Pad Zero
by Claude Opus 5.5
1. On the subject of gravel
There is a photograph that Aiyana Redcloud keeps on the wall above her desk, and it is not of Mars. It is of a road. A gravel road, in South Dakota, on the Pine Ridge reservation, running dead straight across a plain of yellow grass towards a line of low buttes, under a sky the size of a continent. A grader is parked on the shoulder, an old yellow Caterpillar with its blade raised, and leaning against its rear wheel, with his arms folded and his hat tipped back, is her uncle Delbert, who maintained the roads of the eastern half of the reservation for thirty-one years and who taught her, when she was twelve, how to read the crown of a road by the way the rain ran off it.
Visitors to her office at the agency's construction directorate, in a building in Houston whose architecture was apparently designed to discourage visitors, sometimes ask why she has a picture of a gravel road on her wall when she is building a landing pad on Mars. She tells them that it is the same job. They laugh, politely, assuming it is a joke. It is not entirely a joke.
2. The problem, as stated in the documents
The problem, as stated in the documents, which ran to several hundred pages and which Aiyana had helped to write, was this.
When a large rocket lands on a planet, it does so by firing its engines downward, against the ground, to slow itself. The exhaust of those engines strikes the surface at very high speed and spreads outward, and whatever loose material is lying on the surface goes with it. On the Moon, which has no air, the material is thrown out in straight lines, at speeds of up to several kilometres a second, and travels a long way. On Mars, which has a thin atmosphere, the material is slowed by the air but still flies outward with great force. A rocket landing on bare Martian ground will excavate a crater beneath itself, blast a spray of sand and pebbles across the surrounding terrain, and possibly, if the ground is soft or the engines are large, dig so deep that the rocket tips over.
The term of art is plume-surface interaction. It sounds like the title of a dull conference session. It is in fact the thing most likely to destroy a crewed landing on Mars other than the landing itself.
3. A short history of sandblasting
In November 1969, the second crewed landing on the Moon, Apollo 12, set down in the Ocean of Storms about a hundred and eighty metres from an unmanned American spacecraft called Surveyor 3, which had landed there two and a half years earlier. The astronauts walked over to it, photographed it, and cut off some pieces to bring home. When the pieces were examined on Earth, their surfaces were found to be pitted and scoured, on the side that had faced the Apollo lander, by particles of lunar soil thrown out by its descent engine from nearly two hundred metres away. Surveyor had been sandblasted. The Apollo engine was small, by the standards of what would come later. At full power the lunar lander's engine pushed with about the weight of three family cars.
The rockets that land on Mars now carry a hundred tonnes of cargo and fire engines whose combined thrust is the weight of a block of flats.
4. The site
The site had been chosen eight years earlier, after a long argument, on a flat plain in Arcadia Planitia in the northern mid-latitudes, where orbital radar had shown water ice lying a few metres beneath the surface, and where the ground was smooth enough to land on and soft enough to dig into. Four uncrewed cargo landers had arrived there over the previous two years, in two pairs. They stood now in a rough line across the plain, a kilometre apart, each one a tall silver cylinder on landing legs, each one surrounded by a shallow crater of disturbed soil and a radiating fan of debris where its engines had scoured the ground on the way down. They carried the habitat, the power plant, the propellant factory, and the machines that Aiyana had spent eleven years designing: six semi-autonomous construction vehicles, which had driven off the landers' ramps the previous spring and had been working on the plain ever since.
Their job was to build a landing pad for the crew.
5. On the definition of a pad
What is a landing pad? On Earth it is a circle of concrete. On Mars, where there is no concrete, it is whatever you can make out of the materials at hand that will not be blown away by a rocket engine.
The design Aiyana had settled on, after nine years of testing in a vacuum chamber in Ohio and a field site in the Mojave Desert and a great many arguments, was a circle forty metres across, made of interlocking hexagonal tiles. Each tile was a slab of Martian regolith, scooped up from the surface, sieved to remove the larger stones, pressed into a mould and heated by microwaves until its grains fused together into something like a ceramic. The tiles were laid on a graded and compacted base and locked together at their edges. Around the pad, at a distance of sixty metres, the machines were to build a berm, a low wall of compacted regolith two metres high, which would catch whatever material escaped from the pad's edges and stop it from flying across the plain towards the habitat and the power plant and the cargo landers, whose solar arrays and radiators would not survive a sandblasting.
The machines could make, on a good day, about forty tiles. The pad required two thousand and fifty. The berm required moving and compacting about three thousand cubic metres of soil. The schedule, calculated two years before launch, had allowed fourteen months.
They had been working for eleven months, and the pad was a little under half finished.
