Update: The accepted answer and its comments contain good references to equipment retrieval. I have separated off-Earth examination and photography into a new question: Has a lander/rover ever examined or photographed another mission?
Two years after Surveyor 3 landed on the moon, Apollo 12 landed at the same site. The astronauts made observations of Surveyor, took photographs, and brought back parts for analysis on Earth. This helped us understand how equipment can be affected by exposure to the moon's environment.
However, this represented only 2 years of lunar exposure. Have there been any such observations of lunar equipment over a much longer term?
For the purposes of this question:
- "long term" means a minimum of 10 years exposure to the lunar environment, before being observed. Note that we've been putting equipment on the moon for almost 60 years.
- "equipment" is any man-made object left on the moon, whether for an unmanned or manned mission.
- "observation" can be interpreted as broadly as you need, as long as it gives us information about the effects of exposure to the lunar environment.
- Orbital imaging is specifically excluded. It is enough to tell us that the equipment is still there and whether it has moved, but does not have enough resolution to show damage/degradation to equipment.
- Outside of Apollo, I'm not aware of any missions that returned equipment from the moon; but that would hypothetically count for this question.
- I'm not aware of a lunar mission that could photograph itself after 10 years; but that would hypothetically count for this question.
- Surveyor 3 / Apollo 12 is specifically excluded, as the exposure was only 2 years.
edit: Although a retrieval mission would give us the most information, a lander or rover mission taking pictures of a nearby older mission should also be considered. I appreciate the answers and comments about studying effects in Earth orbit, but that wouldn't address the effects of moon dust, which was a big concern during the Apollo program.