The most curious feature of the world dissected in this book is: the temperature at its surface & extremely dense nearby atmosphere varies in a very very narrow band - around the boiling point of water.
This almost fixed temperature is about 370ºC, just a little below the 374ºC that is the "critical temperature" of water (the reason for the title). The most interesting aspects of the story are about water/steam transition when temperature varies even slightly, & that density of water here is scarcely more than that of steam.
The consequences include: complete boiling away of oceans in the morning, & getting refilled each evening; frequent earthquakes; evening "raindrops" several tens of feet in diameter with scarcely higher density than water that lazily drift down & can be boiled away while still high in the air by barely increasing their temperature by holding a torch several meters underneath, ...
A tribe looking for a new home, or a traveler looking for a camping site, has to be very very wary - or the ocean will take you at night even if you chose a rather high elevation place in the evening!
This is not a book to pick up when you want light reading. And some science background will help. If you haven't read Hal Clement before, this note can give you an idea if he is likely to be your kind of author; it provides some caveats to reading Clement.
Table of contents.
- Executive summary.
- Tenebra, the fictional world.
- The various aliens.
- Tenebrites, the natives.
- Drommians, the other aliens.
- Story summary, thread 1: Human exploration of this world.
- Story summary, thread 2: Children stranded on Tenebra.
- Story summary, thread 3: Rescue of the kids.
- Characters & things.
- Related.
- Fact sheet.
Executive summary.
Humans are exploring this world via a robot, controlled from a spaceship in orbit. Robot needed assistants to explore the big world. Their recruitment involved stealing several eggs of the egg-laying intelligent natives at the level of human stone age, raising them, & then using them to explore! In time, this band of human assistants will come in conflict with a native tribe.
Via a separate thread, in circumstances that can only be called fantastic, two kids left alone in a space vehicle drift down to planetary surface near the location of robot! So there is a rescue drama with a happy ending - with the help of natives.
The most interesting aspects of the story are not actors or plot, but the world & its non-intuitive behavior.
Tenebra, the fictional world.
The story is set on a planet called "Tenebra", 16 light years from earth. "Altair" is the name of its sun.
Here are the key characteristics of this world:
- Tenebra is 3 times the diameter of earth, with surface gravity 3 times that of earth.
- It has a very dense atmosphere near the surface - its pressure at surface is about 800 times earth value. To human eye, the place is always rather dark. Local life, however, is well adapted to this level of light.
- It rotates about its axis in about 100 hours. Means one Tenebra day is about 4 earth days.
- Lower atmosphere is mostly steam, with a lot of oxygen & sulfur as impurities. Means chemically extremely volatile. You need especially engineered materials to make a lander; most normal materials humans use will quickly corrode there. Local animals breath oxygen.
- Almost constant surface temperature is 370ºC. Very minor difference between day & night temperatures results in precipitation of a part of lower atmosphere every evening that fills up the oceans for night; these oceans then boil away in the morning!
Small earthquakes are common - several times each day. "The area is not very well mapped, and what there is doesn't stay mapped very well... the question is not whether it will rain tomorrow but whether your pasture will start to grow into a hill... the atmosphere is mostly water near its critical temperature, and silicate rocks dissolve fairly rapidly under those circumstances. The place cools off just enough each night to let a little of the atmosphere turn liquid, so for the best part of two Earth days you have the crust washing down to the oceans... With three Earth gravities trying to make themselves felt, it's hardly surprising that the crust is readjusting all the time."
About the only surface liquid during day time is in
oleum lakes - "oily liquid", "principally sulphuric acid with a heavy lacing of metal ions from the surrounding rocks which had been dissolved in the nightly rain, and an equilibrium amount of the atmospheric gases."
Nightly precipitation turns these oleum lakes into oceans. This "ocean decreases enormously in density" due to evening precipitation. A boat that will float well in day time will likely sink in the evening! "the sea had already been heavily diluted by the rain. That meant there was no point in expecting the raft to float. Its air-filled sacks were nearly half as dense as the straight acid; with this diluted stuff their buoyancy would be negligible... as the water began to boil back out of the sea, the latter's density increased, and the raft began to float".
