Space gear tends to use a realtime OS like VxWorks.
.NET hasn't proven itself for long-term reliability. Java's getting there but I think assembly and C are here to say for that sort of thing.
Curiously, the space shuttle's flight computers actually use a higher-level language, not assembly or C. They're written in a language called HAL/S, which is designed around real-time operation (including task scheduling and data sharing) and vector/matrix
algebra rather than the really low-level stuff that C is designed for.