The NIBIRU mission, scheduled for launch in 2038, seeks to image and characterise Planet 9, a hypothesized planet beyond Neptune in the Kuiper Belt. The mission will travel farther than any human-made object and involves a close approach to the Sun, necessitating precise design and integration of all subsystems. This report details the mission's objectives, design processes, subsystem specifics, and risk assessments. Primary objectives include confirming Planet 9's existence and location, estimating its mass and radius, capturing detailed images, identifying surface and atmospheric features, detecting moons or rings, determining atmospheric composition, analysing oxygen isotope ratios, and identifying potential biosignatures. Secondary objectives involve characterizing planets used for gravity assists, studying Kuiper Belt objects, and analysing the boundaries of the heliosphere. A comprehensive trade-off analysis evaluated four mission concepts based on criteria like scientific objectives, risk, cost, communications, flexibility, and sustainability. Concept C1, featuring a spacecraft that will perform in-situ measurements and communicate directly with Earth, was selected for its high performance and feasibility. Subsystem designs were meticulously developed. The payload includes instruments such as the N'LORRI imager, ISHTAR imaging spectrometer, NCREX cosmic ray telescope, and PSP particle science package. The Electrical Power Subsystem (EPS) relies on two eMMRTGs and additional batteries for power. The Telecommunications subsystem features a large deployable antenna for long-distance communication. The Attitude Determination and Control System (ADCS) uses spin stabilization and thrusters for precise control, as well as star trackers and IMUs. The propulsion system is a liquid bipropellant system using nitrogen tetroxide and monomethylhydrazine. The Thermal Control System (TCS) combines multi-layer insulation and ceramic carbon tiles to manage extreme temperatures. Structural components use aluminium alloys for strength and durability. System integration ensured compliance with mission requirements, with a total spacecraft mass of 40,648 kg and mission cost of €M3441. Risk assessments identified and mitigated 114 risks, with detailed contingency plans in place. The mission's sustainable development strategy emphasizes the need to use reusable launchers, green energy, and sustainable off-the-shelf materials. The detailed design, manufacturing, testing, and integration phases are meticulously planned to ensure mission readiness for a 2038 launch, promising significant contributions to our understanding of the outer solar system.