposed using the heavy hydrogen isotopes Deuterium and Tritium, which require less energy to fuse. The easiest fusion reaction to initiate involves the combination of one Deuterium and one Tritium atom to produce one atom of Helium-4, one free neutron, and 17.6 Mega-electron-volts of energy. The next-easiest reaction combines two Deuterium atoms to produce one Helium-3 atom, one neutron, and 3.268 Mega-electron-volts of energy.

The main advantage of fusion is the significant increase in weapon yield and efficiency it allows. While regular fission weapons are extraordinarily powerful, there is an upper limit to their explosive yield. This is because there is only a limited period during which the fuel in a weapon’s core can undergo fission and be converted into energy before said energy blows the core apart and halts the reaction. While there are several methods for increasing pure fission yield, such as using a larger core; surrounding the core with a heavy lead or uranium shell or tamper to delay its scattering; or using a neutron source to increase the number of fissions, eventually the weapon becomes too heavy or uses too much costly fuel to be practical. Therefore, the upper limit for pure fission devices is around 700 kilotons. However, not only does fusion produce far more energy per unit mass of fuel than fission, but it also generates large numbers of neutrons, which can be directed into the trigger weapon’s core to boost the fission process and release even more energy. This makes the theoretical yield of fusion weapons effectively limitless.

The idea of using fusion to create weapons of nearly limitless power immediately captured the imagination of Fermi’s colleague, Hungarian physicist Edward Teller. Though such a weapon would require a regular – and as yet unproven – fission weapon to trigger the fusion reaction, Teller dismissed fission as a mere matter of engineering, and saw the fusion bomb – which he dubbed the ‘Super’ – as a far more interesting theoretical problem. Indeed, such was Teller’s obsession that he began neglecting his actual assigned duties on the Manhattan Project, leading project director J. Robert Oppenheimer to reassign much of this work to others. Yet while fusion was a low priority compared to fission, Oppenheimer nonetheless allowed Teller to spend much of his time studying the ‘Super.’

However, following the end of the Second World War and the destruction of Hiroshima and Nagasaki, many nuclear scientists began to question whether the ‘Super’ – if it was feasible – should actually be developed. Some, like Oppenheimer, Bethe, Fermi, and Atomic Energy Commission chair David Lilienthal argued that the Super would be too powerful for use against military targets, and could thus only be used against civilian populations. As Fermi and physicist Isidor Rabi wrote in 1949:

“A decision on the proposal that an all-out effort be undertaken for the development of the “Super” cannot in our opinion be separated from considerations of broad national policy…necessarily such a weapon goes far beyond any military objective and enters the range of very great natural catastrophes. By its very nature it cannot be confined to a military objective but becomes a weapon which in practical effect is almost one of genocide… The fact that no limits exist to the destructiveness of this weapon makes its very existence and the knowledge of its construction a danger to humanity as a whole. It is necessarily an evil thing considered in any light.”

Oppenheimer further emphasized the potentially high cost of developing and building super bombs, arguing that building a large stockpile of smaller, tactical fission weapons was a better use of the nation’s resources.

On the other side of the debate, Edward Teller, along with Ernest Lawrence, Luis Alvarez, and others, argued that the development of the Super was inevitable, and that if the United States did not develop it first, then the Soviet Union would. As Teller wrote to Fermi in 1945:

“If the development [of the Super] is possible, it is out of our powers to prevent it. All rear we can do is retard its completion by some years. I believe, on the other hand, that any form of international control may be put on a more stable basis by the knowledge of the full extent of the problem that must be solved and of the dangers of a ruthless international competition.”

The debate raged on until August 29, 1949, when the Soviet Union tested its first atomic bomb – five years ahead of western predictions. This feat, aided in great part by a network of Soviet spies within the Manhattan Project, shattered the United States’ comfortable nuclear monopoly and sent shockwaves through the defence establishment. A month later on October 29, the Atomic Energy Commission convened a special committee of scientists to draw up recommendations regarding the development of the Super. The committee’s report, released the following day, concluded that:

“Although the members of the Advisory Committee are not unanimous in their proposals as to what should be done with regard to the super bomb, there are certain elements of unanimity among us. We all hope that by one means or another, the development of these weapons can be avoided. We are all reluctant to see the United States take the initiative in precipitating this development. We are all agreed that it would be wrong at the present moment to commit ourselves to an all out effort towards its development.”

