Close to Critical is a science fiction novel by American writer Hal Clement. The novel was first serialized in three parts and published in Astounding Science Fiction magazine in 1958. Its first hardcover book publication was in July 1964.
It began with human researchers sending a semi-autonomous robot to the surface of Tenebra. The robot descended on rocket boosters until atmospheric pressure exceeded the pressure in the rockets' combustion chambers. Instead of generating thrust, the rockets began to overheat and melt themselves, so the controllers jettisoned the engines and the robot completed its descent under a parachute.
After acclimatizing itself to the dark world under Tenebra's thick clouds, the robot set out to explore. After months of mapping the territory and observing the plants and animals, the robot encountered a sentient native. The creature resembled a very large, scale-armored fir cone with four pairs of limbs; the lowest pair used for walking, the next pair apparently not used at all, and the upper two pairs used for prehension (used as arms and hands). A set of spines jutting from the top of the creature served as lensless eyes, making maximum use of the thin flow of photons seeping from the sky.
Unseen by the native, the robot followed it, stole ten of its eggs and good selection of stone knives that the creature had set out in booby traps, and then took its loot far from the caves in which the creature and its clan lived...