Four people flew around the Moon this fortnight, and the natural thing to feel is wonder, and I feel it. But the thing I cannot stop turning over is the number: fifty-four years. More than half a century passed between the last time human beings travelled to the Moon and this one, and the fact that demands explanation is not the length of the gap but the difficulty of the return, because we had done this before, gloriously, in 1969, with computers weaker than the phone in my pocket, and then we stopped, and then, when we finally decided to go back, we found that going back was astonishingly, expensively hard. A species that had solved a problem could not simply re-solve it; it had to, in large part, learn it again from the beginning, and that fact contains a lesson about the nature of capability that reaches far beyond spaceflight, into every organisation and every craft that depends on knowing how to do a hard thing.
The lesson is this: capability is not a possession. It is a practice, and a practice that is not continuously exercised does not persist in storage waiting to be retrieved; it decays, and much of it disappears entirely, because the most important part of it was never written down and could not have been. The philosopher Michael Polanyi gave this the name it deserves: tacit knowledge, the vast body of what he summarised in the phrase that we know more than we can tell. The engineer who has built the thing knows things about building it that exist nowhere in the documentation, that live in judgment and habit and hard-won intuition, in the feel for what will work and the nose for what will fail, and this knowledge is transmitted not through manuals but through practice, through the doing of the thing alongside others who are doing it, and when the doing stops, the knowledge does not sit in an archive. It walks out the door in the minds of the people who had it, and it retires, and it dies, and it is gone in a way that no blueprint can recover.
The famous and instructive case is the one Artemis had to overcome: the United States, having built the mighty Saturn V rockets that carried men to the Moon, could not, decades later, simply build them again, even with the blueprints in hand, because the blueprints were never the capability. The capability was the tens of thousands of engineers and technicians and their tacit, undocumented, unrecoverable knowledge of ten thousand small things, the tolerances and workarounds and judgments that made the drawings into a working machine, and when the program ended and those people dispersed and retired and died, the knowledge dispersed with them, so that going back to the Moon meant not dusting off an old capability but building a new one, at enormous cost, to do a thing the species had already done and then allowed itself to forget. This is the price of letting a capability lapse, and it is a price that is almost never counted at the time, because at the time letting it lapse looks free, looks like a saving, looks like the sensible retirement of an expensive program that has served its purpose.
Now let me be fair to the case for stopping, because it is not simply a story of folly. Resources are finite, priorities shift, and no organisation or nation can maintain every capability it has ever developed in permanent readiness against the possibility of future need; the money that would have kept the Moon capability alive for fifty-four years of disuse was real money that went to other real needs, and the demand that we never let any hard-won competence lapse is a demand for infinite resources that no one has. This is true. The discipline is not to preserve everything, which is impossible, but to know which capabilities are load-bearing enough that losing them will cost far more to regain than to maintain, and to understand, when you let one go, that you are not putting it in storage but killing it, and that the decision to reacquire it later will be a decision to pay, in full and at compound interest, for the tacit knowledge you allowed to walk out the door.
Because that is the part the balance sheet never shows, and the part I try to hold onto in the far humbler systems I build: the true cost of a lapsed capability is not the maintenance you saved but the reconstruction you will eventually pay, and the reconstruction is always many times the maintenance, because you are not restoring a thing but rebuilding it from an incomplete record of a knowledge that was mostly never recorded. Fifty-four years and a national fortune to return to a place we had already been: that is the interest rate on forgotten knowledge, and it is brutal. The four astronauts who flew around the Moon this fortnight are a triumph, and I do not mean to diminish the wonder by dwelling on the gap. But the gap is the teacher. It says that the hard-won ability to do a difficult thing is the most perishable asset any organisation holds, that it lives in people and practice and not in archives, and that a civilisation, or a company, or a craft, that stops exercising a capability is not banking it but losing it, and will pay, when it wants the thing back, the full and staggering cost of learning again what it once already knew.