But to the dismay of the anti-Super camp, the die was already cast. Not wanting to fall behind the Soviets in the nuclear arms race, on January 31, 1950, U.S. President Harry S. Truman officially ordered the development of the Hydrogen Bomb.

Unfortunately, the problem of initiating fusion proved far more difficult than anyone could have imagined. In Enrico Fermi and Edward Teller’s original conception of the hydrogen bomb, known as the Classical Super, Deuterium or Tritium fusion fuel was either wrapped around the core of a regular fission weapon or layered within it – a configuration Teller dubbed the “Alarm Clock.” Teller believed that the tremendous heat produced by the fission reaction would be sufficient to set off a fusion reaction within the fuel. But, as Teller’s wartime calculations quickly revealed, this was not the case. The major problem, Teller discovered, was that around 80% of the energy released by a detonating nuclear bomb is in the form of X-rays, to which hydrogen is nearly transparent. This means that almost none of this energy will be absorbed by the fusion fuel. The remaining 20% is in the form of kinetic energy contained within electrons, neutrons, and other subatomic particles. However, generating enough of this energy to initiate fusion requires an extremely large fission bomb in the hundred-kilotons range and an even larger mass of fusion fuel – creating a weapon so large and heavy that, as Robert Oppenheimer joked, it could “only be delivered by ox cart.”

And even if a fusion reaction can be initiated, Teller’s calculations revealed that it cannot be sustained for long enough to generate a useful explosive yield. The problem is that successful fusion requires a certain fuel density and temperature. However, the higher the temperature of the fuel, the more forcefully it expands, and the harder it becomes to compress. And once the reaction begins, the energy released heats the fuel and causes it to expand, reducing its density and halting the reaction. Thus, in order to initiate a successful fusion reaction, the fuel must be compressed to the proper density before it is heated. Unfortunately, the kinetic shock wave from the fission detonation is neither powerful nor fast enough to accomplish this. Even worse, the thermal energy generated by the compression of the fuel escapes the weapon at the same rate as it is added, meaning the fuel never accumulates enough energy to ignite a fusion reaction.

Of course, all this assumes that the fusion fuel is composed entirely of Deuterium. However, adding Tritium to the mix significantly improves the situation since, as previously mentioned, the Deuterium-Tritium fusion reaction requires the least amount of energy to initiate and is 100 times faster than the Deuterium-Deuterium reaction. But this creates an entirely different problem, since not only is Tritium expensive to produce – being generated in small quantities in nuclear reactors – but it is also radioactive, decaying with a half-life of 12.3 years. As a result, incorporating enough Tritium into a Classical Super to make it feasible would make such a weapon prohibitively expensive. The Tritium would also need to be constantly topped up as it decays away in storage. One way around this is to breed Tritium within the bomb itself. This is done by using the isotope Lithium 6 – in the form of lithium deuteride – as the fusion fuel. When Lithium 6 absorbs a neutron from the fission reaction, it splits into an atom of Helium-4 and an atom of Tritium, which can then be burned along with the deuterium. But while this solves the storage problem, it is hardly an ideal solution, for it is far more efficient to use the neutrons from the trigger’s detonation to breed Plutonium from Uranium, a much more efficient and energetic process than breeding Tritium. No matter what Teller and others tried, the Super appeared for all intents and purposes impossible.

Then, after years of fruitless calculations and endless dead ends, in 1951 Teller sought the advice of Polish mathematician Stanislaw Ulam, who immediately made a key insight. Within just a few months, the pair presented their groundbreaking solution to the Super problem in a paper titled On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors. Teller and Ulam’s breakthrough was the principle of radiation implosion, wherein X-rays, rather than heat or shock waves, are used to compress and ignite the fusion fuel. Their bomb design – known to this day as the Teller-Ulam design – consists of two main parts: a primary stage or “trigger” comprising an ordinary fission weapon, and a secondary stage containing the fusion fuel. The second stage consists of a cylindrical metal canister called the radiation case, within which sits a thick cylindrical layer of natural Uranium known as a pusher or tamper. Inside this is a layer of polystyrene foam, and inside this the fusion fuel. And finally, running down the centre of the secondary is a cylindrical Plutonium rod known as the spark plug.

