by Hal Clement
He used five seconds, deliberately and thoughtfully, in analyzing the ship’s motion as well as he could with the unaided eye. It was traveling horizontally by local reference and accelerating toward Tammuz; about half the planet’s disc was above the local horizon. The rate of acceleration was, as nearly as he could tell, the same it had been using toward him, so either that part of his original command had not been affected by the trouble or the disturbing influence was not interested in getting the ship to the planet very quickly.
He aimed the spindle again, and sent an emphatic command to accelerate away from him, holding the aim while he awaited results. They came within about a second. Nimepotea slowed, spun end for end, and headed away from the planet.
Not toward Cunningham at all precisely. Not away from him. It had simply reversed its previous acceleration.
He watched, frowning in concentration, as it passed above him and slightly to one side, about as far as before. Carefully refraining from sending any signals, he waited until it was nearly a kilometer away, now on a line nearly opposite the planet’s direction; only then did he send, at the lowest power he thought had a chance of being effective, an additional order for slow acceleration toward the satellite—downward, from his viewpoint. He was not surprised when the order was obeyed; the question in his mind was how long this would continue.
With hull almost on the horizon—he had been a little slow in realizing that he had better keep it in sight, even if his ideas seemed to be working—he cancelled the downward thrust and once more ordered the ship to approach him. It came, this time low above the surface. He was foresighted enough to place himself on the side of its flight path toward the open lock.
At ten meters distance he ordered it to stop.
He was alert enough to make a leap for the open lock when it didn’t obey, and coordinated enough to score a near hit at four meters. He did not actually fly into the opening, but struck the hull near it; and it would have been difficult to find any spot on Nimepotea’s outer surface out of reach of a climbing rung. Cunningham seized one of these, and an instant later was inside the lock. He closed the outer door with the fixed control, not the spindle, opened the inner the instant it was possible, and hurled himself through it and at the control station without bothering to remove any of his armor.
Naturally, he touched nothing for the second or so it took to assemble a mental picture from the various instruments. Neem was accelerating just as before he had arrived. It was not at once obvious whether the fact that this was also toward Tammuz was anyone’s doing but his own—with the tendency to go that way once displayed, he had naturally tried to use it to solve his immediate problem of getting back aboard. The direction might have been a coincidence before, or this time, or both, or neither.
He keyed controls briefly, sending his little home away from the satellite and at right angles to the former acceleration at a full gravity for five seconds. Then he went into free fall, watching velocity readings and activating sensors for every kind and frequency of radiation the machine could detect, starting with the bands used by the remote controller.
His test could not last indefinitely; he would have had to drive four or five times as long to reach the satellite’s escape velocity, and he certainly did not intend to wait until he struck rock.
He didn’t have to. A command signal came in on the appropriate frequency within twenty seconds, and acceleration toward Tammuz resumed. Cunningham thoughtfully removed his helmet, and more slowly the rest of his armor, as the tiny satellite dwindled behind him. The tunnels would have to wait.
The acceleration, as before, was very low; many hours would be needed to reach Tammuz, if that were in fact where the ship was being led. The man had not succeeded in locating at all precisely the direction of the signal before it had accomplished its presumed purpose and shut itself off; the ship had been assigned a fixed direction, not given a homing location—the signals had been precisely the same as those he himself had been using, except for the “align” command he had employed for a moment when he had first been aware of the problem. Did something or someone want the ship? or him? or was it/he/she simply imitating signals he had used, with no idea of their significance? If that were the case, why not use the “align” one? Because it had not been received? That had been sent from the bottom of the rock shaft, aimed nowhere near Tammuz’ direction, and any diffraction pattern reaching the planet would have been very weak indeed.
Whatever it was had imitated what amounted to “approach” and “depart” signals. An obvious experiment would be to radiate a “home in on this” pattern, which would have to be kept on and would lead the ship to its source—but would the sender know what it meant? If this were blind parroting, would it keep the signal radiating? The others had been short bursts, simply resetting Neem’s directors; why wouldn’t this imitation, if it came at all, do the same—or better, how could it be persuaded to keep sending?
Easily, if imitation were the order of the day. Keep sending the “home” pattern from the ship. Cunningham’s fingers manipulated keys.
There was no twenty-second delay this time. It was much less than one second—a short enough interval to leave the man wondering how much of it could be reaction time of a living being. Most of it, if the response were actually coming from the planet, had to be light-travel time. Maybe the thing was between him and Tammuz, not on the world itself.
If so, the chances were good that it would be changing direction from him, unless a deliberate attempt to hide were being made. There is a limit to the accuracy with which the direction of centimeter waves can be measured without large-aperture detectors or interferometers, but Cunningham did his best for the next ten minutes. As nearly as he could tell, the source was moving with the surface of Tammuz and was most probably on that surface, somewhere well off center from the man’s present viewpoint.
