In Space Weapons, Earth Wars, Preston and colleagues describe directed-energy weapons as:

• A. While they propagate energy at very high speeds—i.e., the speed of light—the speed of effect will be more pedestrian.
• B. They would be ideal weapons for space warfare, but they are prohibited by the Outer Space Treaty.
• C. They are ideally used on orbital platforms because atmospheric attenuation makes them ineffective when postured in the terrestrial environment.
• D. They cannot be used in space where there is no oxygen to carry the energy they generate.

Answer: (A)

Directed-energy weapons deliver energy at the speed of light, but the speed at which they cause damage—how quickly the target is actually affected—is much slower and depends on how the target absorbs and responds to that energy. When a laser or microwave beam hits a target, energy must be deposited into the material, raising temperature, causing melting or vaporization, or producing mechanical effects. That energy buildup and material response take time, so even though the beam reaches the target almost instantaneously, the observable damage unfolds over milliseconds to seconds depending on power, exposure duration, and the target’s properties.

The other statements misstate the situation in important ways. The Outer Space Treaty does not categorically prohibit directed-energy weapons, so claiming they’re banned isn’t accurate. While atmospheric attenuation is a factor for ground-based systems, the idea that directed-energy weapons are inherently ideal only on orbital platforms oversimplifies their tradeoffs and ignores challenges like power supply, aiming, and thermal management. And energy transfer in space does not require oxygen—the energy is carried by photons and interacts with the target’s material regardless of an atmosphere—so that last option is incorrect.

So the best description emphasizes that propagation is incredibly fast, but the damaging effect develops more gradually as energy is deposited and material responds.