> ...you get much better resolution by sending a small telescope to orbit the Moon ...

Just to comment on this part, we already have such a probe! We at NASA Goddard operate tbe Lunar Reconnaissance Orbiter (LRO) which has two a Narrow Angle Camera's (LROC NAC-L and NAC-R), each have a focal length of ~700mm and so are effectively mini telescopes.

Infact, with these camera's we've been able to photograph the old Apollo landing sites

If anyone is interested in diving further, all the imagery taken by the LRO is available to the public and can be explored using the LROC QuickMaps tool. If you zoom in you'll see the actual mosaic of NAC images. You may be wondering why each segment of the image looks really long, and this is because NAC is a push-broom sensor rather than a framing one. A framing sensor (like in most camera's today) is a rectangular grid of pixels where each pixel on the sensor is sampled once and corresponds to one pixel in the final image. A push-broom sensor is a single line of pixels, which each pixel represents a column of the final image. It is sampled at a regular interval, and each time it's sampled, that forms a new row of the image. So as you fly over a surface, you construct the image one row at a time. It maps the surface kinda like you're pushing a broom over the surface, hence the name.

Can depend on what you're interested in. There's lots of books on various celestial bodies or overviews of cosmology/astrophysics. A common goto is "a brief history of time" by Stephen hawking. I havent read that in a long time, but in middle/high school I really liked it.

There's also mission/program specific books like "Chasing New Horizons" (about the New Horizons mission to Pluto), or "Failure is not an Option" by Gene Kranz. Plus tons of biographies in various figures in space.

There's a lot of great books out there depending on what you like.

There's also clothing or other small things like mission patches or pins released for specific missions.

Another cool thing might be rocket/spacecraft model kits.

If you're into gaming and haven't played it yet, maybe Kerbal Space Program. Back in undergrad, I had a professor in orbital mechanics highly recommend the game to us to build an intuition before we dived into the math of everything. It's honestly a fantastic way to learn about how space travel works, and it's a lot of fun!

As others have pointed out, "radiation" is the energy "radiated" away from something. It could be electromagnetic radiation (radiowaves, microwaves, visible light, X-rays, gamma rays, etc.), or it could be particles (alpha or beta particles).

When we talk about washing off radioactive contamination we mean removing the radioactive particles that are emitting the radiation. If you get covered in Uranium dust, you are being exposed to all of the radiation being released by that dust. Washing it off of you removes that source of radiation thus protecting you from further exposure.

As for how there is a half life to the contamination, remember that what we're concerned with is the radioactive material itself. Radioactive material is just material that is unstable at an atomic level. That instability means it will randomly break apart, emitting energy (radiation) in the process. Once a given atom has broken apart though, it can't emit more energy.

With a large collection of radioactive material different atoms will break apart at different times, releasing their energy in a somewhat constant stream. But over time, as more of the individual atoms break apart, there will be fewer unstable atoms left. Eventually, all of the atoms will have broken apart, and so the substance will no longer be radioactive. For many things, like Uranium, this takes a very very long time though.