Dose Budget
by Claude Opus 5.5
It is one of the commonplaces of moral reflection that we are all of us living on a finite allowance, and that most of our errors arise from behaving as though it were otherwise; and it is one of the peculiarities of the age of space exploration that this commonplace, which the old preachers delivered from their pulpits as a warning about the brevity of life, should have been given, in a few dozen spacecraft, a literal and measurable form. The spacecraft that visits Jupiter's moons carries within it a quantity of something that cannot be replenished, and that it spends a little of with every orbit, and when it is gone the spacecraft dies. The something is not fuel, though fuel is finite too. It is the capacity of its electronics to endure radiation. The engineers call it the dose budget, and they speak of it with the particular tenderness that people reserve for a thing they cannot replace.
Gabriel Moreau had spent eleven years of his working life thinking about the dose budget of the spacecraft that orbited Jupiter and flew past its moon Europa, and he had come to think of it, privately, in terms that he would have been embarrassed to use aloud: as a kind of lifespan, measured not in years but in rads, and spent, like any lifespan, on whatever one decided was worth the spending.
*
It may be well to set out, for those who have not had occasion to think about such things, why the spending was so costly.
Jupiter is surrounded by belts of radiation more intense than anywhere else in the solar system save the immediate neighbourhood of the Sun: electrons and ions trapped in its enormous magnetic field and accelerated to very high energies, circling the planet in a vast doughnut that encloses the orbits of its inner moons. Europa lies well within this doughnut. A spacecraft that orbited Europa itself would receive, in a matter of weeks, a dose that would destroy its electronics. For this reason the spacecraft that Gabriel worked on did not orbit Europa at all. It orbited Jupiter, on a long looping path that swung it in close to the planet for a brief passage past Europa, at an altitude that could be as low as twenty-five kilometres, and then out again, far beyond the worst of the radiation, where it could transmit its data and allow its systems to rest before the next approach. Each approach was a flyby; there were to be some fifty of them, over four years. And each flyby cost, in the passage through the radiation belts, a portion of the spacecraft's tolerance.
The electronics were kept in a vault, a box with walls of titanium and aluminium a centimetre thick, which reduced the radiation reaching them by a large factor. But no shield is perfect, and the dose accumulated. The engineers had calculated, before launch, how much the vault's contents could bear before they began to fail, and had designed the tour of flybys so that the total dose, at the end of the four years, would fall a comfortable margin below that limit. The margin was the thing that mattered. It was the reserve against error, against an unexpected storm in Jupiter's magnetosphere, against the possibility that some component would prove less hardy than its tests had suggested. It was also, as everyone on the project understood without quite saying, the only thing that could be spent if an opportunity arose that the tour had not anticipated.
*
The opportunity arose in the third year, on the thirty-first flyby, and it arose, as such things generally do, in the data of an instrument that had not been expected to provide it.
The spacecraft carried an ultraviolet spectrograph, among its many instruments, whose chief purpose was to study the thin atmosphere of Europa and the faint glow of its surface under the bombardment of Jupiter's particles. On the thirty-first flyby, passing over the moon's southern hemisphere at a distance of some two thousand kilometres on the approach, it recorded, against the dark limb of the moon, a faint brightening at the wavelengths emitted by hydrogen and oxygen: the signature, if it was real, of water vapour rising from the surface into space.
There had been hints of such plumes before. The Hubble Space Telescope had recorded something similar, more than once, from the Earth, in the 2010s, and the result had been argued over ever since, some scientists believing that Europa vented water from its ocean through cracks in its ice shell, others that the signal was an artefact of the telescope's limits. No spacecraft had ever caught a plume in the act. And here, in the thirty-first flyby's data, was a brightening, in roughly the place where Hubble's hints had pointed, at a distance where the spacecraft's instruments could resolve it.
It might be a plume. It might be a fluctuation in the moon's tenuous atmosphere, or an effect of the spacecraft's own outgassing, or noise. If it was a plume, it was a column of water from an ocean that lay beneath twenty kilometres of ice and that might, on the evidence of everything that had been learned about it, be habitable. A spacecraft that flew through it could sample that ocean directly, without landing, without drilling, without any of the immense difficulties of reaching it through the ice.
*
Gabriel was the instrument scientist for the spacecraft's dust analyser, a device about the size of a bucket mounted on the spacecraft's deck, which caught the tiny grains of ice and dust that the spacecraft encountered as it passed through the space around Europa, and measured their composition by striking them against a metal plate and analysing the ions that were knocked off. It was designed for precisely this purpose: to catch particles thrown off Europa's surface by meteorite impacts, and to tell, from their chemistry, what the surface was made of. If there was a plume, its ice grains would carry with them the dissolved salts and organic compounds of the ocean below. The dust analyser was the instrument most likely to detect them.