Also, near shore where a river empties, there is more water content. Means the density increases as you move towards deeper seas. Adds some complications to the raft's travel, if I recall correctly.
Daily precipitation not only spontaneously forms raging rivers at night, their location keeps changing due to constant remaking of the planetary crust - but in ways that natives can intuitively predict.
Wind is almost unknown (explanation below). "Three knots is a wild hurricane on Tenebra" & "Tenebra is about the calmest place you'll find inside any atmosphere in the galaxy." A volcano eruption near the end of story will start winds that will play a role in the story.
One of the craziest aspects I could not fully fathom is the size of "raindrops". Evening precipitation doesn't lead to rains we are familiar with. It leads to huge spheres of water - several tens of feet in diameter - to drift down! These are not big bubbles with air filled in - these are water! "The pressure's so high that the atmosphere doesn't even come close to obeying the classical gas laws ... and the small percentage changes in temperature that do occur result in even smaller changes in volume, and therefore in density, and therefore in pressure. Little pressure difference means little wind. Even changing phase, from gas to liquid, makes so little change in density that the big raindrops just drift down like bubbles, in spite of the gravity."
So
you really can physically run around to avoid them - "It's not too hard to dodge raindrops if you have enough light to see them coming". In one scene, a native uses one of these drops for camouflage - let yourself get enveloped in a drop & run at the expected speed of drop's movement while still inside!
There is a peculiar interaction of these big raindrops & the fires human have taught the natives. Nightly fires add just enough heat to a falling raindrop to evaporate it again - so those camping around can stay dry!! But occasionally, a raindrop will get a fire - so the fires need to be tended often.
There is another effect the nightly precipitation has. Early precipitation has a lot of oxygen bubbles suspended within the raindrop. Later ones come from relatively higher atmosphere, & their drift has allowed these bubble to leave their cage - so if you vaporize it, what you get is unbreathable steam without any oxygen. Means a traveler must find a place away from falling raindrops, particularly of later no-oxygen variety.
A variant effect of these no-oxygen raindrops is: if you immediately rush to a fire that has just been claimed by a raindrop, you will suffocate - oxygen near that place takes a while to replenish. "Several times ... human-educated natives had to make a rescue as someone ran blithely into the dead-air zone of a boiled-away raindrop to relight a fire. When they finally realized that a newly destroyed raindrop was like a newly boiled lake in the early morning, some of them took to waiting a long time before venturing near the extinguished fires, so that the fuel cooled too far to let the blaze spring back at the mere touch of a torch."
Frequent earthquakes & daily water cycle make any kind of useful map making very hard.
The various aliens.
Story features two races of intelligent aliens - Tenebrites are natives of Tenebra at a stone age stage of development; Drommian are space farers that are about peers of humans in technology development. Former are key to plot; introduction of later just complicates the story without adding value.
Among the natives featured are two different tribes - one raised by humans & an independent native tribe.
All aliens can speak to humans via their normal vocal/auditory apparatus.
Tenebrites, the natives.
Like other stories by Clement, the local life is as abundant as earth - plants, animals; vegetable eaters & predators. Among the flying animals, only "floater" is mentioned; it will play a key part near end of the story.
Plus, of course, there are intelligent natives. By & large, indistinguishable from humans except in physical appearance, but at a level of technology where they live in caves & don't even make fire (but can work with basic stone tools).
There is one aspect of the natives' behavior that is key to the story, & I could not accept as reasonable; it's there only because the author says so. These natives with near human intelligence are egg-layers, but leave their eggs unguarded in a communal nursery on a world that has predators!!
On physical appearance, we are told they are "fir-cone shaped beings", have scales, & have eight limbs. First adult the human robot saw after landing was "fully nine feet in height, and on that planet must have weighed well over a ton. It conformed to the local custom as regarded scales and number of limbs, but it walked erect on two of the appendages, seemed not to be using the next two, and used the upper four for pretension."
OK - so our counterparts use 4 hands, walk erect on 2 legs, & have 2 unused limbs.
Drommians, the other aliens.
Apparentlyfrom planet "Dromm". Evolved apparently from carnivores.