He wondered whether he should speed up. He was certainly curious, and wanted to see what was imitating his signals. He was very dubious about living things on the planet which could be doing it deliberately, though he had encountered life on equally hot and airless worlds; but life takes time to adapt, and for most of its existence this planet had been more or less Earthlike. Its present blistered state, barely outside the atmosphere of a red giant, was a brief and recent part of its history. At the same time, life is rather ubiquitous; its key characteristic, the ability to replicate molecular and energetic patterns, shows up in an incredible variety of chemical machines through a range of temperatures and concentrations which would startle anyone from a pre-star-travel culture.
Even Cunningham considered it more likely that any living creatures at the Tammuz end of the beam were visitors than natives, but he could not guess why Ishtar explorers would be doing it. The situation was a healthy one for curiosity.
If the echo effect were caused by something non-living, however, hurrying might not be so wise. Cunningham had always, so far, managed to get on comfortably with intelligent life forms, at least after long enough and careful enough preparation. More simply natural phenomena tended to be less sympathetic with rational curiosity. He was hoping for intelligent interaction with whatever was going on, but if he had to supply all the intelligence himself then experience suggested that he should learn a good deal more before contact became too intimate.
He compromised by eating before making up his mind. He finished the meal with his cautious personality uppermost, and rather than sending his ship along the beam at higher acceleration he decided to make a few more tests. The reaction time question seemed the most pressing. It also occurred to him that there was no real need to confine test signals to patterns which would control the Nimepotea; this could be a nuisance. If the echo-thing would imitate anything else, experiments could be carried out at a less personal and less risky level.
He set his board to transmit fairly simple multibit patterns on the control channel, beaming them toward Tammuz. with a timer set to determine as precisely as electronics pe
rmitted the interval between transmission and any return of the same pattern. Results were immediate, and suggested continued caution. The signals came back after, to within a millisecond or so, the round-trip light travel time to the apparent edge of Tammuz’ visible disc. If the responder were actually at the center of that disc, it was showing about a fiftieth of a second reaction time—quicker, down to practically zero, if it were anywhere else on the near hemisphere.
Maybe it was somewhere in space between him and the planet after all. A tenth of a second was about the fastest response Cunningham could recall observing in a living being. If it were in space, though, it had to be moving in a way very suggestive of deliberate concealment; it was hard to believe that the apparent match to Tammuz’ surface velocity could be accidental or due to anything less complex than intelligence. If it were trying to hide, its intentions were at least suspect. Cunningham was friendly and outgoing, as well as optimistic, by nature; but he had been around much too long to be naive. He did not speed up. He sat, quite relaxed all things considered, at Neem’s control console and thought, while Tammuz grew larger on the forward vision screen.
He was still thinking when another copy of one of his earlier signals suddenly arrived, applying acceleration at right angles to the principal one. Instantly alert, the man tried to make sense out of the new vector, which would bring the ship to a point a good deal closer to the north pole of the planet than it had been heading. Before he succeeded, the reverse of the last signal sent it on a path which would cause it to miss Tammuz’ limb by several hundred kilometers. He was still trying to work this out without doing anything to the new course itself—there was nothing obviously dangerous about it; if anything, it was safer than before—when the beam his ship was following began to flicker. It weakened, brightened again, then cycled three or four times through a steadily fading pattern which might have been a set of diffraction fringes. Then it went out entirely; and Nimepotea, which had followed it mindlessly as long as there was anything to follow, stopped its acceleration toward Tammuz and maintained only the latest sideways thrust it had been given.
Cunningham had long since located by Doppler the rotation plane of the planet below. He had no real doubt about what had happened, and he applied two gravities of normal-space thrust in the direction which would bring him back into view of that part of the planet which had just been carried out of his sight by rotation. Simultaneously, he shut off the remote control receptors; he thought he knew enough to make his next tests much closer to the planet.
What had been a half-moon shape rapidly grew gibbous, and expanded as he drove the little vessel closer. He watched its image as closely as he could as it grew. Tammuz had long since lost atmosphere and ocean, of course, as its sun aged; but it was far from featureless. There were few prominent shadows except very near the terminator, because of the huge angular size of Anu; but there were albedo features—areas ranging from blinding white, since the man’s sight had long ago adapted to the local color temperature, through brownish yellows and reds to dark brown and virtually black. What substances were represented he could only guess—except that the white was certainly not snow. Not at Tammuz’ temperature. Variations of the silicate pattern which makes up most Terrestrial worlds, most likely, since Tammuz had once been a Terrestrial planet.
Nothing offered the slightest clue, to his eyes, as to the whereabouts of the radiation source. Nothing suggested, even remotely, anything artificial. No works of intelligence could be expected to have survived a billion years even of ordinary planetary environment, of course, far less what must have happened while Anu was turning up his output nearly three hundred times. It would be fun to try to work out in detail what had gone on as atmosphere and oceans were stripped away, tectonic forces readjusted to the changed restraints on the crust, and rising temperature altered the rocks themselves; but that would have to be faced later. Any normal person could get years of recreation out of Tammuz. Some of the Ishtar colonists already had, and Cunningham expected to; but the planet would wait. This radiation problem might not.