When the primary stage detonates, the X-rays produced are channeled through an interstage into the radiation case, where they are absorbed by the tamper. This heats the outside surface of the tamper to extremely high temperatures, causing that surface to rapidly slough off or ablate.The rapid outwards expansion of this ablated material induces an equal and opposite reaction that causes the tamper to collapse inwards, compressing the fusion fuel. However, due to its relatively large mass, the tamper acts as a thermal barrier, preventing the fuel from being heated too rapidly and allowing it to be compressed to critical density before ignition. At the same time, rapid heating converts the polystyrene foam into hot hydrocarbon plasma. This serves two main functions. First, the plasma prevents the inside surface of the tamper from ablating, which would create an outward reactive force that could prematurely blow the secondary stage apart. Second, the plasma, being transparent to X-rays, maintains an open channel for x-rays streaming in from the primary stage.

The implosion of the tamper not only compresses the fusion fuel, but also the spark plug, which soon reaches criticality and undergoes a nuclear chain reaction. This suddenly increases the temperature inside the secondary stage to around 300 million Kelvin, triggering a fusion reaction in the now-compressed fuel. This fusion reaction in turn releases a large burst of neutrons, further boosting the fission rate of both the spark plug and the tamper and creating a positive feedback loop that releases enormous amounts of energy. Eventually, the outward pressure generated by the fusing fuel overcomes the inward momentum of the collapsing tamper, and the secondary detonates.

The genius of Teller and Ulam’s design lies in the harnessing of X-rays, which, as previously mentioned, make up 80% of the energy released by the primary stage. As X-rays travel at the speed of light, they can trigger fusion sooner – and sustain it for longer – than either the heat or the shock wave from the primary. But while this is impressive enough on its own, there is another, even more terrifying advantage to the Teller-Ulam design. While the basic design only includes a single fusion stage, if a second fusion stage of identical construction is placed next to the first, the X-rays produced by the first fusion stage will trigger the same radiation implosion process in the second, releasing even more energy. Indeed, while no practical thermonuclear weapon has ever used more than three stages, one could theoretically stack any number of fusion stages together, making the explosive yield of the Teller-Ulam design effectively limitless.

While this design is traditionally credited to both Teller and Ulam, there is considerable debate as to which man contributed what. Some, like weapons designer Ted Taylor, credited Ulam with developing the key principles of radiation implosion, arguing that Teller would not have gotten far without his help. Teller, on the other hand, being prone to self-aggrandizement and eager to be seen as the “father” of the atomic bomb, took full credit for the breakthrough, stating in a 1999 interview with Scientific American that:

“I contributed; Ulam did not. I’m sorry I had to answer it in this abrupt way. Ulam was rightly dissatisfied with an old approach. He came to me with a part of an idea which I already had worked out and difficulty getting people to listen to. He was willing to sign a paper. When it then came to defending that paper and really putting work into it, he refused. He said, “I don’t believe in it.”

Whatever the case may be, the first full-scale tests of Teller and Ulam’s ideas took place in the spring of 1951 as part of Operation Greenhouse, conducted at Enewetak Atoll in the Marshall Islands. The third nuclear device of the series, codenamed George, comprised a ring-shaped fission weapon with a capsule of gaseous Deuterium-Tritium fusion fuel placed in its centre. Detonated at 8:30 PM on May 8, 1951, Greenhouse George had a yield of 225 kilotons – significantly greater than the trigger but still far below the potential yield of a true thermonuclear weapon. Indeed, most of George’s extra yield came not from the fusion itself but the flurry of neutrons it produced, which boosted the fission process within the trigger’s core. The fourth and final test in the series, codenamed Item, would exploit this effect more fully by injecting a small amount of deuterium-tritium gas into the core of a conventional nuclear weapon just prior to detonation. This resulted in an explosive yield of 45.5 kilotons – about twice that of the original fission weapon. Such “fission boosting” is still used today, and is a useful method for enhancing the yield of smaller nuclear weapons.

