This question has been covered in great depth over on Security Stack Exchange:

http://security.stackexchange.com/questions/6424/what-would-one-need-to-do-in-order-to-hijack-a-satellite

A very quick read of the answers there shows that while encryption is used for military satellites, security of many devices sent to space is mostly reliant on esoteric protocols, unlisted frequency bands, accurate location finding etc.- security by obscurity rather than strong security mechanisms.

As a threat model for deep space probes this could well be appropriate. For satellites, probably not, but read that question anyway.

This question has been covered in great depth over on Security Stack Exchange:

https://security.stackexchange.com/questions/6424/what-would-one-need-to-do-in-order-to-hijack-a-satellite

A very quick read of the answers there shows that while encryption is used for military satellites, security of many devices sent to space is mostly reliant on esoteric protocols, unlisted frequency bands, accurate location finding etc.- security by obscurity rather than strong security mechanisms.

As a threat model for deep space probes this could well be appropriate. For satellites, probably not, but read that question anyway.