MJOLNIR armor is a subset of powered armor worn by the Spartan-II super soldiers in the Halo universe. Most recognizable as the armor the Master Chief wears in three of the Halo games, MJOLNIR armor comes in several colors. MJOLNIR armor is a technological marvel, but can only be deployed to SPARTAN units, because, regular soldiers do not have the strength or reflexes required to operate the suits.
Early MJOLNIR Powered Armor Systems
The earliest incarnations of MJOLNIR armor were more like powered shells than actual armor that a soldier would wear into battle. They were not internally powered, requiring them to remain stationary in order to maintain a constant connection to an outside power source. This greatly limited their mobility and functionality, and they were further hampered by a limited armament. Although the armor featured a high-powered cannon at its disposal, this was a mounted weapon and could not be replaced in the middle of battle, should the need arise. These design flaws made it necessary to develop new systems that were better suited for a live combat situation, as these were never fit for direct warfare. Out of this necessity came the new series of internally powered and combat-ready armors seen in the video games. The early MJOLNIR prototypes were eventually developed into the UNSC Cyclops, a large mechanized suit capable of being operated by normal Marines. The UNSC Spirit of Fire was outfitted with a contingent of Cyclops' and they are a usable unit in Halo Wars.
This was the first MJOLNIR prototype to be tested on SPARTAN-II soldiers. It greatly enhances speed, strength, and reaction times. However, this variant of the MJOLNIR armor did not have self-regenerating energy shields, as the technology was not feasible at the time the Mark IV was developed. The Mark IV armor can be seen on the Spartans in Halo Wars. The MJOLNIR suits in Halo Wars are depicted as having energy shields, but this is likely a gameplay conceit to make the Spartans more similar to the Master Chief and the main Halo franchise. Spartan Gray Team was also depicted as using this armor in the novel The Cole Protocol, and may have continued using it past the advent of the Mark V as they were deployed too deep into enemy territory to be refitted easily.
The Mark V armor had many improvements over the Mark IV, it's most notable new feature being the addition of self-regenerating energy shields, re-engineered from captured Covenant shield technology. The Mark V also allowed for a neural interface with an AI, which throughout the games has been Cortana. The AI neural interface allowed for quicker reaction times and advanced situation assessment in battle, as the AI can preform many more calculations per second than even a SPARTAN-II can, using the suit's various sensory equipment. All SPARTAN-II's had been assembled on Reach to be re-equipped with Mark V MJOLNIR suits, which were to be utilized during a stealth assault on the Covenant home world. However, the Covenant fleet began attacking Reach as they were making preparations, which means that the Master Chief had only recently been equipped with a Mark V suit prior to the events of Halo: Combat Evolved. All other operational SPARTAN-II's, besides Gray Team (who were on a distant battlefield at the time of the upgrade), are presumed to still be using this variant.
Pieces of this suit are available as armor permutations in Halo 3. The Mk. V also appears in Halo: Reach, being worn by members of Noble Team, a group composed of 5 SPARTAN-IIIs and one SPARTAN-II. SPARTAN-III's usually wear Semi-Powered Infiltration (SPI) armor, which is capable of camouflage and much cheaper to produce than MJOLNIR. Noble Team, however, was an elite group and was thus issued the more powerful MJOLNIR armor. They were, however, not issued the latest version of the Mark V that the Chief has as they lack the ability to interface with an AI which is why Cortana's fragment had to be transferred in a "package" by Noble Six in Halo: Reach.
The Mark VI armor variant followed the Mark V, but was only ever given to the Master Chief because it had just finished testing before the Covenant's invasion on Earth. The Chief was equipped with this armor at the beginning of Halo 2, during the level The Armory. The Mark VI suit includes several incremental improvements to the Mark V, such as improved jump height, faster regenerative shields, and zoom functionality built into the heads-up display. The suit also has an internal mechanism which injects the wearer with biofoam upon detecting injury, explaining the lack of a health meter in Halos 2 and 3. The Mark VI suit can also track the status of two equipped weapons simultaneously, thus making an explanation as to why dual-wielding capabilities were present in Halo 2 but not its predecessor.
The Master Chief wears the Mark VI suit throughout all of Halo 2 and Halo 3, and it is the standard armor permutation for online multiplayer.
The specs, outlined by Dr. Halsey in her journal, included AI transfer protocols, atmospheric insertion and slipspace de-insertion capabilities, and shaping of energy shields. In the novel Halo: Glasslands Naomi-010, a SPARTAN-II who survived the Human-Covenant war was issued the Mark VII armor in February 2553, however, the only details offered were that it was lighter than the Mark VI, it is not known if it has any of the features outlined by Halsey. While it's yet to appear in a game it's very possible it could, at some point, appear in 343 Industries' "Reclaimer" trilogy, which begins with Halo 4.
Halo 3 Multiplayer Armor Permutations
Helmet - Default Unlocked
Shoulders - Default Unlocked
Chest - Default Unlocked
Close Quarters Battle
Helmet - Default Unlocked
Shoulders - Default Unlocked
Chest - Default Unlocked
Helmet - Unlock Achievement for completing Campaign Mode on at least Normal Difficulty
Shoulders - Complete the level 'The Ark' on at least Normal Difficulty
Chest - Complete the level 'Tsavo Highway' on at least Normal Difficulty
Explosive Ordnance Detection
Helmet - Unlock Achievement for completing Campaign Mode on Legendary Difficulty
Shoulders - Complete the level 'The Ark' on Legendary Difficulty
Chest - Complete the level 'Tsavo Highway' on Legendary Difficulty
Helmet - Collect all Skulls in Campaign Mode
Shoulders - Collect at least 9 Skulls in Campaign Mode
Chest - Collect at least 5 Skulls in Campaign Mode
Helmet - Unlock all Achievements in Halo 3
Shoulders - 750 / 1000 Gamerscore in Halo 3 (Due to inconsistencies this may require as much as 800 Gamerscore in some cases)
Chest - No Security Chest Plate Exists
Orbital Drop Shock Trooper
Helmet - Unlock the 'Spartan Graduate' Achievement (online)
Shoulders - No ODST Shoulder Pieces Exist
Chest - No ODST Chest Plate Exists
Helmet - Unlock the 'UNSC Spartan Achievement' (online)
Shoulders - No Mark V Shoulder Plates Exist
Chest - No Mark V Chest Plate Exists
Helmet - Unlock the 'Spartan Officer' Achievement (online)
Shoulders - No Rogue Shoulder Plates Exist
Chest - No Rogue Chest Plate Exists
Not obtained within the confines of the game, a player must be sought out by Bungie to obtain this extremely rare armor. It will be available to players who obtain all the vidmaster challenge achievements after the release of Halo 3: ODST
Helmet - Unlock the 'Used Car Salesman' Achievement
Shoulders - Unlock the 'Mongoose Mowdown' Achievement
Chest - Unlock the 'Too Close to the Sun' Achievement
Halo: Reach Armor Permutations
Armor customization in Halo: Reach is far more flexible and complex than that of Halo 3. Customization takes place in the Armory, where players may purchase armor components using credits (cR) they have earned by completing campaign missions, firefight rounds and multiplayer matches, and well as completing achievements and challenges and garnering commendations. The new customizable features are Wrist, Utility, Visor Color, Knee Guards, Armor Effect, and Firefight Voice. Players' gender choice is now reflected in the physical appearance of the Spartan instead of only in voice. Players are no longer permitted to use Elites as their default player model, possibly due to gameplay imbalances.