Regardless. If a piece of space junk crashes onto your property you know it is not yours so taking it would be stealing. That's pretty much the definition of stealing.
If after a few weeks NASA doesn't come to claim it then their ownership of it is forfeit and then you can keep the potentially radioactive piece of space debris.
If after a few weeks NASA doesn't come to claim it then their ownership of it is forfeit and then you can keep the potentially radioactive piece of space debris.