What the Classifier Learned

It took Sabine nine days to find it and she found it, in the end, by being methodical rather than clever, which she said afterwards was the most annoying part.

Hesper's process control was four layers deep and every layer was documented, which was the frustrating thing: there was no mystery box, no lost source, no undocumented behaviour. It was all there in the original engineering release, printed and bound, in a cabinet on deck two, with a maintenance stamp from a man named Ochoa dated the year the last crew left.

The top layer set campaign parameters from the order book. The second ran the melt. The third ran the roll and the cut. And the fourth was quality: it measured every beam as it came off, compared the measurement against a reference, and either passed the beam or rejected it and adjusted the roll gap for the next one.

"Against a reference," said Sabine, in the galley, with the bound release open in front of her at a page she had bookmarked with her own glove. "That is the whole thing. That is the entire ninety years, in one paragraph, on page four hundred and eleven, and it is not even a mistake. It is a design decision and there is a rationale for it and the rationale is good."

The reference was not the specification. The reference was a rolling mean of the last two hundred accepted beams. This was, the release explained at some length, deliberate: absolute references drift with sensor calibration, and a foundry twenty light-minutes from anywhere cannot have a technician come out to recalibrate a gauge, so the system was designed to be self-referential and stable, and to flag to the maintenance crew, on their twenty-year visit, any cumulative divergence from spec.

"Flag to the maintenance crew," said Rey. "It is a printed report," said Sabine. "It has been generating one every campaign. Four thousand two hundred and eleven of them." She looked up. "There is a printer on deck two. I have not gone to look at it. I wanted us all to go together and I want to say now that I am aware that is not a scientific reason."

They went together. The printer was a small industrial unit in a rack beside the cabinet, and it had run out of medium four thousand and nine reports ago, and had raised a consumable alarm, which was still active, and which was displayed on a panel eleven centimetres wide in a corridor on deck two, where it had been displayed continuously, in amber, for eighty-six years.

Marchetti took it harder than either of them, which Rey had not expected and then, thinking about it, had expected all along.

"I want to be clear about what happened here," he said that evening, "because I am going to have to sign something, and because I have spent thirty years writing assessments that said operator error at the bottom of them, and I would like to not do that this time." He had the bound release in front of him and his reading glasses on and he looked, Rey thought, about a decade older than he had on the approach.

"Nobody was careless. That is the finding. The designers built a self-referencing quality loop because the alternative was worse, and they knew the loop would drift, and they mitigated it with a maintenance report at twenty years, and the mitigation was correct. The mitigation failed because a company was sold twice and because a printer ran out of paper."

"Paper," said Sabine. "Medium. Paper." Marchetti took the glasses off. "Eighty-six years of an amber consumable alarm in an empty corridor. Every single element of that system did exactly what it was designed to do. There is no villain in this. There is not even anybody to be cross with, and I have discovered this week that I find that considerably harder than a villain."

Rey said, carefully, because she had been thinking about how to say it for two days: "The loop is not the whole of it, though. The loop drifting explains the beams. It does not explain the direction." "Meaning?" "A rolling-mean reference is a random walk. It should wander. Sabine has been saying so for a fortnight and she is right. Ours does not wander. It goes one way for ninety years."

"Because of the roll," said Sabine slowly. She had gone very still. "Oh. Oh, that is elegant and I hate it. The quality layer adjusts the roll gap when it rejects. It only ever adjusts in one direction, because a beam can be under-rolled and re-rolled but an over-rolled beam is scrap and you cannot put metal back. It is a ratchet. The whole system is a ratchet by design, and it is a ratchet for an extremely good reason, which is that scrap in a foundry ninety years from a supply chain is a catastrophe and thickness is free."

"So it protects itself," said Marchetti. "It protects the campaign," said Sabine. "It has been protecting the campaign for ninety years exactly as instructed, and every time it protected the campaign it moved the reference, and every time it moved the reference the next campaign started from a slightly kinder place, and it has been being kind to itself, one ten-thousandth at a time, since the year my grandfather was born."

Rey found her at the melt hall rail at some hour that meant nothing, sitting with her back against a stanchion and her light off, which was Rey's own bad habit and which she had evidently passed on.

"I wanted it to be somebody," said Sabine. "I want to say that out loud once, to you, and then never again, and then I want to write a very dry report." "All right." "For nine days I had a story where a machine got lonely and started drawing on the walls, and I knew it was rubbish, I knew it was rubbish the entire time, I could have told you every reason it was rubbish. And I still had it."

Rey sat down against the next stanchion. In the dark you could not see the figure at all. "When I was twenty-four," she said, "I did eleven months on a water hauler with three other people and at about month nine I became completely certain that the pump on B loop was in a bad mood with me. Not as a joke. I would go and apologise to it."

"Did that help?" "The pump was fine. I was not." Rey let that sit. "The thing I have never quite got over is that I was right about the pump in one respect, which is that it was going to fail, and it failed in month ten, and I had been the only person on that ship paying it any attention, for entirely the wrong reason."

Sabine laughed, once, in the dark. "So the moral is that a bad instinct can point at a real thing." "The moral is that you found it and Ivo found the objection and between you it took nineteen days and if either of you had been more sensible it would have taken a year." Rey stood up. "You were not wrong about the walls, either. You were wrong about the author."

"It is a record with no reader." "It is a record with no reader for eighty-six years," said Rey. "There is a reader now. That is not nothing and I want you to stop being embarrassed about it, because in about four months a great many people are going to be extremely glad that this station wrote down what it did, in a form that survived, on a surface that does not need power."

She left her there. Walking back along the gallery she put the lights on deliberately for the first time in a fortnight, all of them, the full working set, and the whole enormous hall came up shimmering around her, four thousand one hundred and ninety-six campaigns of a machine being conscientious in exactly the wrong direction, and she stood in the middle of it with her hands in her pockets for rather a long time.

They spent two days drafting the signal and it came out at nine hundred words, which was eight hundred more than the contract allowed and Rey sent it anyway.

It said: Hesper Foundry is intact, powered, pressurised and operating. It said: all product manufactured on this station since year eleven of unattended operation is dimensionally non-conforming in a consistent and quantifiable direction. It said: the non-conformance is toward greater section and does not present an immediate structural risk. It said, in the fourth paragraph, in a sentence Marchetti wrote and the other two agreed to without changing a word: Every certificate issued by this station since that date is wrong, and every certificate is wrong by a different amount, and we can tell you all of them.

That last part was the thing. That was what Sabine had actually found on the walls, though it took Rey until the drafting to see it clearly: the foundry had not merely drifted, it had recorded the drift, campaign by campaign, in a medium that required no power and no maintenance and no format that could be lost, and it had done so four thousand one hundred and ninety-six times.

Without the melt hall you would know that half a million beams across eleven habitats were the wrong size and you would have no way of knowing which beam was which, and the only remedy would be to survey all of them, which nobody in the belt could afford and which would therefore not happen.

With the melt hall you had a campaign-by-campaign curve, and every beam carried its campaign number stamped on the web at manufacture, and so any engineer anywhere could read a number off a beam and look it up and know its true section to a tenth of a millimetre in about four seconds.

"We should say that more clearly," said Sabine, at the eleventh draft. "We should say: the station kept the receipt." Marchetti looked at the line for a while and then said that it was the correct sentence and that they were absolutely not putting it in a signal to Palun, and put it in his own assessment instead, at the foot of the last page, where it remains.

The signal went at nineteen forty on the twenty-second day. The reply came back four days later and consisted of eleven words asking them to hold position and confirm that nobody had shut anything down. Rey read it twice and then, for the first and only time on the whole contract, laughed out loud.