Nuclear blackout

When those warheads emerge from the blackout area there may not be enough time for the defensive system to develop tracking information and attack them.

Later missile defense designs used radars operating at higher frequencies in the UHF and microwave region to mitigate the effect.

When a nuclear bomb is exploded near ground level, the dense atmosphere interacts with many of the subatomic particles being released.

This energy heats the air, promptly ionizing it to incandescence and causing a roughly spherical fireball to form within microseconds.

[2] Proceeding at a slower speed is the actual explosion, which creates a powerful shock wave moving outward.

The energy released by the shock wave is enough to compression heat the air into incandescence, creating a second fireball.

So, a 1 megatonne of TNT (4.2 PJ) bomb exploded at a burst altitude around 5,000 feet (1,500 m)[a] will expand to about 1 kilometre (3,300 ft).

[3] So the same burst at 50,000 feet (15,000 m) will be at a pressure of about 0.1 atmospheres, resulting in a fireball on the order of 2,150 metres (7,050 ft) in diameter, about twice the size of one near the ground.

[4] When the bomb is exploded outside the atmosphere, generally any altitude above about 100 kilometres (330,000 ft), the lack of interaction with the air changes the nature of the fireball formation.

In this case, the various subatomic particles can travel arbitrary distances, and continue to outpace the expanding bomb debris.

The first is due to the gammas, which arrive as a burst directly below the explosion and promptly ionize the air, causing a huge pulse of downward moving electrons.

These gammas and neutrons are the source of the nuclear electromagnetic pulse, or EMP, which can damage electronics that are not shielded from its effects.

These are constantly being created by the radioactive decay of the uranium tamper that surrounds the fusion core, so the magnitude of this effect is largely a function of the size of the bomb and its physical dispersal in space.

This causes a much larger but spread out current pulse of lower energy electrons released from these air molecules.

[9] When bound to atoms and molecules, quantum mechanics causes electrons to naturally assume a set of distinct energy levels.

In metals the energy levels are so closely spaced that the electrons in them will respond to almost any radio frequency photon, which makes them excellent antenna materials.

The latter so strongly refract radio waves that it forms a mirror-like surface when the electron density is above a critical value.

As the fireball radiates away energy and cools, the ions and electrons re-form back into atoms and the effect slowly fades over a period of seconds or minutes.

Since the fireball can expand to hundreds of kilometers at high altitude, this means that a typical attenuation of 1 dB per kilometer through a fireball at mid to high-altitudes which expands to 10 km will completely attenuate the signal, making tracking objects on the far side impossible.

This process takes as long as several minutes, and as there is less air at higher altitudes, the fireball remains ionized for longer periods.

[12] At higher altitudes, from 100,000 to 200,000 feet (30–60 km), the density of air is not enough to be a significant effect, and the fireball continues to cool radiatively.

[12] For purely exoatmospheric explosions, the betas causing the blackout disk are continually produced by the fission events in the bomb debris.

To create a complete blackout, with 109 free electrons per cubic centimeter, requires about 10 tons of fission products per square kilometer.

[13] Blackout is a special concern in missile defense systems, where the effect can be used to defeat ground-based radars by producing large opaque areas behind which approaching warheads cannot be seen.

[7] For short-range interceptors like Sprint, blackout is not a serious concern because the entire interception takes place at ranges and altitudes below where the fireballs grow large enough to block a significant area of the sky.

[7] Only an attack consisting of a few dozen large warheads would be significant enough to cause a short range interceptor to have a problem.

The conclusion from these tests was that the only solution to such an attack profile would be to use multiple radar systems netting them together, and selecting whichever one has the clearest view of the targets.

In this case, the missile was expected to be carrying out interceptions at ranges as great as 500 kilometres (300 mi), a distance that took some time to reach.

This means there is an advantage to using higher frequencies for search radars, as they will be able to resolve a given sized object, like a warhead or booster fragments, from a smaller antenna.

[7] This means that exoatmospheric explosions are very effective against long-range early-warning radars like PAR or the Soviet Dnestr.