Stealth ablative coating
From Halopedia, the Halo wiki
Black in color, the stealth ablative coating serves to refract both UNSC radar and Covenant sensors, making the ship invisible to radar, similar to the way active camouflage cloaks something from eyesight. However, the coating also doubles as a visual cloak, enabled by the ship's texture buffers. The coating must be maintained perfectly, or it will return radar signals. This necessitates constant maintenance on the entire coating. Therefore, virtually all UNSC prowlers have relatively small surface areas, so that less has to be maintained. An exception to this is the UNSC Point of No Return, which is a prowler the size of a UNSC UNSC destroyer. Stealth ablative coatings can also be found on Long Range Stealth Orbital Insertion Pods.
During a mission in early 2526, the armors of Spartan-II John-117 and his compatriots were tinted with the stealth ablative coating. While the UNSC's understanding of Covenant sensor technology was limited at the time, the hope was that it would help conceal the Spartans as they went EVA to board an alien vessel.
By early 2557, stealth ablative coatings had begun to see applications outside the UNSC. Ace of Spades, a Mariner-class transport ship captained by Rion Forge, was equipped with such a coating along with other stealth technologies.
List of appearances