Plasma Grenade

Blue Gas

After a Plasma Grenade is ignited and thrown, it begins to leak a densely small and bright cloud of plasma blue gas, believed to be between 60 to 75 degrees. This occurs due to the explosive reaction taking place inside.

This cloud will gradually grow to a larger diameter after each second of the countdown, and also causes a trailing effect, seen in all games.

Stickiness

The Plasma Grenade is a specially designed grenade that sticks to its targets. Once it hits a biological creature or a vehicle, the Plasma Grenade will immediately bond to it, and beep to tell you that it has stuck, making any removal impossible. The bonding mechanism holds its attraction, except to seemingly static objects, such as walls, until detonation, making it impossible to avoid its deadly explosion. Therefore, if you have been "stuck", it is a good idea to take out as many opponents around you to reclaim points. Although it will bond to any target it hits, if it attaches to a piece of armour or a weapon, removing said articles will allow one to distance themselves from the explosion. Once the Plasma Grenade has adhered to the target, it kills instantly. All one can do is to try and get as close to an enemy as possible, with the hope of killing them or at least damage them, thus leaving them vulnerable to any attack from another hostile. Although the Plasma Grenades can stick to the enemies or friendlies, in Halo: Combat Evolved, it can be seen clearly that the Frag Grenade is much more powerful, but due to the enemy or friendly being stuck the explosion is close range thus the enemy being stuck has a higher chance of dying. It can easily be identified by its blue glow.

The "sticking" mechanism is complex enough to allow the Plasma Grenade the ability to distinguish between targets and inanimate objects(the technology behind this is unknown, although it is thought to be vaguely Forerunner). For example, it will stick to a soldier or a vehicle, but not a tree or a wall etc. This does not, however, allow the grenade to act selectively. The Plasma Grenade will not distinguish between friend or foe so accuracy and caution is strongly advised when throwing it, so as not to betray your allies.

Theories

Note: The following is speculation and theory.

Sticking to Targets

The Plasma Grenade may stick by producing a thin layer of plasma which, when in contact with a target, fuses to the target. The reason the Plasma Grenade sticks to vehicles and characters may be due to an ability to sense body heat and the heat from vehicles' engines. Since a "dead" wall doesn't emit heat, it will, however, stick to the "living" walls of a Flood Hive. One problem with this theory is that it will stick even to exposed skin (on Grunts, Jackals, or Marines as an example) even though there is no metal to fuse to - it would be nearly impossible to fuse the grenade to flesh using heat alone.

Another theory is that when the grenade is thrown it will be covered by a thick layer of extremely hot plasma that when stuck to anything, will burn into the object, sticking there, and then explode.

It is also theorized that there is a complex internal mechanism built into the grenade that gives it this characteristic. A gravity generator may be built into the core of the device, giving it its own force of attraction. This artificial force of attraction is highly concentrated, which is exactly why the grenade is impossible to remove.

Another theory is that the Plasma Grenade has a energy shield hybrid shell to bounce off of walls and an internal magnet to pull it to vehicles and armour.

Yet another theory states that the Plasma Grenade detects low-level and medium level electrical pulses that are in living things and that are in shield and vehicle technology. Jackal Gauntlet Defense systems emit a high rate of energy, enough to repel Human Projectiles, while the MJOLNIR Armour, Elite Combat Harness, and Brute Combat Harness all are damaged by projectiles. This could mean that the Plasma Grenade cannot stick to a device emitting high level electrical pulses, such as the ones that are in Jackal Gauntlets, but has sufficient energy to stick to low to medium power energy shields as well as vehicles. This would also explain why Brute Chieftains cannot be stuck, as their armour has extremely powerful energy shielding, albeit the fact that it can still be damaged by human projectiles.

Another theory is that the grenade has a small A.I. that can allow it to understand what and what not to stick to. This would allow it to stick to any kind of flesh material and also let it to stick to the walls of the level Cortana. This would also allow it to stick to vehicles.

Sticking to Throwers

An interesting thing to note is the fact that when a Plasma Grenade is primed, it does not stick to the thrower. A theory for this could be that the grenade must be primed touching something emitting heat, and then leaving the heat-emitting object, thus activating its "stickiness", and finally landing and sticking to another heat-emitting object (i.e. the target). This is even better proven by the fact that it will stick to the person who threw it if it bounces back at them (such as hitting a wall).

It's possible that the grenade has an adhesive activated by a timer. Once the grenade is activated, this timer is initiated, giving a few seconds for the thrower to release the grenade. Once the timer has run out, the adhesive activates and the surface of the grenade is made as sticky as glue by the activation of the adhesive, although this theory doesn't explain the environment/character principles of the grenade.

It is also possible that it will not stick to the thrower because the user may have to hold down the activation switch and apon releasing the switch it will become sticky.

Detonating Timer

The Plasma Grenade possesses a timer. It has a three-second fuse that counts down at the point of activation. A manual activation via a small button or a stimulated activation, such as another explosion or gunfire, will set the detonator and start the countdown. Delayed activation can also set the detonator after it either sticks to a target or otherwise comes to a rest. It is possible to tell if the grenade has been activated. A densely small and bright cloud of plasma emission will leak around the grenade as the explosive reaction takes place inside. This cloud will gradually grow to a larger diameter after each second of the countdown. A low sound is emitted by the grenade when activated, followed by two rapid beeps and a high-pitched whine just seconds before the explosion. This is the Plasma Grenade's standard process of detonation.

The Plasma Grenade appears to have two settings: the standard three-second fuse and a delayed manual detonation. The latter is seen in Halo 3 by suicidal Grunts hoping to ensure their enemy's death by exploding right next to them. In addition, the Plasma Grenade can be destroyed by being shot (Easily seen with the Sentinel Beam).

Effects on Health

It has been shown that prolonged exposure to the radiation emitted by Plasma Grenades, without proper protection, can cause a deviation in the neural electric pathways of the user, a disorder called Boren's Syndrome. This disease can be fatal if the person does not get treatment. Sergeant Avery Johnson is the only known living person to have Boren's Syndrome. This explains why he was immune to infection by the Flood during the attack on his team on Installation 04 while trying to extract a weapons cache. The reason for this is because the Flood cannot copy the organisms nerve endings because they are "messed up" (as said by Master Chief in Halo First Strike), so the host never gets infected, although it can still be killed by one because of attack. Boren's Syndrome is not simply immunity to the Flood, it is known to cause mental instability as well as other things, making it impossible as a weapon of self defense against the Flood (except in the case of Staff Sergeant Avery Johnson who, for unknown reasons, seems to be immune to the side-effects, even though he rejected the treatment and cure).