With the basic principles now fully understood, scientists were now ready to conduct a full-scale test of the Teller-Ulam design, codenamed Ivy Mike. As the technology for producing sufficiently pure lithium deuteride had not yet been fully perfected, it was decided to use pure liquid deuterium as the fusion fuel. This also simplified the fusion reaction from a physics standpoint, allowing the design to be more easily analyzed. However, as hydrogen liquefies at 20 Kelvin or -253º Celsius, this design required the construction of a massive 3,000 kilowatt refrigeration plant and a 50-ton, 1,000 litre Dewar – essentially a giant thermos flask – to cool and store the cryogenic deuterium. This, along with the test weapon itself, was installed in a two-storey corrugated aluminium building known as a “shot cab”, located on the island of Eugelab at Enewetak Atoll. More a laboratory experiment than a practical weapon, the Mike device was derisively dubbed a “thermonuclear installation” by Soviet scientists. To gather vital data from the test, dozens of cameras and other instruments were arranged around Enewetak Atoll. This including an aluminium-coated plywood tunnel filled with helium balloons, which stretched 2.7 kilometres along an artificial coral causeway from the shot cab to an unmanned observation station on Boken Island. Known as a Krause-Ogle box, the device provided a free path for the x-rays, gamma rays, and neutrons from the blast to travel to scientific instruments inside the station.

Ivy Mike was detonated at 7:15 AM on November 1, 1952. The blast, measured at 10.4 megatons, was the largest man-made explosion up to that time, creating a fireball 3.3 kilometres wide and a mushroom cloud 33 kilometres tall, and completely obliterating the island of Eugelab, leaving behind a crater 2 kilometres in diameter. The world had entered the thermonuclear age.

As the success of Ivy Mike was not guaranteed, a contingency weapon was prepared in case the test failed or the Teller-Ulam design took longer than expected to perfect. Codenamed Ivy King, this was a high-yield conventional fission weapon which, unlike the Mike device, was light enough to be delivered by aircraft and could be immediately added to the U.S. nuclear stockpile. Though the Mike shot was ultimately successful, Ivy King was nonetheless tested on November 16, 1962 with a yield of 500 kilotons. It remains the most powerful pure-fission weapon ever detonated by the United States and one of the most powerful overall, second only to the British 720-kiloton Orange Herald warhead.

In the wake of Operation Ivy, scientists raced to create a practical, air-droppable dry fuel thermonuclear weapon. This effort produced an experimental, 10,000 kilogram device code-named “Shrimp”, whose 1.3 metre diameter, 4.5-metre long cylindrical case contained a 61 kiloton fission trigger and 400 kilograms of dry Lithium Deuteride fusion fuel. The fuel was enriched to contain 40% fusible Lithium-6 isotope and 60% the more common Lithium-7 isotope, which scientists believed was inert and would take no part in the reaction. Based on the data obtained from Ivy Mike, the yield of Shrimp was estimated at 5 megatons.

Shrimp was tested as the Bravo shot of Operation Castle, conducted at Bikini Atoll in the spring of 1954.

Like the Mike device, Shrimp was installed in a metal shot cab on Namu island and surrounded by an array of scientific instruments, including dozens of high-speed cameras and a 2.3 kilometre-long evacuated tunnel or “light pipe” to channel x-rays and neutron radiation to an unmanned instrument station.

Castle Bravo was detonated at 6:45 AM on March 1, 1954. Right from the start, it was clear something had gone horribly wrong. The flash from the detonation was unusually bright even by the standards of a nuclear weapon – so bright that observers reported seeing the shadows of bones in their hands. And so powerful was the thermal pulse that sailors on ships anchored 20 kilometres felt like they were being hit with a blowtorch, while the x-rays streaming through the light pipe incinerated the instrumentation station, creating a second, smaller fireball.

Meanwhile, the main fireball kept growing, and growing….and growing, until it seemed to engulf the entire lagoon. Buildings and instruments meant to survive the blast were incinerated, and entire islands swept clean. As the fireball continued to expand to a monstrous 7 kilometres in diameter, one sailor recalled that:

“We soon found ourselves under a large black and orange cloud that seemed to be dropping bright red balls of fire all over the ocean around us. I think many of us expected that we were witnessing the end of the world.”

Then came the shock wave, which was so powerful it nearly knocked a circling observation aircraft from the sky. On Enyu Island, 30 kilometres away from ground zero, the reinforced-concrete bunker protecting the bomb’s firing party shook so violently that its occupants feared they would be swept into the sea. When the noise and shock soon subsided, the observers were presented with the awesome sight of a white mushroom cloud 100 kilometres in diameter and towering 40 kilometres into the sky.

