The Mars Surveyor Program will launch a lander vehicle to the southern polar regions of Mars in January, 1999 to evaluate Martian earth sciences and climatology. The lander vehicle will examine Martian soil samples take pictures of the Martian surface in the vicinity of the landing site and record weather data. As a “Faster, Better, Cheaper” program exposed to widely variant thermal and gravity environments, a robust yet cost-effective thermal design is mandatory. The need is amplified further by the extraordinary resource constraints on vehicle mass and power. A capillary pumped loop design has been selected because it provides a flight-proven adaptive control capability to match the range of environments encountered on the Martian surface, as well as during Earth-to-Mars cruise. During pre-launch spacecraft operation, the two phase system will be operated at low power in 1-g field. During the ten month cruise, the system will provide heat rejection in a 0-g environment in a fixed conductance mode. The Mars landed mission will rely on fixed conductance operation at tilt of up to ±16° in a 3/8-g field. Due to vehicle power consumption constraints, the two phase system will be designed to shut down at the end of each day to minimize heat leaks as the external night environments approach -73°C. This paper describes the unique requirements and how a fixed conductance ammonia Capillary Pumped Loop (CPL) was designed and analyzed to meet all of the thermal requirements.