Mark 4 decoy

The Mark 4 decoy was a single-use deployable anti-plasma countermeasure, first used by the United Nations Space Command Navy during the later years of the Remnant War. The decoy consisted of an unguided, roughly cylindrical pre-fragmented steel canister, shaped to reduce radar cross-section, containing a central 450 kilogram warhead and a torus-shaped capsule. The decoy was magnetically ejected from the AN/SLE-311 decoy launcher, directed towards incoming guided plasma bolts, or 'torpedoes'. The decoy was remote-detonated with a backup proximity detonation fuze, optimally within 500 metres of the target torpedo. The detonation would destroy the internal capsule and explosively disperse its contents, a 'dust' composed of carbon nanosphere particles, upon which the torpedo would prematurely detonate. The system, comprising shipboard guidance, launcher and decoy, was said to be more than 97% effective under optimum conditions.
The AN/SLE-311 decoy launcher operated essentially as a specialised railgun, ejecting aimed decoys at a relatively low speed of three kilometres per second. In order to keep costs low, as the decoy was a single-use device, the Mark 4 was unguided and not capable of self-propulsion. This was offset by its use as a short range device, its high linear speed, and an effective range sufficient enough to intercept a single torpedo with reasonable margin of error. Optimium effectiveness was achieved within 500 metres, but plasma torpedoes could be detonated or destabilised within a kilometre of the decoy detonating. Initial target prediction, aiming and firing was carried out by automated systems under direction of the shipboard AI.
Carbon nanospheres were the chosen active component as they functioned similarly to titanium dust utilised in older systems, while providing less of a radar return, reducing disruption to UNSC radar and detection by the enemy. As plasma torpedoes were guided from the launching vessel, often from extreme range, it was virtually impossible that the decoy could be detected before intercepting the torpedo. The decoy did not incorporate stealth materials but, due to its radar cross section-minimising design, unpropelled and unguided nature and comparably tiny size, the decoy was virtually immune to enemy detection before it intercepted a torpedo.
The Mark 4 decoy, along with the AN/SLE-311 launcher, finally saw the replacement of improvised anti-plasma countermeasures in UNSC service, that had been developed piecemeal during the closing years of the Human-Covenant War. The Mark 4, unlike the various systems it replaced, provided a widely used and dependable standard countermeasure system. The ancestor of the decoy's operating principle was born from experience gained at the , where a UNSC refit station was sacrificed to plasma torpedoes to spare the UNSC fleet. Early attempts at countermeasure systems, such as the Tactical Intercept Station, essentially a redesigned refit station specifically to absorb enemy plasma, were followed by more sophisticated systems, including the AN/SLE-159, a shield-projecting, self-propelling satellite.
The Mark 1 decoy, a prototype capsule in service by November 2552, consisted of an innaccurate explosively-ejected capsule containing titanium dust, and saw limited success during the . The Mark 2 was an abortive attempt at using an unfurling titanium lattice to intercept plasma torpedoes, though this proved too complicated and limited in unfurled range. The Mark 3, the first decoy to enter mainstream service, was introduced during the post-war period in 2557, supplementing and eventually superseding the Bulwark satellite. The Mark 4, redesigned to function with the new AN/SLE-311 magnetic launcher, entered service in 2561, being part of the Remembrance-class frigate's refit of that year, and later being fitted to all new and serving UNSC ships.
===Classes equipped===
*Marathon-class cruiser: refitted from 2563 onwards, which equipped it with six AN/SLE-311 decoy launchers, as well as improved reactors, low output shield generators and improved armour plating.
*Remembrance-class frigate: existing vessels were equipped with the AN/SLE-311 during their refit between 2561 and 2567, while it was added to newly produced vessels as part of the fitting out stage of their construction.
*Vengeance-class destroyer: existing examples of the class were equipped with three AN/SLE-311 from 2563 onwards, while newly produced vessels had been fitted with the launchers beginning the previous year.