This paper discusses using impulse radio ultra-wideband (IR-UWB) ranging technique to realize high-accuracy inter-spacecraft ranging in future distributed space networks. First, it gives the Cramér-Rao lower bound (CRLB) on root mean square error (RMSE) of IR-UWB-based ranging under inter-spacecraft channels. It also discusses the effects of pulse parameters on CRLB and gives the asymptotic optimum maximum likelihood (ML) range estimation algorithm. Second, it analyzes the IR-UWB-based inter-spacecraft link budget and illustrates its feasibility and high-accuracy ranging performance. In addition, it analyzes and summaries the technical challenges on using IR-UWB technique to future distributed space networks missions.