The speed at which the sphere of a nuclear blast travels depends on several factors, including the size of the explosion and the surrounding atmospheric conditions. When a nuclear explosion occurs, it releases an immense amount of energy in the form of an expanding shockwave.
The initial expansion of the blast wave can occur at speeds of several thousand meters per second (m/s), which is roughly the speed of sound in air. This is known as the "incident shockwave." The exact speed depends on the energy yield of the explosion and the distance from the epicenter.
As the shockwave propagates outward, it decelerates due to the drag and interaction with the surrounding air. The rate of deceleration depends on various factors, such as the altitude, air density, and the type of explosion (airburst or surface burst). Generally, the shockwave slows down significantly as it moves farther away from the point of detonation.
It's important to note that the sphere of a nuclear blast is not a literal physical object that moves uniformly. Rather, it refers to the expanding front of the shockwave, which carries the energy and destructive force of the explosion. The shockwave interacts with the surrounding medium, causing damage and destruction through a combination of blast pressure, heat, and radiation.
To summarize, the initial expansion of the blast wave from a nuclear explosion can occur at speeds comparable to the speed of sound in air. However, the speed of the blast wave decreases as it propagates outward due to deceleration caused by atmospheric drag and other factors. The exact speed varies depending on the specific circumstances of the explosion.