Even in absolutely dark space, there are kinds of things around. A tiny spec of dust, a large fragmented rock, radiation...anything. this was a largely unshielded device as we know it today. It had some foil to reflect radiation from the sun, and I think that's about it. Even in space, everything will degrade over time from one thing or another given enough time. This thing has had 45+ years.

I saw this documentary once where a spaceship picked up one of them and it was crazy nuts.