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Project THERMOPYLAE was a United Nations Space Command project running concurrently with the SPARTAN-II and SPARTAN-III programs, with the aim to develop effective prosthetics for Spartan-grade supersoldiers.
Knowing the missions for which her supersoldiers would be used, Doctor Catherine Halsey was aware of the risk that the spartans would, sooner or late, lose body parts in the line of duty, and thus set out to create prosthetics that would be able to mimick - and in some areas improve - the capabilities of Spartans' lost body parts. She also hoped that it would allow for crippled spartans to get back up to combat-ready status. Her studies were approved by HIGHCOM and codenamed THERMOPYLAE.
The first Thermopylae-grade prosthetics were completed alongside the Mark IV MJOLNIR Assault Armor. However, all (known) crippled spartans were too disfigured or mentally ill to allow for the use of prosthetics to get them back into the field, and the spartans didn't receive any injuries to warrant their use for some time. Secretly, however, Team Black's leader, Black-One had her previous replacement eye upgraded to a Thermopylae-grade eye prosthetic, which managed to perfectly mimick spartan-grade sight, allowing One to fight without difficulty.
Gradually, several secret sub-cells within the project itself were formed in order to produce similar prosthetics for the top secret SPARTAN-III commandos. Some time after Noble Team's formation, one of its members, Catherine-B320, suffered a grievous injury, forcing her to replace the arm with a Thermopylae prosthetic.
Around the same time, Class II SPARTAN-II Amanda-208, inflicted with elephantiasis by the augmentation procedures, had her legs amputated and replaced with Thermopylae prosthetics, allowing her to be fielded some time later.
Following a run-in with Hunters on Sigma Octanus IV, James-005 had to replace his arm with a prosthetic of Thermopylae-grade.
During the catastrophic Pyrrhic victory of Operation: BLACK GOLD during the Great Schism, Arelynn-G056 lost the lower portion of her left arm after a fight with several Covenant Revanchist Sangheili: despite her protests to having robotics attached to her body, Arelynn was outfitted with a Thermopylae arm prosthetic soon after.
Aiding in the mission to finish off the Acolytes of Devotion once and for all, Viggo-G132 suffered a blow from an Energy Sword that, while missing the intended destination of his heart, chopped off his hand and cut almost entirely through his leg. Evacuated for medical attention, Viggo's leg was fully amputated and both his leg and his hand were replaced with Thermopylae prosthetics
During the Battle of Chigago in 2556, Hunter-G241 was slashed in the left hand from a Jiralhanae Chieftain's Gravity Hammer, almost dismembering his hand. His hand was later replaced with a THERMOPYLAE Prosthetic.
In 2536, when the first SPARTAN-IIIs were still being trained on Onyx, an accident with live explosives cost Theodore-A399 both of his arms. With damage too extensive for him to integrate with a flash-cloned limb, from the shoulder down his arms were replaced by prosthetics, and caused him to wash out of Alpha Company.