The momentum wheel (MW) plays a significant role in ensuring the success of satellite missions, the reliability information of MW can be provided by collecting degradation data when there exists certain performance characteristics that degrade over time. In this paper, we develop a reliability modeling and life estimation approach for MW used in satellites based on the expectation maximization (EM) algorithm from a Wiener degradation model. The degradation model corresponding to a Wiener process with the random effect is first established using failure modes, mechanisms, and effects analysis. Afterwards, the first hitting time is employed to describe the failure time, and the explicit result of the reliability function is derived in terms of the Wiener degradation model. As the likelihood function for such a model contains unobserved latent variables, an EM algorithm is adopted to obtain the maximum likelihood estimators of model parameters efficiently. Finally, the effectiveness of the developed approach is validated using the degradation data from a specific type of MW.