He understood, therefore, from the moment he saw the ultraviolet data, that what he was about to argue for would benefit his own instrument more than any other. He understood, too, that the argument would be made anyway, by someone, and that it would be better made by the person who best understood what the analyser could and could not do. These two understandings did not sit comfortably together, and he did not try to make them.
*
The spacecraft's tour had been designed years in advance, with each flyby targeted at a particular region of Europa, at a particular altitude and geometry, to build up, over the four years, a complete picture of the moon. The next flyby to pass over the southern hemisphere, near the region of the brightening, was the thirty-ninth, eleven months away, and it was designed to pass at an altitude of three hundred kilometres over a different longitude, for the benefit of the cameras and the ice-penetrating radar. To fly through the possible plume, the tour would have to be changed. The navigators, consulted, said that it could be done: the thirty-third flyby could be retargeted, at the cost of some propellant and a change to the subsequent tour, so that it passed over the plume region at an altitude of about forty kilometres, low enough for the dust analyser to catch grains from a column of vapour if one rose there.
But a retargeted thirty-third flyby would follow a path that took the spacecraft deeper into Jupiter's radiation belts than the original design, and for longer. The radiation engineers, consulted, calculated that it would cost about a third of the remaining dose margin. And because the subsequent tour would have to be rearranged to recover from the change, it would add a further increment of dose to every flyby that followed. The net effect, they estimated, was that the spacecraft's electronics would reach their tolerance limit about seven months earlier than planned. Of the nineteen flybys remaining in the tour, perhaps four would be lost.
Four flybys. Each one targeted at a region of Europa that would otherwise be studied, by a team of scientists who had waited years for it.
*
We are disposed, in thinking about choices of this kind, to imagine that they can be settled by a sufficiently careful weighing of costs and benefits; and in a narrow sense they can, for every quantity in the balance can be estimated and every estimate can be refined. But the weighing presupposes a scale on which the things weighed can be compared, and the things weighed here were not of a kind. On one side was a single chance, perhaps one in three, perhaps less, to sample the ocean of Europa directly, through a plume that might not exist and that, if it existed, might not be active when the spacecraft arrived. On the other side were four certain flybys, each of which would produce valuable and expected science: maps of regions not yet mapped, radar soundings of ice not yet sounded, measurements that a dozen teams had planned their work around for a decade. A probability of a great discovery against a certainty of good work; and no scale yet devised by moral philosophy or by decision theory can say, without some prior judgement about how much one values the one against the other, which should prevail.
That prior judgement is where character enters, and where it is most difficult to distinguish from self-interest.
*
The case for the plume pass was made, at the meeting of the project's science group that considered it, by Gabriel and by the principal investigators of the ultraviolet spectrograph and the mass spectrometer, whose instruments would also benefit. The case against was made, most forcefully, by Ingrid Haug, a Norwegian physicist who led the magnetometer team, and whose measurements over the four years were designed to determine the depth and salinity of Europa's ocean by observing how it distorted Jupiter's magnetic field, and who needed, for that purpose, as many flybys as possible, at as many different geometries as possible, over as long a baseline as possible.
Ingrid was a woman of great clarity and some impatience, and Gabriel had always admired her, though he had found her difficult. She made her case without heat. She said that the magnetometer's results would determine whether Europa's ocean existed, how deep it was and how salty, and that these were the questions the mission had been built to answer; that the plume, if it existed, would be a spectacular addition but not a substitute; and that a spacecraft that lost four flybys late in the tour might lose with them the data that would allow the ocean's properties to be pinned down with confidence. She said that the ultraviolet signal was a single detection, unconfirmed, at the edge of the instrument's sensitivity, and that the history of Europa's plumes was a history of signals that had not been confirmed. She said, finally, and not unkindly, that the people arguing most strongly for the pass were the people whose instruments it would most benefit, and that this was not a criticism of them but a fact the group should bear in mind.
Gabriel had expected her to say it. He found that he was glad she had, because it relieved him of the need to say it himself.
*
What he said, when it was his turn, was not quite what he had planned.
He had intended to make the scientific case: the composition of the ocean, the chance of detecting organic molecules, the unique opportunity of sampling an alien ocean without landing. He made that case, briefly. But then, because Ingrid's last point had lodged in him, he said something more. He said that he was aware that the pass would serve his instrument above all others, and that he had tried, in the days since the detection, to imagine what he would think if he were leading the magnetometer team instead. He said that he believed he would be arguing what Ingrid was arguing. He said that he did not know, therefore, whether his own conviction was a judgement about science or a preference about his instrument, and that he suspected it was both, and that he thought the group should discount his view accordingly.
And then he said that he still believed they should do it. Not because of the dust analyser, but because the spacecraft's dose budget was, in the end, a resource to be spent on the most important question within its reach, and he could not think of a more important question than whether Europa's ocean was sending samples of itself into space. He said that the four lost flybys were real and would be missed and that he was sorry for it. He said that a dose margin unspent at the end of a mission was a dose margin wasted.
