Contamination of spacecraft surfaces by deposition and ablation can potentially limit the performance and lifetime of solar panels, optical systems and microsatellite structural elements. However, instrumentation for on-orbit contamination assessment is limited by payload requirements and the experiments which can be conducted are limited by spacecraft geometry and mission lifetime. We present the design of a fiber optics evanescent wave sensor capable of real time detection of the contamination of spacecraft surfaces during flight. While other evanescent wave detection schemes rely on special coatings to selectively and reversibly absorb a target analyte, in the present context, such coatings would themselves be considered undesirable forms of contamination, and are therefore prohibited. The sensor described here is capable of detecting contamination by direct exposure of the evanescent wave to the environment through the use of a reduced-cladding fiber. The sensor can measure contamination from foreign substances, and ablation from the impact of space debris or ion thruster exhaust. In this paper, we briefly discuss the major forms of contamination. We describe the operating principles of the fiber optic evanescent wave sensor we have constructed for monitoring these forms of contamination, and provide preliminary results indicating sensor performance characteristics. We demonstrate the feasibility of the sensor for detecting the deposition of a variety of substances, and for observing the effects of ablation from thruster exhaust. In all cases, we make a qualitative comparison between sensor performance and theory.