Close to Critical

Close to Critical by Hal Clement.

SOL, seen at a distance of sixteen light-years, is a little fainter than the star at the tip of Orion's sword, and it could not have been contributing much to the sparkle in the diamond lenses of the strange machine. More than one of the watching men, however, got a distinct impression that the thing was taking a last look at the planetary system where it had been made. It would be a natural thing for any sentient and sentimental being to do, for it was already falling toward the great dark object only a few thousand miles away. Any ordinary planet would have been glaringly bright at that range, for Altair is an excellent illuminator and was at its best right then: Altair is not a variable star, but it rotates fast enough to flatten itself considerably, and the planet was in a part of its orbit where it got the maximum benefit from the hotter, brighter polar regions. In spite of this, the world's great bulk was visible chiefly as a fuzzy blot not very much brighter than the Milky Way, which formed a background to it. It seemed as though the white glare of Altair were being sucked in and quenched, rather than illuminating anything.

But the eyes of the machine had been designed with Tenebra's atmosphere in mind. Almost visibly the robot's attention shifted, and the whitish lump of synthetic material turned slowly. The metal skeleton framing it kept pace with the motion, and a set of stubby cylinders lined themselves up with the direction of fall. Nothing visible emerged from them, for there was still too little atmosphere to glow at the impact of the ions, but the tons of metal and plastic altered their acceleration. The boosters were fighting the already fierce tug of a world nearly three times the diameter of distant Earth, and they fought well enough so that the patchwork fabrication which held them suffered no harm when atmosphere was finally reached. The glitter faded out of the diamond eyes as the world's great gas mantle gradually enfolded the machine. It was dropping slowly and steadily, now; the word cautiously might almost have been used. Altair still glowed overhead, but the stars were vanishing even to the hypersensitive pickups behind those lenses as the drop continued.

Then there was a change. Up to now, the thing might have been a rocket of unusually weird design, braking straight down to a landing on outboard jets. The fact that the jet streams were glowing ever brighter meant nothing; naturally, the air was growing denser. However, the boosters themselves should not have been glowing.

These were. Their exhausts brightened still further, as though they were trying harder to slow a fall that was speeding up in spite of them, and the casings themselves began to shine, a dull red. That was enough for the distant controllers; a group of brilliant flashes shone out for an instant, not from the boosters themselves but from points on the metal girders that held them. The struts gave way instantly, and the machine fell unsupported.

For only a moment. There was still equipment fastened to its outer surface, and a scant half-second after the blowoff of the boosters a gigantic parachute flowered above the falling lump of plastic. In that gravity it might have been expected to tear away instantly, but its designers had known their business. It held. The incredibly thick atmosphere - even at that height several tunes as dense as Earth's - held stubbornly in front of the parachute's broad expanse and grimly insisted on the lion's share of every erg of potential energy given up by the descending mass. In consequence, even a gravity three times that of Earth's surface failed to damage the device when it finally struck solid ground.

For some moments after the landing, nothing seemed to happen. Then the flat-bottomed ovoid moved, separating itself from the light girders which had held the parachute, crawled on nearly invisible treads away from the tangle of metal ribbons, and stopped once more as though to look around. It was not looking, however; for the moment, it could not see. There were adjustments to be made. Even a' solid block of polymer, with no moving parts except its outer traveling and handling equipment, could not remain completely unchanged under an external pressure of some eight hundred atmospheres. The dimensions of the block, and of the circuitry imbedded in it, had changed slightly. The initial pause after landing had been required for the distant controllers to find and match the slightly different frequencies now needed to operate it. The eyes, which had seen so clearly in empty space, had to adjust so that the different index of refraction between the diamond and the new external medium did not blur their pictures hopelessly. This did not take too long, as it was automatic, effected by the atmosphere itself as it filtered through minute pores into the spaces between certain of the lens elements,

Once optically adjusted, the nearly complete darkness meant nothing to those eyes, for the multipliers behind them made use of every quantum of radiation the diamond could refract. Far away, human eyes glued themselves to vision screens which carried the relayed images of what the machine saw. It was a rolling landscape, not too unearthly at first glance. There were large hills in the distance, their outlines softened by what might have been forests. The nearby ground was completely covered with vegetation which looked more or less like grass, though the visible trail the robot had already left suggested that the stuff was far more brittle. Clumps of taller growths erupted at irregular intervals, usually on higher ground. Nothing seemed to move, not even the thinnest fronds of the plants, though an irregular crashing and booming registered almost constantly on the sound pickups built into the plastic block. Except for the sound it was a still-life landscape, without wind or animal activity. The machine gazed thoughtfully for many minutes. Probably its distant operators were hoping that life frightened into hiding by its fall might reappear; but if this were the case they were disappointed for the moment. After a time it crawled back to the remains of its parachute harness and played a set of lights carefully over the collection of metal girders, cables, and ribbons, examining them all in great detail. Then it moved away again, this time with a purposeful air.

For the next ten hours it quartered meticulously the general area of the landing, sometimes stopping to play its light on some object like a plant, sometimes looking around for minutes on end without obvious purpose, sometimes emitting sounds of varying pitch and loudness. This last always happened when it was in a valley, or at least not on the very top of a hill; it seemed to be studying echoes for some reason.

