Prowler

"If you saw a prowler, it probably wasn't a prowler."

- Anonymous Prowler Corps officer

A prowler is a line of warship, represented by the hull classification symbol PRO. Originally, the term properly referred to the slipspace-capable reconnaissance ships used by the Office of Naval Intelligence. However, it has been adopted by many human organisations as a moniker for any ships that are similar in size to a corvette which are designed for covert operations.

Technology
Up until the late 25th century, the notion of a stealth spacecraft was considered a pipe dream. Although visual and radar stealth were viable, any ship would be blatantly visible on infrared sensors a system away the moment it fired its thrusters. Although trickery involving decoys and electronic countermesures could confuse or disable enemy sensors, no ship (or at least any ship with human crew expected to move about a star system) could be made truly undetectable to opponents with access to appreciable telescope equipment.

The heightened tensions of the UNSC-CMA Cold War, coupled with advances in spacecraft thermal management, fusioneering and slipspace theory, saw the UNSC reopening research on thermal emissions masking. The solution, as it turned out, arose from a novel application of slipspace. As early experiments with slipspace-based communication had shown, moving energy into slipspace is not particularly difficult; the trouble is keeping that energy coherent and controlling its trajectory past entry. Any radiation transmitted into slipspace (including radio waves or tightbeam) rapidly decoheres, then radiates back into normal space predominantly as random patterns of microwave radiation scattered across wide swathes of space in what is known as the Muratovski effect.

Prowlers rely on the Muratovski effect for concealment, using an array of fine-tuned emissions baffles to shunt drive emissions and the rest of the ship's thermal output into non-spatial dimensions, which then scatter it widely enough to obfuscate the presence of a ship. When operating properly, the radiation backscatter is virtually indistinguishable from the cosmic microwave background. Though this does not make the ship completely invisible — an observer can still detect the minute variance in black-body radiation if they look hard enough, and persistent active scans will reveal the presence of an object — it does make prowlers very difficult to detect.

The baffling arrays have to be coupled with a highly specialized "dumb" AI, which calculates the precise output of the engines at any given time along with numerous other onboard heat sources, such as life support, which is then synchronized with the baffle field. This is coupled with a system of hull cooling, insulation and internal heat sinks. For this reason, prowlers and their engines have to be constructed to far stricter tolerances than normal ships, which increases their costs and makes manufacturing a specialized and expensive affair.

Emissions baffling has its limits, however. The early baffling devices were finicky and had to be constantly tuned, with even a minor miscalibration or failure to account for a drive flare exposing the ship's presence. Even with advances in the technology, baffles are not completely efficient and cannot be used indefinitely. As a rule, any burn of the main engines is generally regarded as a costly maneuver with an ever-present risk of stray particles escaping. Prowlers use cold-gas thrusters over their fusion engines as much as they can, but the inefficiency of said thrusters (even compared to the resistojets mostly used on non-stealth craft) means they can only be used for attitude control and local maneuvers. By the Human-Covenant War, even the prowler's slipspace exit can be convincingly masked, though this means fissile materials cannot be carried onboard as their Cherenkov radiation aura interacts with slipspace in ways too unpredictable to be accounted for by the baffles.

Visual camouflage is provided by a relatively simple chameleoflage stealth coating, though advances during the Human-Covenant War have seen prowlers shift increasingly toward photoreactive metamaterials.