Adults are "nearly ten feet tall" when "standing on their hind legs". "Their general build was that of a weasel - or better, an otter, since the slender digits which terminated their five pairs of limbs were webbed. The limbs themselves were short and powerful, and the webs on the first two pairs reduced to fringes of membrane along the fingers - a perfectly normal evolutionary development for intelligent amphibious beings living on a planet with a surface gravity nearly four times that of the earth". Their limbs are "ten-clawed", & they normally "walked on all ten limbs, even those whose webs were modified to permit prehension".
Story features one adult - a diplomat named Councilor Aminadabarlee, & his little son Aminadorneldo aka 'Mina - "four years old, about the social equivalent of a human being of seven".
Adult is extremely hot tempered. Child is far more reasonable, but still violent.
Story summary, thread 1: Human exploration of this world.
The plot has some similarity with "
Mission of Gravity" - in that human explorers in an orbiting ship use natives' help to explore their world & will later seek their help in locating & recovering something precious.
A big human ship, "Vindemiatrix", is in orbit around Tenebra, & has sent down a robotic explorer. By the end of the story, some 16 earth years would have lapsed since this landing.
Ship is 160 thousand miles above the surface - so communications with robot has a 2-second round-trip delay. This delay makes humans helpless in helping their proteges defend themselves during a violent fight - because fight needs instant reaction time. This delay also plays a part elsewhere in the story.
We don't know much about the robot except that,
- It can move around on the planetary surface, even in liquid parts of it. But more slowly than natives. It cannot fly.
- It either has a built in power source that can last very long, or is somehow able to recharge. I don't recall any mention of what powers it, but it's still going strong 16 years after landing.
- Its parts normally exposed to local atmosphere are made from some kind of plastic that can withstand local atmosphere.
- It has appendages that can act like arms & manipulate the environment. But they are of material that corrodes easily in local atmosphere. So it avoids using them except when absolutely necessary.
- It has a built in video phone. So humans orbiting the planet can see what the robot sees, or make it move around to explore the place. Robot is also is fitted with floodlights that can be remotely switched on or off - when low light local conditions aren't good enough. It can also pick up local sounds, including natives' language, for humans; or enable humans to speak to natives.
Robot will be known as Fagin to natives who will consider it alive. Only in later parts of the story do they learn that speech coming out from it is being relayed from elsewhere. To one group of locals, robot will be "the Teacher" - it has been teaching the life skills & technology way beyond the local knowledge.
Story begins with robot's landing. A year into its exploration, it will identify one kind of native animals as intelligent, discover that they lay eggs & leave the nursery unguarded in the open, steal some eggs & hatch them. These hatchlings will be his pupils. Idea is to teach the life skills & technology to these raised natives, & use them for exploring the world - for how much exploring can a single expensive robot do? I'm sure you can find some holes with this way of exploring, but that's the story - so let's continue.
16 years have lapsed since landing. Hatchlings are adults, live in a village, know how to make fire, raise cattle, speak English, etc. And they have began mapping the neighborhood for humans, via the Teacher. Nick Chopper is the leader of this group.
One fine day, Nick is on an exploration mission where he is attacked by an animal & saved by another of his kind. Nick grew up speaking English; other one speaks his own language. Nick follows him to his cave. This tribe of
cavemen is led by someone called "Swift" - an autocratic but very able leader. Nick will spend a while with this tribe.
On seeing & hearing about the wondrous things Nick can do like making "fire and keeping herds of animals and raising plants for food", the leader wants to learn too. Nick innocently tells him about the Teacher but regrets it - for Swift now wants Teacher for exclusive use of his own people. Nick will be placed under guard, but will escape in dangerous conditions.
But Nick is an amateur at escape; Swift is an expert tracker. Nick reaches his village, & wants to leave since Swift might arrive. Teacher agrees to the danger, but says escape is futile - since they will be quickly tracked to new home too.
Attack from Swift's tribe happens at night. Two of Nick's people will be killed, & the Teacher "kidnapped".
Nick & the remaining crowd leave their village looking for a safer location; they will then work out a rescue plan for Teacher. This is where we learn what the nightly precipitation can do. They find home on a hill protected by a lake of oleum & connected to mainland by a narrow passage. Nightly precipitation will turn the lake into ocean that reaches near the top of the hill. They won't lose a group member, but will lose several cattle to predators of the sea.