The yield of Castle Bravo was measured at an astonishing 15 megatons – triple what scientists had predicted and 1000 times more powerful than the bomb dropped on Hiroshima. This unexpected increase in yield resulted from a fundamental physics mistake. Scientists had assumed that the Lithium-7 isotope which made up the majority of the fusion fuel would simply absorb neutrons to form Lithium-8 and play no part in the fusion reaction. In reality, however, when bombarded by the high-energy neutrons from the primary, the Lithium-7 fissioned into an alpha particle, a neutron, and a nucleus of Tritium, generating significantly more fusion fuel and tripling the explosive yield.

But the consequences of this miscalculation went far beyond a few destroyed buildings and instruments. The oversized blast carved out a 2-kilometre wide crater in the lagoon floor – the rock, coral, and water from which was pulverized, drawn up into the fireball, and irradiated.This material then rained back to earth as vast amounts of highly-radioactive fallout. Even worse, the wind suddenly changed direction, so that rather than safely blowing north over open ocean, the fallout was instead carried east, creating a 450 kilometre long radioactive plume that engulfed the Navy support fleet as well as the inhabited atolls of Rongelap, Ailinginae, Utirik, and Rongerik. Unaware of the danger, native Marshallese children played in and even licked the radioactive dust, leading to hundreds of cases of radiation sickness and other health effects. Back on Bikini, the firing crew on Enyu found themselves trapped in the bunker by the fallout, forcing them to seal themselves inside for hours until a helicopter could be dispatched to rescue them. When it finally arrived, the crew ran out of the bunker holding blankets over their heads to prevent fallout from contaminating their bodies.

Also caught in the fallout plume was the Daigo Fukuru Maru or Lucky Dragon No.5, a Japanese fishing boat that was supposed to be outside the official exclusion zone. Blanketed in fine radioactive dust, the ship’s crew suffered severe burns and radiation sickness, with the radio operator later dying from his injuries. This incident severely strained diplomatic relations between the United States and the Japanese, who likened the disaster to a second Hiroshima and Nagasaki. In the wake of Castle Bravo, the U.S. military expanded the exclusion zone around the Pacific Proving Grounds to nearly 1.5 million square kilometres; however, this only served to disrupt the Japanese fishing industry, further escalating tensions.

Within 48 hours of the Castle Bravo test, the U.S. military evacuated the inhabitants of Rongelap, Rongerik, Ailinginea and Utirik to Kwajalein Atoll. However, this proved too little, too late for many Marshallese – especially the inhabitants of Rongelap, who received high doses of radiation and began suffering from high rates of thyroid and cervical cancer, birth defects, and other chronic health problems. In 1957 the U.S. Government declared Rongelap safe for human habitation and allowed 82 people to return. Unfortunately, many staple food items like arrowroot, coconut, and fish were found to be heavily contaminated with Plutonium and other radioactive isotopes, causing further long-term health problems. But while the inhabitants of Rongelap appealed to the U.S. Government to evacuate them back to Kawjalein, these pleas fell on deaf ears. This, along with a 1957 declaration by the US Atomic Energy Commission that “…the habitation of these people on the island will afford most valuable ecological radiation data on human beings,” led to widespread speculation that the inhabitants were being deliberately kept on Rongelap as radiological guinea pigs. Finally, in May 1985, international environmental organization Greenpeace launched Operation Exodus, using their flagship the Rainbow Warrior to relocate all 300 islanders and 100 tons of building materials to the islands of Mejato and Ebeye, 180 kilometres away. The following year, the U.S. Government approved the creation of a $150 million trust fund to pay compensation to the Marshallese victims of the Castle Bravo disaster. This fund, however, was quickly depleted, and despite repeated calls for the amount to be increased, in both 2005 and 2012 the U.S. Government asserted that it had paid sufficient compensation to the Marshallese and had no legal responsibility to pay more. The former inhabitants of Rongelap and other atolls contaminated by Castle Bravo remain in exile to this day, wondering when they will finally be able to return to their ancestral homes.

Meanwhile, the U.S. Government agreed to pay more than $15 million in compensation to the crew of the Daigo Fukuru Maru and their families, re-normalizing relations between the United States and Japan. However the Castle Bravo incident proved deeply traumatizing to the Japanese people – so much so that it inspired filmmaker Ishiro Honda to write and direct a film about the looming danger of nuclear annihilation, symbolized by a giant radioactive reptile rampaging across Japan. You may know it better as Godzilla.

