Fantasy / Humour / Sci-Fi. Astronauts crash land on a planet, and struggle to reach civilisation to warn of impending doom. Hitchhiker's-Guide-style of humour.

[Srđan]

This should be free of spoilers. I speed-read through a few dozen pages of the book several years ago, and then forgot the title, so I couldn't find the book again. The details might not be completely accurate.

Plot: Mad emperor of a medieval civilisation on a faraway planet builds a giant slingshot (balista? catapult?), planning to use it to destroy the sun, or something equally ridiculous. He ends up shooting down a spaceship, and the crew crashes in a remote frozen wasteland.
The astronauts establish a colony/village. They are aware of some cataclysmic danger heading toward them (asteroid?), and decide to send one of their number, who, I believe, they called a tribute, to try and warn the rest of the planet. They keep sending tributes every X years, but none ever return. Several decades or centuries later, the protagonist is about to be the next tribute.

The only thing other than the plot that caught my attention was that the humour was similar to that of Douglas Adams or Terry Pratchett.