I'd bet, though, that many things aren't very well protected, because people designing space systems assume that nobody is going to try to mess with them, and also because they probably use proprietary communication protocols.
But just imagine how cool it would be to ssh into a satellite :)
A few years ago I worked for a satellite imagery company. I can tell you that communications with space vehicles in orbit are heavily encrypted using very strong and very proprietary algorithms that require special hardware. Because of this encryption, I had to acquire a government security clearance.
But just imagine how cool it would be to ssh into a satellite :)