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''Close to Critical'' is a science fiction novel by 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. ==Setting== Revolving on an orbit about the star Altair, Tenebra provides the reader with an excellent example of one of Hal Clement's well-crafted hellworlds. 27 times the mass of Earth, Tenebra is three times Earth's diameter (38,226 kilometers; 23,746 miles) and has three times Earth's gravity at its surface (29.43 meters per second per second; 96.6 feet per second per second), which gives Tenebra an escape velocity of 33.558 kilometers per second (nearly 21 miles per second). Clement also describes the planet as rotating on its axis in a little less than four Earth days. Tenebra has an atmosphere "consisting of water heavily laced with oxygen and oxides of sulphur". At the surface the temperature comes close enough to the critical point of water (temperature = 374 Celsius; 705 Fahrenheit : pressure = 22.1 megaPascals; 217.755 atmospheres) that during the night the atmosphere begins to condense into the liquid state and falls as raindrops 9 to 15 meters (30 to 50 feet) wide. The naively expected density of the surface atmosphere stands at about 322 kilograms per cubic meter (compared to 1.2 kilograms per cubic meter at sea level on Earth). But the raindrops descend slowly, even in Tenebra's stiff gravity. That latter statement necessitates, as Clement states in the story, that the surface pressure on Tenebra be 800 atmospheres, not 218. At 800 atmospheres of pressure the surface atmosphere, with its load of dissolved oxygen and sulphur oxides, is compressed to a density a little less than the density of liquid water (1182 kilograms per cubic meter). Only when the density of the planet's air is a little less than the density of liquid water will the raindrops descend slowly; otherwise, they would come down like meteors and make life on Tenebra impossible. But life is possible on Tenebra. Although Clement doesn't say so explicitly, Tenebra's plants do not use photosynthesis. The optical depth of the atmosphere is too great for enough of Altair's light to reach the ground. Instead, Tenebra's plants use chemosynthesis based on the transformation of sulphur oxides. The process is much like that used by plants growing around the hydrothermal vents known as black smokers on the bottom of Earth's oceans. Altair's abundant ultraviolet radiation, striking the top of Tenebra's atmosphere, restores the balance among the various sulphur oxides. Because Tenebra's atmosphere contains large quantities of sulphur, the planet's lakes and small oceans are filled with sulphuric acid. Throughout the day-night cycle the lakes and oceans rise and fall like tides in the Bay of Fundy, due to the heavy nightly rainfall and the easy evaporation of the liquid water (its heat of vaporization stands close to zero under Tenebran conditions). The constant ebb and flow of hot, pressurized sulphuric acid weakens the planet's crust, resulting in frequent earthquakes and minor rearrangements of the landscape. For two decades humans have been observing this world from the space station Vindemiatrix, which revolves about Tenebra on a synchronous orbit, one with a period just under 96 hours. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Close to Critical」の詳細全文を読む スポンサード リンク
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