This paper describes the design of a two sided aluminum tracking mirror and mechanism for the CONTOUR Remote Imager and Spectrometer (CRISP) instrument flown on NASA's COmet Nucleus TOUR (CONTOUR) spacecraft launched in July 2002. The tracking mirror mechanism was designed to operate in a -60°C high vacuum space environment. The primary structure of the instrument and the tracking mirror assembly were all fabricated from solid billets of magnesium ZK60A-T5 alloy. Interfacing materials with drastically different coefficients of thermal expansion required significant analysis and close attention to details. By maintaining close symmetry, tight but not unrealistic tolerances, and simultaneous machining of interfaces, we were able to keep thermal distortions virtually identical on both ends of the mirror axis and thus maintain mirror flatness to mission requirements.