Fallout from Castle Bravo drifted around the world, being detected as far afield as Australia, Europe, and the United States. But while this attracted international outrage and calls to ban further development of thermonuclear weapons, the strategic demands of the ever-escalating Cold War pushed the U.S. Government to press on with the hydrogen bomb project. As with the earlier Ivy Mike test, scientists prepared a contingency weapon to provide stopgap thermonuclear capability in case the dry-fuel bomb took longer than expected to perfect. Known as the Mark 16 or “Jughead”, the bomb was effectively a scaled-down version of the Mike device, using cryogenic liquid deuterium as its fuel. Measuring 1.5 metres in diameter and 7.5 metres long, weighing 19,000 kilograms, and with an estimated yield of 5 megatons, the Mark 16 could only be carried by a specially-modified Convair B-36 Peacemaker strategic bomber fitted with dewar flasks to keep the deuterium topped up. However, the success of Castle Bravo made this cumbersome weapon unnecessary, and it was withdrawn from the U.S. stockpile after only a few months.

The first proper thermonuclear device to be officially deployed by the United States was instead the Mk. 14 bomb, which was first tested on April 26, 1954 in the Castle Union shot with a yield of 7 megatons. However, the Mark 14’s secondary stage was fuelled by 95% enriched Lithium-6 deuteride, which was extremely expensive to refine. Since the Castle Bravo shot had demonstrated that the much cheaper Lithium-7 isotope was just as effective, only five Mark 14s were built and deployed before being replaced in 1956 by the 15-megaton Mark 17 and Mark 24, the world’s first mass-produced thermonuclear weapons.

The urgency with which these weapons were deployed was a response to alarming developments in Soviet nuclear technology. On August 12, 1953 – none months after Ivy Mike but seven months before Castle Bravo – the Soviets detonated a 400 kiloton device codenamed RDS-6S at the Semipalatinsk Test Site in Kazakhstan. The Soviet Government announced that the device was a “dry” thermonuclear weapon which, unlike the building-sized Ivy Mike, was small enough to be carried by aircraft. But while this supposed tipping of the thermonuclear scales alarmed U.S. military planners, the Soviet claim was not quite accurate.

The Soviets first learned of the hydrogen bomb concept through documents supplied by Manhattan Project physicist and atomic spy Klaus Fuchs. Unfortunately, these documents described very early concepts for the “Classical Super,” and Fuchs was arrested in 1950 before the crucial Teller-Ulam breakthrough was made. The Soviets therefore had very little to go on when developing their own hydrogen bombs. However, they had their own Edward Teller in the physicist Andrei Sakharov, who independently worked out all the same design principles as his American counterpart. Sakharov’s “first idea”, which he dubbed the Truba or “pipe”, was very similar to Teller’s initial concept for the Super, consisting of a regular fission bomb attached to a long cylinder filled with deuterium and tritium – though Sakharov replaced Teller’s lightweight Beryllium tamper with one made of heavier Uranium-238 in order to increase compression of the fuel. However, like Teller, Sakharov soon realized that this configuration was not feasible, and instead came up with a “second idea” similar to Teller’s abandoned “alarm clock” design, wherein alternating layers of Uranium 238 and Lithium-6 Deuteride were wrapped around a fission bomb’s core. Dubbed Sloika after a type of Russian layer cake, it was this design and not a proper thermonuclear weapon which was detonated in the 1953 RDS-6S test. Nonetheless, sloika allowed the 40 kiloton yield of the primary to be boosted tenfold, giving the Soviets a powerful – and more importantly – deployable – new weapon.

But while a step in the right direction, the sloika design was bulky and could not be scaled up further. But Sakharov soon discovered the principle of staged radiation implosion and developed his “third idea” – effectively a version of the Teller-Ulam design. The first true Soviet thermonuclear weapon, codenamed RDS-37, was tested on November 22, 1955, with a yield of 1.6 megatons. The Soviets had finally caught up with the Americans in the thermonuclear race.

What followed was an intense three-year period of testing in which the United States and Soviet Union detonated a total of 177 nuclear warheads – 40 of them thermonuclear. This period also saw the United Kingdom become a thermonuclear power, detonating its first hydrogen bomb, codenamed Grapple X, over Christmas Island in the South Pacific on November 8, 1957.