Anyway, they'll later find safer home near this lake/sea, & Nick will effect a daring rescue of Teacher. Only, they will be quickly tracked down again by Swift's gang. But the rest of the story is best described as third thread - where this first thread meets the second to eventual climax.
Story summary, thread 2: Children stranded on Tenebra.
This part of the story is best described as
fantastic - in its realism. But it will make the story progress - in the next part involving rescue.
Humans have made some progress developing a vehicle that can take them down to the surface of Tenebra. Ordinary rockets cannot work - "booster that will unload its exhaust against eight hundred atmospheres hasn't been built yet. They melt down - they don't blow up because the pressure's too high." So the plan is to go down in a "
bathyscaphe", an aerial version of the old undersea vehicle that will lift via a balloon. High up where the pressure allows a rocket to function, bathyscaphe will be mated with a rocket-powered shuttle that will ferry the duo to orbiting Vindemiatrix. But the contraption is still in last stages of testing - not quite ready to go down.
Vindemiatrix has had two diplomat visitors - Councilor Rich (a human), & Councilor Aminadabarlee (a Drommian). Rich is accompanied with his 12 year old daughter Easy Rich, & Aminadabarlee has brought an even younger son Aminadorneldo (aka 'Mina).
While the adults are talking, someone is giving a tour of the ship to kids. As part of this tour, they will visit the bathyscaphe. Something goes wrong, leaving only the two kids in the vehicle while it drifts away from mothership. Later the vehicle has been captured by Tenebra's gravity & is hurtling down. Only thing the controllers on spaceship can do is guide it to land near the robot - so a rescue with the help of natives can be mounted. Landing is on on liquid rather than ground surface.
Problem with the stranded vehicle is some kind of external damage that comes in the way of inflating the balloons to take it up to a height where shuttle can pick it up. Kids inside cannot get out because "their space suits won't stand the pressure". So, the hope is: the natives will be able to repair the external damage based on instructions from humans in the ship, relayed via robot. As it turns out, a different solution will actually work near the end.
Story summary, thread 3: Rescue of the kids.
Something that struck me as abnormal was the supersmart human kid, Easy. She is not only composed most of the time she's stranded (several weeks), she seems to be far smarter than adults.
Anyway, Nick's people are mobilized for rescue via the robot. Before Swift's gang is upon them, they capture one of Swift's spies. Some discussion, & humans make an offer to Swift via this spy - join the search & rescue, in return for not only human knowledge but physical goodies that can be
traded via future missions. But Swift seems to have other ideas. In any case, Nick & Swift's people will individually search for the stranded vehicle, but Swift will locate it first.
In the mean time, there is a volcanic eruption nearby - a very unusual event on Tenebra. Temperature difference near the volcanic vent & elsewhere will trigger wind - another unusual phenomenon. This wind will play a role near the climax, but otherwise, a part of volcano related adventures is simply about examining the effects of such temperature differences on surface & local life - it doesn't really advance the rescue drama.
One drastic effect of this wind is a hurricane-like effect on the lake/sea on whose coast Nick's group has been camping. I didn't quite follow the argument on how volcano-induced wind inundated the high lands: "With the sea not much denser than the air, the way it is late at night," "the two-mile hurricane had changed from gas to scarcely denser liquid".
Nick's group will lose a lot of their belongings & some members will be nearly killed by the event.
We also see some of the effects of nightly precipitation. New rivers that often come into being. For a while, the vehicle kids are stranded in will be dragged at the bottom of a newly formed river.
Part of the rescue effort involves building a boat - a raft-like structure fitted with balloon-type things underneath - balloons made from water-tight leather. "it was a rectangle of some fifteen by twenty feet, made of about three dozen rods of wood which an Earthman would probably have described as saplings, lashed at right angles to each other to form a reasonably solid grillwork." This part illustrates the effects of changing sea density due to daily water precipitation & evaporation.
