In Issue #31, the Edge ran a "Reality Check" concerning explosions. An anonymous reader noticed a glaring discrepancy in our physics and emailed us the following response. (P.S. If the author is out there, send us your real name so we can credit you.)

In SHADIS #31, Rob Vaux (with help from others) made the claim that "...space just doesn't allow for all those cool Star Destroyer explosions." Unfortunately, this is incorrect and his subsequent explanation perpetuates several fallacies about space.

Webster's New Collegiate Dictionary defines "explode" in two ways that apply to the situation under discussion: "2 a: to undergo a rapid chemical or nuclear reaction with the production of noise, heat, and violent expansion of gases.... b: to burst violently as a result of pressure from within."

Let's examine the second case first. Flash heating of either the ship's atmosphere or, more likely, the ship's water supply could result in a catastrophic pressure increase within the ship, and a resulting explosion. One possible cause of this sort of explosion would be a "catastrophic reactor excursion" (melt-down) resulting in one or more steam explosions. If enough energy is liberated fast enough, the ship's internal pressure could rise more rapidly than weak spots in the ships hull could fail. The result would be quite a spectacular explosion of the ship.

Another cause of this sort of explosion could be the liberation of electrical energy. When an electrical circuit with lots of energy is shorted (say by a missile bashing in the side of the ship) the energy will all be dumped into whatever is shorting the circuit. If enough energy is dumped, it can result in vaporizing the short, sometimes in a very short time. The resulting gases will be quite energetic and can cause an impressive explosion, with sparks and a tremendous shock wave.

Clearly, a vacuum would be no impediment to a nuclear explosion caused by fusion or fission warheads on missiles. Neither requires an atmosphere of any sort. That just leaves rapid chemical reactions as a cause of explosions. There is in fact a category of explosive which require an atmosphere. Suspensions of fluids or solids in air (fuel-air explosives or dust explosions as in grain elevators) are the cause of some intentional or unintentional explosions. Most explosives, however, require no atmosphere. Among chemical "explosives", there are two broad types.

In the first, there are two or more components which combine to form a new compound and at the same time liberate quite a lot of energy. This type normally requires confinement to build up pressure to the point where an explosion occurs. Into this class fall many rocket fuels and early gunpowders. One of the components of this sort of mixture is an "oxidizer" (accepts electrons) and one is a "reducer" (donates electrons).

Oxygen is a fairly good oxidizer (hence the name), but free gaseous oxygen does not normally work in propellants or explosives, as it is too diffuse, the only exception is in the suspensions noted above. Nor is oxygen the only possible oxidizer. As an example, hydrogen and fluorine (reducer and oxidizer respectively) will liberate a great deal of energy when brought into contact with each other. If the resulting energy is confined, a quite satisfactory explosion will result.

The second type of explosive is a compound which is stable under normal conditions, but which becomes unstable under the proper stimulus. When it becomes unstable, it breaks up into other compounds which are more stable and liberates energy. Examples include trinitrotoluene (TNT), nitroglycerine, torpex, and any number of other explosives. These also don't require an oxygen atmosphere to explode. In fact, the explosion propagates so quickly that even in an atmosphere were available, none of the atmospheric gases could possibly reach the site of the explosion before the reaction finished.

Thus missiles, bombs, or even hand grenades will work just fine in a vacuum, though not quite as well as they would in an atmosphere. Since there is no medium to conduct their shock waves, the resulting concussion will not be as impressive. On the other hand, fragmentation weapons will actually be more lethal. With no atmosphere to slow the particles down and no gravity to change their trajectories, a grenade could be capable of killing people at very great distances. The effect would also be increased because of the problems caused by perforation of environmental enclosures (venting suit or ship atmosphere into space). As far as sound goes, there would actually be perceptible sounds from explosions in the distance in space. The sound would travel at the same speed as the shock wave, which would probably be faster than the sound would travel in atmosphere. The absence of an atmosphere to diffuse the wave front would result in a short thump or crack as the fairly sharp wave front passed the listening ship. If the engagement took place at tens, hundreds, or thousands of kilometers, the sound would be so diffuse as to probably be completely inaudible, but at those ranges, ships would at best be tiny points of light anyway. If you can see it well enough to pick out details on a movie or TV screen, it's probably close enough to hear the shock wave pass.