All this atmospheric testing, however, raised public fears over the possible global health effects of fallout, especially when high levels of the radioactive isotope Strontium-90 were detected in milk and baby teeth across the United States. This prompted Nobel laureate Linus Pauling and 9,000 scientists across 43 countries to sign a petition calling for the end of nuclear testing. While Edward Teller and others tried to downplay the fallout risk, in October 1958 the United States, Soviet Union, and United Kingdom agreed to a moratorium on atmospheric nuclear testing. Instead, testing moved underground, with the United States conducting its first underground weapons test – codenamed Plumbbob Rainier – on September 19, 1957, at the Nevada Test Site.

But if anti-nuclear activists hoped that the moratorium would immediately lead to a comprehensive test ban – or a reduction in nuclear weapons stockpiles – they were to be bitterly disappointed. Throughout the moratorium, both American and Soviet nuclear stockpiles continued to grow, while innovation in warhead size, yield, and efficiency stagnated due to lack of testing. It was only a matter of time before the uneasy truce was broken.

In the end, it was the Soviets who broke the moratorium – in the most spectacular fashion possible. On July 10, 1961, Soviet leader Nikita Khrushchev summoned Andrei Sakharov and other nuclear physicists to the Kremlin and instructed them to resume nuclear testing in the fall. Sakharov, who by now harboured serious doubts as to the morality of thermonuclear weapons, protested that further testing was not necessary, as he and his colleagues had already learned all they needed to know about nuclear weapons design. Khrushchev was furious, barking that:

“Sakharov, don’t try to tell us what to do or how to behave. We understand politics. I’d be a jellyfish and not Chairman of the Council of Ministers if I listened to people like Sakharov!”

In reality, Khrushchev was in a delicate political position. Not only had he just survived an attempted coup by pro-Stalinist hardliners, but the Soviet Union lagged far behind the United States in its nuclear arsenal. The resumption of nuclear testing was Khrushchev’s attempt to look strong to both his countrymen and on the world stage. Thus, it was to his delight that one of the physicists in the meeting suggested building and testing a 100-megaton bomb as the ultimate demonstration of Soviet nuclear power. Khrushchev immediately seized upon the idea, declaring:

“Let the 100-megaton bomb hang over the capitalists like a sword of Damocles!”

Though officially designated Product 602, the 100-megaton bomb later became known as the Tsar Bomba or “Emperor Bomb”, in the tradition of other giant Russian objects like the giant bronze Tsar’-pushka cannon and Tsar’-kolokol bell on display at the Kremlin. 2 metres in diameter, 8 metres long, and weighing 27,000 kilograms, the Tsar Bomba could only be carried by a specially-modified Tupolev Tu-95 Bear strategic bomber, which had its bomb bay doors removed so the bomb hung mostly outside the fuselage. The bomb was a three-stage device, with a combination first and second fusion stage on one side of the fission primary and a third stage on the other side, each module contributing 50 megatons of energy for a total of 100 megatons. Sakharov, however, soon grew concerned about the massive amounts of fallout the bomb would produce, and ordered the Uranium tamper in the third stage replaced with one made of inert Lead, cutting the yield in half.

Khrushchev revealed the existence of the Tsar Bomba on October 17, 1961 in a speech to the 22nd Congress of the Communist Party, announcing:

“Since I have digressed from the prepared text, I might as well say that the testing of our new nuclear weapons is going on very successfully. We shall complete it very soon—probably by the end of October. We shall evidently round out the tests by exploding a hydrogen bomb equivalent to 50 million tons of TNT. We have said that we have a bomb as powerful as 100 million tons of TNT. And we have it, too. But we are not going to explode it, because, even if exploded in the remotest of places, we are likely to break our own windows. We will therefore not do it yet. But by exploding the 50-million bomb, we shall test the triggering device of the 100-million one. However, God grant, as people said in the old days, that we never have to explode those bombs over any territory. That is our fondest dream.”

Though this announcement was immediately condemned by the international community, the test went ahead as planned. On October 30, 1961, the modified Tu-95V took off from Olenya airfield on the Kola Peninsula and flew north towards the test site on the arctic island of Novaya Zemlya. At 11:32 AM the bomb was released at an altitude of 10.3 kilometres, whereupon the pilot, Major Andrei Durnovtsev, immediately pulled his aircraft into a sharp bank. The bomb was fitted with a parachute to slow its descent, allowing the aircraft to fly 45 kilometres before detonation – a distance which, according to calculations, would give the crew a 50:50 chance of survival.