A radar scan showed no sign of metal, but did bring response in the form of a similarly patterned beam of much lower power. The imitator was still at work. Cunningham located the source as precisely as he could in terms of the visual images. From his present distance, the uncertainty was ten kilometers or so. A survey specialist ship would have been able to locate it within a meter; retirement-hobby craft didn’t carry such equipment unless the hobbyist made it, and Cunningham’s interest lay more in the direction of personal communication with nonhuman intelligences.
What he did manage was a set of good photographic images, the equivalent of a map for practical purposes, for ten kilometers or so around the most probable location of the emission. He could have done better from a lower altitude, and if he had been sure that this was a matter of communication he would have; but he was still inclined to suspect that some natural, and hence basically unsympathetic, phenomenon was behind the whole affair. He remained cautious.
There is, of course, a perfectly good technique for determining whether a response to a question is a simple echo or not. Cunningham knew it perfectly well—he had even used it more than once—and was extremely annoyed with himself afterward for not thinking of it at the very beginning. He had, he decided, attached too much weight to the reaction time.
He was just finishing the picture mosaic when it occurred to him, and with an exclamation of irritation he snapped off the viewing projector and took the few steps which brought him back to the controls—nothing inside Nimepotea was more than a few steps from anything else; the ship was no place for a claustrophobe, especially after Cunningham had been exploring and collecting for a few years without visiting home.
He turned the radar beam back on, aiming it at the area which had now become quite familiar to him from the photographic work, and waited briefly for the response.
And waited longer for the response.
And longer still.
There was no echo, other than normal radar ones. For a moment the man was even more annoyed; then he brightened. Maybe this was intelligence, and they were tossing him the ball—expecting something new. He was rational, more or less, but human.
He turned the beam off. Then he turned it on once, briefly, and once again. He waited a few seconds, then turned it on twice, paused briefly, and four times. Another wait, three flashes, then nine.
Then he waited.
Nothing happened. X was asleep, or out to lunch, or had lost interest in playing with strange signals, or couldn’t see in this direction.
The last idea could conceivably be checked. Cunningham reached for the photomosaic, then firmed his lips and turned back to the control console. Risks or no risks—and he didn’t really believe there were any—the time had come for a close, personal look at that area. Nimepotea plunged downward, and within minutes was hanging a kilometer above the center of what seemed to be the important section of Tammuz’ landscape while its pilot’s fingers played over direction and field controls of the view screen and his eyes rapidly scanned the screen itself. Almost by reflex he oriented his image with respect to the planet’s rotation axis; being able to apply terms like “north” and “east” helped the memory a lot.
The eastern edge of the imaginary square which seemed so important was cut by the rim of a ten-kilometer impact crater which had clearly formed later than most of the rest of the landscape. Its nearly perfect circle was unaffected by hills, plateaus, and an ancient graben which postdated the first two. The plateaus were the most significant features; except for one detail they resembled the mesas of the North American southwest, and covered fully half the local landscape.
The exception was their edges, which were angle-of-repose slopes rather than steep cliffs. The rock was light-colored, as close to white as anything Cunningham had seen on Tammuz even from a distance which permitted a view of most of a hemisphere. It might conceivably be limestone, and the area represent the erosion r
emnant of a region of horizontal sediment—except, of course, for graben and crater.
The question was whether any caves or overhangs, which could easily have formed in such country during the early stages of weathering as they had on Earth, would still be present when the destruction had reached a general stage of sand slope. This was especially true since the weathering had presumably occurred while Tammuz still had an atmosphere, while the nearby impact feature might not have—and its arrival would probably have shaken down anything with room underneath for a hundred kilometers around. He could look for holes, or he could fly a search grid over the area while broadcasting signals and listening for possible response from X.
He was going, of course, to have to get out and do some foot exploring sooner or later, and one part of his feeling suggested that it might as well be sooner; but native caution had not faded out completely. If nothing else, he should certainly not step outside until he had a good map of the general area firmly in his head.
He could memorize the map while flying the search, so he started the latter. He dropped the ship to five hundred meters above the general plateau tops, which in turn was about seven hundred above the level ground separating the mesas and perhaps a thousand above the bottom of the crater. Starting at the northwest corner of the key area, he headed east at a low speed, about fifty meters a second; he wanted to give himself time for a good look at everything. At the eastern edge of the area, north of the crater, he turned south for two hundred meters and then made the westward cast of his grid. Patiently, back and forth, keenly watching for any detail which might suggest the presence of X, with the ship automatically broadcasting the sequence he had set up earlier and which any intelligent being should reasonably answer with “four-sixteen.” Presumably.
The three-hour flight produced no response to the signals, but left Cunningham ready to spend time out on the planet’s surface if he could only find something worth checking closely. Nothing seemed to offer the slightest danger to a reasonably equipped explorer. Landslides could no longer occur, since all slopes seemed to have reached angle of repose in the mesa area. It was furiously hot, naturally, but his armor could handle that. There was no air, unless a few hundred molecules of carbon dioxide per liter deserved the name, but again that was what space armor was for.