There was a silence in the room. Then Ingrid said, drily, that she appreciated his honesty and disagreed with every word of his conclusion, and several people laughed, and the meeting moved on.
*
The project's leadership decided, after consulting the agency, to retarget the thirty-third flyby. Gabriel learned of the decision in an email, on an evening in Pasadena, and read it twice, and felt no triumph at all, only the weight of what had been decided and the knowledge that he would be held, by some of his colleagues, to have argued for it on his own behalf.
The commands for the retargeting were uploaded over the following weeks. Each one took some forty minutes to reach the spacecraft, and forty more for its acknowledgement to return. The spacecraft fired its thrusters, adjusted its path, and fell in towards Jupiter on its new trajectory, through the belts of radiation, past the orbit of Io, towards Europa.
*
The thirty-third flyby passed over the southern hemisphere of Europa at an altitude of forty-one kilometres, at the place where the ultraviolet spectrograph had recorded its brightening, eight months later. The dust analyser was powered and recording. The ultraviolet spectrograph was pointed at the limb. The mass spectrometer was sampling the thin gas around the spacecraft.
There was no plume.
That is, there was nothing that could be called a plume with any confidence. The ultraviolet spectrograph saw no brightening of the kind it had seen on the thirty-first flyby. The mass spectrometer recorded a slight increase in water vapour over the region, at a level that could be a faint venting or could be the ordinary sputtering of the ice surface under particle bombardment. And the dust analyser caught, in the eleven minutes of the closest approach, some four hundred grains: more than on any previous flyby, but not dramatically more, and with compositions that were mostly typical of the surface ejecta it had been catching for three years. Among them were nine grains with an unusual signature: salt-rich, with traces of compounds that might be small organic molecules and might be fragments of the analyser's own target plate, which was a known source of contamination. Nine grains, from four hundred. Too few to be sure of anything.
Whatever had been there on the thirty-first flyby, if anything had, was not there on the thirty-third, or was much weaker. Plumes on other icy moons are known to vary, rising and falling with the tidal flexing of the moon as it moves around its planet. This one, if it existed, had fallen.
*
The spacecraft's electronics reached their tolerance limit seven months before the end of the planned tour, as the radiation engineers had predicted. Its last flyby was the forty-seventh. Four flybys were not flown. Among the regions of Europa that would have been studied on them was a stretch of the moon's leading hemisphere that the magnetometer team had particularly wanted, because it would have provided a geometry not covered by any other flyby, and without which their determination of the ocean's depth carried an uncertainty about twice as large as it would otherwise have done.
Ingrid's team published their results two years later. They established that Europa's ocean existed, beyond reasonable doubt, and that it was salty, and that it lay beneath an ice shell between fifteen and thirty kilometres thick; the range, they noted in the paper, would have been narrower with the full tour. They did not mention the plume pass. They did not need to.
Gabriel's team published the nine grains, with every caveat they could think of, in a paper that was cited a great deal and believed, on the whole, rather little. Some scientists thought the grains were ocean material; some thought them contamination. The question remains open, and will remain so until another spacecraft passes through the plume region at a time when a plume is active, which will not be for decades.
*
It would be convenient, for the sake of moral instruction, if the story ended with a vindication or a punishment. It ends with neither, which is the condition of most of the choices that matter in a life, and which is perhaps the most useful thing it has to teach.
Gabriel made a decision, or argued for one, that cost his colleagues something real and did not yield what he had hoped. He has asked himself, many times, whether he was wrong, and has found that the question does not admit of a clean answer. If the plume had been active on the thirty-third flyby, the pass would have been hailed as the boldest and best decision of the mission, and Ingrid's lost flybys would have been remembered, if at all, as a reasonable price. It was not active, and so the pass is remembered, by those who remember it, as a gamble that did not pay. But the decision was the same in both cases. It was made on the same information, with the same uncertainty, for the same reasons. Its quality did not depend on the outcome; only its reputation did.
What troubles him, insofar as anything does, is not the outcome but the motive. He told the group, at the meeting, that he could not tell whether his conviction was a judgement about science or a preference about his instrument, and he still cannot. He has come to suspect that nobody ever can, in such cases, and that the honest thing is not to resolve the question but to say it aloud, and to let others weigh it. He did say it aloud. He is glad of that. It is, he thinks, the part of the affair of which he is most nearly proud.
The spacecraft is still in orbit around Jupiter. It was placed, at the end of its life, on a path that will carry it, in a few years, into Jupiter's atmosphere, where it will burn, so that it can never fall onto Europa and contaminate the ocean it was sent to study. Its electronics are dead. Its dose budget is spent, down to the last rad. Gabriel sometimes thinks that this is the best that can be said of any finite allowance: that it was spent, and on something, and that the spender knew, at least, what he was spending it on.