Thank you guys for all your comments.
@Mr. Cryns: Considering our common interest in ancient history, I totally agree with your preferences for pre-modern subjects. But this absurd project just had to be done …
The question of the backgrounds: In earlier years, I actually did quite a lot of digital fiddling around, to combine the diorama = foreground with some suitable background photo. In the
space pictures that I had presented as a first batch, I had to do this again. But the in main pictures presented here, everything was done in the conventional way by setting up the correct background already during the shoot.
To print pictures on genuine photo paper can be done in poster-size at a reasonable price. And to place those pictures behind the dio during photographing gives some „real“ feeling that would require a tremendous amount of work, if I would try to do this at the computer afterwards.
Both photos I used are showing the Trona Pinnacles in California (© Bob Wick / BLM, CC BY 2.0). One of them has a rather diffuse illumination that gave the freedom to set the light on the dio almost as you want. The other one – used on the last 4 pics shown here – has quite distinct shadows. Here I positioned the halogen lamps accordingly, to achieve a good match.
@Wiking: Initially, I really thought about using some genuine extraterrestrial landscape pic; the Gale Crater on Mars had caught my attention. But then I decided that it looks too familiar (i.e. too terrestrial). So for some really alien landscape, I had to search on Earth. Strange, isn’t it? The Trona Pinnacles are wonderful in this respect, and afterwards I have learned that other people had this idea before me: They have featured, for instance, in Star Trek.
@Susofrick: One of my favorite pics is the one with the critter hiding behind the leg of the spaceship:

Here the missing dialogue is easy to fill in: It would be "oh-oh!“ (If you have seen the Alien movies, you'll know what I mean).