Isso é "Camelot 30K" de Robert L. Forward.
Os astronautas não se tornam pequenos, mas constroem uma pequena sonda na forma de um dos locais.
Out on the far boundaries of the Solar System, in the Oort Cloud, intelligent life has been discovered by humanity's space probes. Now the first humans to venture beyond the planetary system have been sent to make contact with this incredibly strange race and to tour the center of their civilization. What they discover is explosive beyond their wildest dreams.
Os alienígenas se chamam kerack e os humanos lhes dão nomes do ciclo de Arthur: o principal personagem alienígena é Merlene , outro é Launcelot . O guarda-costas da Rainha Une é chamado de Mordet , e a cidade-estado é chamada Camalor .
O kerack passa o tempo lutando com as cidades vizinhas para que as almas mais corajosas possam descer ao céu. O céu é uma grande câmara sob o palácio da Rainha, e suas almas são pequenas balinhas de urânio - porque o kerack tem uma biologia bio-nuclear que lhes permite prosperar a meros 30 Kelvin, perto do zero absoluto. Sua única grande conquista racial é a construção do Céu, uma câmara semiesférica cuidadosamente construída que, não acidentalmente, atua como um perfeito refletor de nêutrons. Dizem que quando bastante almas desceram ao Céu, Camalor ascenderá à grandeza em um pilar de fogo.
Esta é a cena da explosão (e encontrei a referência aos hexágonos, que acho que é a única no livro):
...stored under the altar, resulting in a large thermonuclear fusion explosion.
Surrounding the explosion was a thick cylinder of lead, surrounded in turn by the heavy metal contents of the privies. The inertia of this thick cylindrical wall of heavy metals constrained the explosion and directed the explosive force upward, where it struck the under-surface of the Palace o' Princesses under the Plaza o'Dance.
The Palace o Princesses had been fabricated of seamless black boron carbide, which was almost as strong as crystalline diamond and could withstand higher temperatures. The rooms in the Palace o'Princesses had been made hexagonal in shape, producing a structure that had the greatest strength for the least weight. As a result, the Palace o'Princesses maintained its physical integrity as the shock front of the thermonuclear explosion hit it.
The palace accelerated rapidly upward, driven by the directed thermonuclear explosion, until it and the cargo of spores on its upper surface attained escape velocity and were thrown into interstellar space.
"Faster, Hiroshi! Faster!"
Elizabeth was watching the view from the monitor camera on the pole overlooking the plaza. "The princesses have stopped bringing uranium pellets to the Queen and are sliding the doors back into place. Now I know why the doors had to be so massive."
Spurts of yellowish smoke shot from cracks around the loos, and a heartbeat later, the plaza floor rose as a single body carrying with it the piles of spores on its upper surface. There was a blinding flash of light, and the camera monitor over the plaza stopped working.
(Sim, sim. Mas como o próprio Robert L. Forward escreveu no prefácio, nunca deixa o fato atrapalhar uma boa história . Este livro exigiu quatro ou cinco vezes meu nível usual de suspensão da descrença, e só posso dizer que valeu a pena).
O dispositivo termonuclear
