I just learned about SpaceWire in this great answer about the slip-ring rotating electrical connection between its solar panel and attached sun-pointing instrumentation, and the body of the craft with its Earth-pointing instrumentation.
The Wikipedia article suggests that SpaceWire should have nine conductors; differential pairs each for data in, data out, strobe in, strobe out, and a single ground.
It looks like a lot of thought has gone into SpaceWire, and it seems to have been developed specifically for space applications.
While Ethernet gets by on a single coaxial cable or (usually shielded) twisted pair, SpaceWire specifies nine conductors. Sometimes the weight of wires is considered a small but significant factor in spacecraft weight minimization. Why were nine wires considered optimal?