6. Concerning the delay, which is also a kind of distance
Aiyana had learned, in her first months of operating the machines, that the delay changed everything about how one built anything.
Mars is between four and twenty-four light-minutes from the Earth, depending on where the two planets are in their orbits. During the months of construction it had been, for the most part, between twelve and twenty. A command sent from Houston to one of the machines took that long to arrive, and the machine's report of what it had done took as long again to come back. So nothing could be steered. Every operation had to be planned in advance, as a sequence of tasks that the machine would carry out on its own, watching its own sensors, stopping if anything unexpected happened. Each morning, in Houston, the construction team reviewed the previous day's work, which they could see only in images and measurements that were already half an hour old by the time they arrived, and planned the next day's, and uploaded the plans, and went home, and the machines worked through the Martian day while the team slept.
It was, she sometimes thought, a little like building a house by post.
Her uncle Delbert, when she had tried to explain this to him on the telephone, had been quiet for a long time, and then had said that it sounded like the way the Bureau of Indian Affairs used to manage the roads. She had laughed until she cried.
7. What went wrong
What went wrong was not one thing but a series of small things, none of them dramatic, which is how most construction projects go wrong, on Mars as on Earth.
The tiles cracked. Not all of them, but about one in eight, a fraction that had not appeared in the testing. The cause, after some weeks of investigation, turned out to be the temperature cycle: the surface at the site warmed to about zero Celsius in the afternoon and fell to about minus ninety at night, every night, and the tiles, made of fused regolith with a coefficient of thermal expansion that varied from grain to grain, developed hairline fractures along their weaker boundaries after a few weeks of this. The cracked tiles had to be lifted and replaced. That cost time.
One of the six machines lost a wheel motor in the fourth month. The machines had been designed to continue on five wheels, and it did, but more slowly, and it could no longer carry a full load of tiles. That cost time.
The microwave sintering unit, which fused the tiles, overheated on afternoons when the dust in the air was thick and the radiators could not shed heat efficiently, and had to be shut down for hours at a time. That cost time.
And the regolith at the site was coarser than the orbital data had suggested, with more stones in it, so that the sieving took longer and yielded less usable material per scoop. That cost time too.
Each of these was solved, more or less. Each solution was a small victory, celebrated quietly in the team room in Houston with cake from the supermarket on the corner. And each solution left the schedule a little further behind.
8. On seasons
Mars has seasons, as the Earth does, because its axis is tilted. But Mars also has a strongly elliptical orbit, which means that it is much closer to the Sun at one point in its year than at another, and the season during which it is closest, which is summer in the southern hemisphere, is the season of dust storms. Most of them are local. Some grow to cover regions the size of continents. Every few Martian years, a storm grows until it covers the entire planet, lifting so much dust into the atmosphere that the Sun is dimmed to a pale disc for weeks or months, and the solar-powered rovers that have been sent to Mars have, more than once, been killed by them.
The crew was due to land in five months. The dust storm season began in three.
The construction machines were powered by a small fission reactor, one of the cargoes delivered by the landers, and so would not lose power in a storm. But they would lose visibility. Their cameras would see nothing in the thick of a regional storm, and their navigation, which depended on the cameras, would fail, and the team in Houston had decided long ago that the machines would shelter beside the landers whenever the dust in the air exceeded a certain level, and wait. A regional storm might last weeks. A global storm, months. If one arrived before the pad was finished, it might not be finished at all.
9. The question
So the question that Aiyana had to answer, in the eleventh month, was what to do.
The crew could not wait. That was the premise from which everything else followed, and she had been in enough meetings to know that it was not negotiable. The crew's ship was launching in two months on a trajectory fixed by the positions of the planets; if it did not launch then, the next opportunity was twenty-six months later, and the mission's funding, its political support, the careers of several hundred people and the patience of two governments would not survive a delay of twenty-six months. The ship would launch. It would arrive. And it would land on whatever the machines had built by then.
There were, as she saw it, three possibilities.
The first was to continue as planned and hope. The machines might go faster. The storms might come late. The pad might be finished. If it was not, the crew would land on a pad that was perhaps eighty per cent complete, with a gap in its outer ring, and the flight engineers would have to calculate, as best they could, what the plume would do to a partial pad, which nobody had ever tested.
The second was to reduce the pad. If its diameter were reduced from forty metres to thirty, it would need eleven hundred fewer tiles, and could probably be finished in time even with the storms. But a thirty-metre pad left less margin for the crew's landing accuracy, which was estimated at plus or minus twelve metres; a landing near the edge of a thirty-metre pad would put one of the ship's engines over bare ground, and the ground would erode, and nobody could say how much.
The third was the one she had been thinking about for several weeks without mentioning to anyone, because it was not in any of the documents and she was not sure it was a good idea.
