There was remarkably little in the way of security for early satellites (or space probes).<p>I recently encountered here on HN the suggestion that, the main reason you can't find particularly in-depth details on the Voyager space probes, is project security. Security through obscurity, mostly. If amateurs can detect the signal from the Voyager probes with a small dish antenna, it's at least conceivable someone might hook up a really powerful transmitter, aim it in the probe's direction, and start issuing commands. There's no cryptography, of course. The resources required to hijack like that in the 1970s would have been much greater, and I doubt being hacked was much on the designer's minds.<p>The Apollo Program was the same; today anyone with the documentation, a small dish antenna, a software radio, and some nerd dedication, would be able to hack Apollo midflight via its radio link. It was the equivalent of a root prompt with no password on an exposed port.<p>A bit closer to home, there's a tremendous amount of semi-functional orbital junk with a similar lack of security, decades-old computers still waiting for telecommands.