Around a minute after release, the bomb detonated at an altitude of 4.2 kilometres, lighting up the sky with a blinding flash that lasted for nearly a minute. As one cameraman filming the test later recalled:

“The clouds beneath the aircraft and in the distance were lit up by the powerful flash. The sea of light spread under the hatch and even clouds began to glow and became transparent. At that moment, our aircraft emerged from between two cloud layers and down below in the gap a huge bright orange ball was emerging. The ball was powerful and arrogant like Jupiter. Slowly and silently it crept upwards … Having broken through the thick layer of clouds it kept growing. It seemed to suck the whole Earth into it. The spectacle was fantastic, unreal, supernatural.”

Another witness remarked that “…it was as if the Earth was killed.”

Within seconds, the fireball expanded to nearly 10 kilometres in diameter – large enough to engulf all of midtown and downtown Manhattan – but was prevented from reaching the ground by its own reflected shock wave. This shock wave caught up to the TU-95V at a distance of 115 kilometres, causing it to drop 1 kilometre in altitude. However, the aircraft survived and made a safe landing back at Olenya. Ten minutes later, the fireball had given way to an enormous mushroom cloud 96 kilometres in diameter and 67 kilometres tall. At 58 megatons, it was the largest manmade explosion in history. The flash was observed more than 1,000 kilometres away in Norway, Greenland, and Alaska; the mushroom cloud could be seen 800 kilometres from ground zero, and the shock wave shattered windows 780 kilometres away. Even before the test’s success was officially announced, it was detected by seismographs and other instruments all over the world, the seismic and atmospheric pressure waves from the blast having circled the globe three times. Interestingly, the Tsar Bomba was also one of the “cleanest” nuclear weapons ever tested, with 97% of the total yield coming from nuclear fusion alone.

Unsurprisingly, the international reaction to the Tsar Bomba was one of outrage and condemnation. Strangely, however, the United States’ reaction was more subdued and dismissive, with an official White House statement declaring that:

“There is no mystery about producing a 50-megaton bomb. … The United States Government considered this matter carefully several years ago and concluded that such weapons would not provide an essential military capability.”

U.S. military planners argued that not only was the Tsar Bomba unwieldy to deploy, but that such high-yield bombs made for inefficient weapons. This is because blast damage scales according to the cubic root of a weapon’s explosive yield, meaning a 100 megaton bomb inflicts only around twice the damage of a 10 megaton bomb, despite being around 10 times heavier. Thus, it is far more efficient to shower a target with multiple, lower-yield warheads than one massive one.

Yet despite such dismissive statements, in the wake of the Tsar Bomba test, the United States seriously considered developing its own high-yield bombs to attack deeply-buried targets like mountain command bunkers. This led to the development of the Big Test Vehicle or BTV and the Flashback, which were even larger than the Tsar Bomba and had planned yields of between 50 and 100 megatons. However, no realistic role could be found for such monster weapons, and both projects were quickly cancelled. To date, the most powerful nuclear weapon ever deployed by the United States is the Mk. 41 bomb, which was introduced in 1961 and had a nominal yield of 23 megatons.

While the Tsar Bomba heralded the resumption of atmospheric nuclear testing, this period was thankfully short-lived. In the wake of the 1962 Cuban Missile Crisis, which brought the world to the brink of nuclear armageddon, on August 5, 1963, the United States, Soviet Union, and United Kingdom signed the Partial Nuclear Test-Ban Treaty, banning all nuclear testing on the ground, underwater, and in space. Underground testing, however, continues to this day.

Today, there are nine nuclear-armed nations: the United States, the Russian Federation, the United Kingdom, France, the People’s Republic of China, India, Pakistan, the Democratic People’s Republic of Korea, and Israel. Between them, they hold some 19,000 nuclear warheads – the majority of them thermonuclear or boosted-fission weapons – with a combined yield of some 4 gigatons of TNT – enough to destroy the world several times over. On witnessing the test of the first atomic bomb, J. Robert Oppenheimer famously said to have declared “Now I am Become Death, the Destroyer of Worlds.” But it was Edward Teller, Andrei Sakharov, and all the others who cracked the secret of the hydrogen bomb who truly gave mankind the power to destroy themselves. A power we will hopefully never use.