Periodically it went back to the abandoned harness and repeated the careful examination, as though it were expecting something to happen. Naturally, in an environment having a three hundred-seventy-degree temperature, about eight hundred atmospheres pressure, and a climate consisting of water heavily laced with oxygen and oxides of sulphur, things started to happen soon enough; and great interest was shown in the progress of the corrosion as it steadily devoured the metal. Some parts lasted longer than others; no doubt the designers had included different alloys, perhaps to check this very point. The robot remained in the general area until the last of the metal had vanished in slime.

At irregular intervals during this time, the surface of the ground shook violently. Sometimes the shaking was accompanied by the crashes which had first greeted the robot's "ears"; at other times it was relatively silent. The operators must have been bothered by this at first; then it became evident that all the hills in the neighborhood were well rounded with no steep cliffs, and that the ground itself was free of both cracks and loose stones, so there was little reason to worry about the effect of quakes on the fabulously expensive mechanism.

A far more interesting event was the appearance of animal life. Most of the creatures were small, but were none the less fascinating for that, if the robot's actions meant anything. It examined everything that appeared, as closely as it possibly could. Most of the creatures seemed to be scale-armored and eightlimbed; some appeared to live on the local vegetation, others, on each other.

With the harness finally gone, the attention of the robot's operators was exclusively occupied by the animals for a long time. The investigation was interrupted a number of times, but this was due to loss of control rather than distraction. The lack of visible surface features on Tenebra had prevented the men from getting a very precise measure of its rotation period, and on several occasions the distant ship "set" as far as the important part of the planet was concerned. Trial and error gradually narrowed down the uncertainties in the length of Tenebra's day, however, and the interruptions in control finally vanished. The project of studying a planet three times the diameter of Earth looked rather ridiculous when attempted with a single exploring machine. Had that been the actual plan, of course it would have been ridiculous; but the men had something else in mind. One machine is not much; a machine with a crew of assistants, particularly if the crew is part of a more or less world-wide culture, is something very different. The operators very definitely hoped to find local help - in spite of the rather extreme environment into which their machine had fallen. They were experienced men, and knew something of the ways of life in the universe.

However, weeks went by, and then months, with no sign .of a creature possessing more than the rudiments of a nervous system. Had the men understood the operation of the lensless, many-spined "eyes" of the local animals they might have been more hopeful; but as it was most of them grew resigned to facing a job of several lifetimes. It was sheer chance that when a thinking creature finally did turn up it was discovered by the robot. Had it been the other way around - if the native had discovered the machine - history could easily have been very different on several planets.

The creature, when they did see it, was big. It towered 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 prehension. That was the fact that betrayed its intelligence; two long and two shorter spears, each with a carefully chipped stone head, were being carried in obvious readiness for instant use. Perhaps the stone disappointed the human watchers, or perhaps they remembered what happened to metals on this planet and refrained from jumping to conclusions about the culture level suggested by the material. In any case, they watched the native carefully.

This was easier than it might have been; the present neighborhood, many miles from the original landing point, was a good deal rougher in its contours. The vegetation was both higher and somewhat less brittle, though it was still virtually impossible to avoid leaving a trail where the robot crawled. The men guessed at first that the higher plants had prevented the native from seeing the relatively small machine; then it became apparent that the creature's attention was fully occupied by something else.

It was traveling slowly and apparently trying to leave as little trail as possible. It was also making allowance for the fact that to leave no trail was not practicable; periodically it stopped and built a peculiar arrangement consisting of branches from some of the rarer, springy plants and the sharp stone blades which it took in seemingly endless supply from a large leather sack slung about its scaly body. The nature of these arrangements was clear, after the native had gotten far enough ahead to permit a close inspection. They were booby traps, designed to drive a stone point into the body of anything attempting to follow in the creature's footsteps. They must have been intended against animals rather than other natives, since they could easily be avoided merely by paralleling the trail instead of following it. The fact that the precaution was being taken at all, however, made the whole situation extremely interesting, and the robot was made to follow with all possible caution. The native traveled five or six miles in this fashion, and during this time set about forty of the traps. The robot avoided these without trouble, but several times tripped others which had apparently been set earlier. The blades did no harm to the machine; some of them actually broke against the plastic. It began to look as though the whole neighborhood had been "mined," however.

Eventually the trail led to a rounded hill. The native climbed this quickly, and paused at a narrow gully opening near the top. It seemed to be looking around for followers, though no organs of vision had yet been identified by the human watchers. Apparently satisfied, it drew an ellipsoidal object from its sack, examined it carefully with delicate fingers, and then disappeared into the gully.

In two or three minutes it was back, this time without its grapefruit-size burden. Heading down the hill once more, it avoided with care both its own traps and the others, and set off in a direction different from that of its approach.

The robot's operators had to think fast. Should they follow the native or find out what it had been doing up the hill? The former might seem more logical, since the native was leaving, and the hill presumably was not, but the second alternative was the one they chose. After all, it was impossible for the thing to travel without leaving some sort of trail; besides, night was approaching, so it wouldn't get far. It seemed safe to assume that it shared the characteristic of Tenebra's other animal life, of collapsing into helplessness a few hours after nightfall.