The only known use of a photon grenade was against a group of Gorns who were attacking an away team from the USS Enterprise. The Gorns had destroyed a new Federation outpost on Cestus III, and they had lured Kirk and his senior officers to the surface, where they planned to ambush him while their ship attacked the Enterprise, which would be vulnerable without its captain.
Kirk found a photon grenade launcher in the ruins of the base and used it to fire a grenade at the estimated position of the Gorn troops. The weapon detonated in a bright flash, silencing the Gorn attacks; the Gorn ship quickly beamed up its landing party and warped away from the planet.
The exact nature of the weapon isn't clear, although its effects rule out a massive energy release: there was no concussion blast, nor was there any noticeable effect on the terrain. The flash from its detonation was very bright, suggesting it may have been intended to blind enemies in the target area. Other hypotheses are that it may have released an electromagnetic pulse to disable enemy equipment or released a wave of energy similar to that of a phaser set to stun in order to incapacitate enemy troops without seriously damaging the area.
Photon Grenades in Debates
Trektards try to use the following statement to generate large energy estimates for the weapon.
- KELOWITZ: If I were them, I'd go to the high ground on the right. I make it twelve hundred yards, azimuth eighty seven. It's pretty close for one of these little jewels, Captain.
They state that since the weapon was still a threat at 1200 yards, they can calculate the energy needed to elevate a hemisphere of that radius to an energy level that would be dangerous to humans. They ignore that sheer energy release is only one of many ways a weapon can be a threat.
- TOS "Arena"