Since Swift's people are first to locate the vehicle, Easy sees hope in them. She quickly learns local language & gets on good terms with him. She also tells him the story of Nick's people - how the eggs stolen by robot were probably of Swift's own people - so the two communities are kin. This will soften Swift, & aid reconciliation later when Nick's people reach the place.
Rescue eventually happens with the ship stranded a bit from shore - with both Nick's & Swift's people to help. Plus Nick's raft. They'll eventually drag the vehicle to shore, but won't actually fix whatever is wrong with machinery. Since the idea is to fill up the balloons with hydrogen, kids will find another method with the help of Swift & engineers aboard the orbiting ship.
I didn't quite follow how the procedure worked, but here it is. Catch some "floaters", local flying animals. They probably float because they have hydrogen pouches inside. Use them to fill the vehicle's tanks - by somehow connecting the animals' pouches to tubes leading to tanks.
Actual lift will be aided by volcano: "When the ship lifted, it naturally rode the wind toward the volcano; and it rose so slowly at first that the children had a good look at the terrifying sight. They dipped frighteningly toward the glowing mountain as it entered warmer air, but recovered in ample time as the hydrogen in its cells also warmed up. Gradually the glow faded out below them, and Easy and her friend waited happily to meet the shuttle" in upper atmosphere.
Characters & things.
- Fagin: Natives' name for human exploration robot.
- Nick Chopper: Leader of the group Fagin raised from stolen eggs. He & his group is a about 15 (earth) years old at the time most of the story unfolds. Fagin has been around a little over 16 years, but it took may be a year to locate the intelligent natives.
- John Doolittle aka Johnny; Alice; Tom; Jim; Dorothy; Nancy, Betsey: Some of the other members of Nick's group. Alice & Tom will be killed during the first attack of Swift on Nick's village.
- Swift: Leader of the natives' tribe that wants to claim Fagin from Nick's people.
- Dr Helven Raeker: Human biologist on board the spaceship. He is the main person we see interacting with & teaching the natives via robot.
- Vindemiatrix: Big human space ship from which the robot Fagin is being controlled.
- Councillor Aminadabarlee: The visiting Drommian diplomat. Very short tempered, & with a very big ego. Managing him will be a big headache for human all through the rescue drama.
- Aminadorneldo aka 'Mina: Little son of visiting Drommian diplomat, & one of the two kids who will be stranded on the planet. "four years old, about the social equivalent of a human being of seven".
- Councillor Rich: Visiting human diplomat whose daugher is among the two children stranded.
- Easy Rich: 12 year old daughter of Councillor Rich who will be stranded with Aminadorneldo. Supersmart & very cool headed. Not only does she behave way beyond her years, she seems to be very competent with technical stuff!
- "Crystal Mechanic Second Class Flanagan": The man showing the kids around on spaceship who left them alone in "bathyscaphe", leading to their eventual stranding. Has a very small role in the story.
- "bathyscaphe" aka 'scaphe: "a dirigible balloon". The vehicles designed to take humans to surface from upper atmosphere; trip from upper atmosphere to spaceship is via a rocket-powered shuttle (also called "tender"). Stranded kids are in a vehicle of this type. I don't think their vehicle is actually named. "a two-hundred-foot-long, cigar-like shell, and the only really heavy part was the forty-foot sphere in the center which held the habitable portion".
Related.
- All stories by Hal Clement.
Fact sheet.
"Close to Critical", novel, review
First published:
As a 3 part serial in Astounding Science Fiction, May/June/July 1958.
Rating: A
Series: "
Mission of Gravity" (A), "Close to Critical" (A), "
Star Light" (A).
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This is an action packed adventure. Like many other stories of author, it's set on a world with atmosphere toxic to humans but not to their alien friends, only the
world is earth! And unlike vast majority of stories of author, this one
has real villains, in addition to his standard "nature is adversary
enough".
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This is often listed as one of the two sequels to "
This is noteworthy among Clement's stories for 2 reasons:
With the exception of "
Key premise is: space within the orbit of asteroid belt has a 
This is the second story I've read yet of a human spaceship approaching Sun: this story is about conscious travel to "within one hundred fifty thousand miles" of photosphere of Sun for scientific observations; 
This book marks a departure from all Clement stories I've read:
