Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


18.6  Nuclear Explosives

Nuclear explosives don‘t fit neatly into the above categories, for the simple reason that the blast products consist of virtually every brand of destructiveness discussed so far. About half of the energy of a thermonuclear device comes off as sonic energy, in the form of high pressure shock waves and gross mechanical vibration. The other half consists of electromagnetic radiation of all kinds, primarily x-rays but including ultraviolet, visible and radio emissions. Nuclear radiation, primarily fast neutrons, constitute a few percent of the total. Fallout comprises various amounts of poisonous radiochemicals. Nuclear weapons are very messy affairs.

Many fires are started at once by the flash of an H-bomb. The majority of casualties result, not from the effects of the blast itself, but by the firestorm sweeping the victim city. Only at the very outermost edges of the hellish thermal maelstrom are individuals able to drag themselves to safety (Table 18.4).

Out in space where there is no atmosphere to absorb and scatter radiation, the destructive range is vastly increased. It has been estimated that a 20 megaton thermonuclear device would be sufficient to kill astronauts in unshielded spacecraft within a sphere nearly 1000 kilometers in diameter. A lethal sphere the size of the Earth itself would require a 10,000 megaton bomb, "a weapon which probably could be built with today’s technology."561


Table 18.4 Thermonuclear Explosive Blast Effects
Thermal Ignition and
Second Degree Burns
Severe Structural
Blast Damage
50% Blast
1 kiloton
0.8 km
0.8 km
1.0 km
0.5 km
10 kilotons
1.3 km
2.3 km
2.3 km
1.3 km
100 kilotons
1.8 km
4.0 km
4.5 km
3.1 km
1 megaton
2.4 km
16 km
10 km
7.7 km
10 megatons
3.7 km
37 km
22 km
19 km
*yield is commonly measured in tons of high explosive (e.g. TNT)


Various aerospace magazines have given rough estimates of the state-of-the-art of H-bomb building technology -- roughly 45 kilograms of bomb weight for each megaton of destructive power.563 According to Theodore Taylor in The Curve of Binding Energy, a well-designed A-bomb could be built as small as a grapefruit. Thermonuclear devices could be disguised as color television sets! Cost? Ten kiloton bombs can reportedly be purchased (authorized customers only!) for $350,000, but two-megaton devices can be picked up for a paltry $600,000.573 Bombs are cheap.

The aliens may be cleverer than we imagine, however. They may choose to attack our nuclear power plants or our nuclear weapon depositories, and get us with our own stuff! Or they may turn to the N-bomb.

The neutron bomb is a clean, low-power H-bomb. Now, A-bombs use the principle of nuclear fission -- a mass of U-235 or plutonium undergoes a chain reaction explosion. In the H-bomb, fissile components are used to achieve the high temperatures necessary for the initiation of fusion reactions between deuterium and tritium. Explosive yields are further increased by wrapping the H-bomb in a jacket of U-238, which is highly fissionable only in the presence of high neutron fluxes.

On the other hand, the neutron bomb would be a pure fusion bomb, employing no dirty fission detonator. The N-bomb would release large numbers of 17 MeV neutrons over limited areas -- perhaps a few kilometers in diameter. These neutrons have high penetrating power, and produce secondary radiation by colliding with atomic nuclei. These secondary particles are responsible for the lethality of the neutrons. There would be no ordinary fallout, although the invading aliens might have to wait a short while before entering our cities. Buildings and land would be left virtually unscathed, although the passage of neutrons would make walls and metal slightly radioactive for a few days. The fusion of as little as one milligram of deuterium could produce enough neutrons to kill 100,000 humans even through one meter of solid concrete.

But an N-bomb is more than just a pure fusion bomb. Normal thermonuclear devices have such large blast effects that the neutron damage would be lost in the greater general destruction. The range of the neutrons depends on their energy, and this is relatively constant. Hence, the trick is to build a very low yield fusion bomb, in the kiloton range. If the N-bomb was of proper size, and was exploded at just the right height, it would spray the area with deadly neutrons without causing widespread firestorms and structural damage. The N-bomb would be an excellent tactical weapon for aliens who wanted us out of the way but wished to examine our undamaged artifacts at their leisure.

Of course, H-bombs can always be made as large as desired. Gigaton yield devices (1000 megatons) have been seriously discussed, and are usually referred to as "doomsday bombs." Robert Salkeld suggests that a gigaton bomb burst in our upper atmosphere would ignite all combustible substances (forests, buildings, humans) in a 600 kilometer circle below.561 Figures released by the Atomic Energy Commission in the early 1960's show that a one gigaton warhead detonated about 16 kilometers up could be expected to start fires over an area of more than 700,000 square kilometers.525

Others have estimated that 20-gigaton devices could be "salted" to make them dirtier. A bomb wrapped in sodium would release intense, quick-killing radioactive fallout with a half-life of only fifteen hours (24Na). If the wrapping were cobalt instead, the fallout (60Co, half-life 5.3 years) would kill very slowly, but would last a long time. Or both could be incorporated in a single weapon. As one writer dryly pointed out: "The sodium-24 would knock out those who didn‘t get into deep shelters, and the cobalt-60 would immobilize those who did."525

But the true doomsday bomb is the one which, if used, could destroy all human life on Earth. Physicist W.H. Clark, a nuclear weapons technologist, has estimated that at least 1000 gigatons of nuclear explosives would be required to wipe out the human race. Freeman J. Dyson, at Princeton, comes up with more conservative figures: 3000 gigatons to kill half the human race.525

The strongest, and certainly the most obvious, argument against doomsday bombs from a military standpoint is that the attacker is destroyed along with the attacked. But would this argument be persuasive for marauding off-world aliens intent on planetary destruction?


Last updated on 6 December 2008