Did US know about the destructive power of the atomic bomb?

Answer 1

Absolutely. In fact, there was actually much fear that the atomic reaction would continue in a chain reaction that would consume the planet. And yet they dropped one anyways; They should have dropped the polititians out of the bomber instead.

The whole "atmosphere will catch on fire and destroy all life" probability is a good example of something being blown wildly out of proportion. Historical research (and interviews withe people involved) revealed that this was merely a possibility floated during private discussions amongst the scientists, and was discounted as an extremely unlikely event; it was never mentioned as a possibility to the military or political leadership.

As far as the actual knowledge of the destructive power of the atomic bomb, most initial estimates of the explosive power for the Trinity Device were in the single-digit kilotons (the scientists' betting pool on the yield showed the expected yield would be 1-2 kT, and Robert Oppenheimer put money on well under 1kT), as there was significant concerns around the "pre-ignition" problem. The actual yield of the Trinity Device was around 20 kT. The test was considered a great success; however, the actual results were not significantly unexpected by scientists.

Other than blast effects, the thermal effects were generally expected and understood, but the radiation effects were very poorly understood, even after the Trinity test.

While the general blast and thermal destructiveness of the atomic bomb was well understood by those attending the Trinity test, this knowledge was not well grasped by others not in attendance. Truman and other members of the military not specifically involved with the Manhattan Project did not seem to grasp the true nature of the atomic bomb; much of this lack of comprehension is understandable, as the a-bomb was a truly revolutionary weapon, and those without direct evidence can't be faulted for their lack of apocalyptic vision as to its effects.

In fact, several very senior U.S. military leaders (including General Marshall, and Admirals King and Nimitz), who, while informed of the weapon's capability, later recorded that the actual power of it was considerably different than they assumed (or imagined).

So, the best answer to this question is this: after the Trinity Test demonstrated the actual operational power of an atomic weapon, the destructive power of such a weapon was well understood by the scientific community of the Manhattan Project, but was only vaguely understood by the upper echelon military and political leadership responsible for the actual use of the weapons.

Think of it this way: up until then, the largest explosive bomb was about 10,000 kg, or 10 tonnes. When Truman was informed of the existence of the Manhattan Project, and the subsequent success of the Trinity test, he was told it was a weapon of "immense power". He can be forgiven if he did not grasp that this meant something THREE ORDERS OF MAGNITUDE (2,000 times) of anything done up until that time. That is simply an unimaginable leap in power; indeed, the atomic bomb is a unique event in human military history; at no other time was such a quantum leap in destructive power made via a single invention. The verbal and written descriptions of the Trinity test to the political and military leadership simply was not suitable to impart the truly revolutionary power of these weapons.

For comparison, the increase in explosive power was equivalent to taking a modern 1000 lbs bomb, and cramming that explosive power into something the size of a hand grenade.