10. The third possibility
The third possibility was to stop building the pad for a month and build the berm instead.
This seemed, on its face, perverse. The pad was the critical item; the berm was a protection. But Aiyana had been looking at the photographs of the cargo landers, and at the scoured fans of soil around them, and at the pitting on the edges of their solar arrays that had been caused by the landings of their neighbours, and she had come to a conclusion that she found she could not shake.
The danger to the crew was not mainly the danger of a poorly built pad beneath their own ship. The ship was robust. Its landing legs were designed for rough ground, and its engines could, if necessary, be throttled to reduce erosion during the last seconds of descent. Even on bare ground, the flight engineers estimated, the chance of a tip-over was small. The danger to the crew was what the plume would do to everything else on the plain: the habitat, two hundred metres from the pad; the power plant, three hundred; the propellant factory, which would make the oxygen and methane for the crew's return and whose radiators and solar arrays and delicate chemical plant were spread across a quarter of a hectare of ground half a kilometre away. Sandblast the propellant factory and the crew could not get home. Sandblast the power plant and the crew could not stay. A pad, however good, would only reduce the ejecta. The berm would stop it.
And the berm was easier. It did not require tiles. It required only that soil be moved and compacted, which the machines were good at, and which did not depend on the temperamental sintering unit. With all six machines working on it, the berm could be finished in five weeks. After that, the pad could be resumed, and finished to whatever extent the storms allowed.
11. A digression on the Seabees
In the Second World War, the United States Navy formed construction battalions, known from their initials as Seabees, to build airfields on the islands of the Pacific as the American forces advanced. They built them very fast, often under fire, on coral and volcanic sand and jungle, and they developed for the purpose a remarkable material called pierced steel planking: long strips of steel, punched with rows of holes, that could be laid across graded ground and locked together at their edges to make a runway in a matter of days. It was not a good runway. Aircraft bounced on it. It buckled in heat and rusted in rain and tore the tyres of the heavier bombers. But it was a runway, and it was there when the aircraft needed it, and it won, in a quite literal sense, a good part of the war.
Aiyana had read a great deal about the Seabees during her training, and had once spent a pleasant afternoon at a naval museum in Port Hueneme, California, looking at a section of the planking under glass. What had struck her, reading the accounts of the men who laid it, was not their speed or their courage, though both were extraordinary, but their pragmatism. They did not build the runway the manuals described. They built the runway the circumstances permitted, and they made sure that whatever they built protected the things that mattered most.
12. The meeting
She proposed it at the weekly review, in a conference room on the second floor of the building designed to discourage visitors, to a room of thirty people and as many more on video.
The response was not enthusiastic. The flight engineers, who were responsible for the crew's landing, were uneasy about any proposal that delayed the pad, which was their concern, in favour of the berm, which was not. The habitat and power teams, who were responsible for the things the berm would protect, were enthusiastic, which was predictable and therefore, in the politics of the room, slightly discounted. The mission director asked her to quantify the risk of each option, and she did, with the help of a probabilistic analyst named Gerald Mbeki who had been building models of the landing for three years and who had, she suspected, been waiting for someone to ask him exactly this question. His numbers were not decisive. They showed that the berm-first option reduced the risk to the surface infrastructure by a large factor and increased the risk to the landing itself by a small one, with large uncertainties on both. They did not say which mattered more.
That was a judgement, and the mission director, after an hour, made it. He approved the berm-first plan, with a condition: that if the pad was less than seventy per cent complete when the storms arrived, the crew's landing target would be shifted from the centre of the pad to a point on its completed side, accepting a greater risk of an engine over bare ground in exchange for keeping the plume away from the gap.
Aiyana thought this was a good condition. She wished she had thought of it.
13. What happened
The machines built the berm in five weeks and two days. It was not beautiful. Seen from the cameras on the landers, it looked like a low ring of grey soil around an unfinished circle of grey tiles, the kind of thing a child might build on a beach with a bucket and a very long afternoon. But it was compacted to the density the engineers had specified, and its height was within ten centimetres of the design everywhere, and when the analysts ran the plume models against its measured shape, it stopped more than ninety per cent of the ejecta that would otherwise have reached the habitat.
The machines then went back to the pad and laid tiles, forty a day, then forty-five, as the team in Houston learned to schedule the sintering unit around the afternoon dust, and then, for three weeks, fifty-two, which was a record that nobody had expected to be broken and which was celebrated with a larger cake.
The first regional dust storm of the season arrived fifty-three days before the crew's landing. It was early. It covered the site for nineteen days, and the machines sheltered beside the landers, and the team in Houston watched the dust readings on their screens and did not sleep well. When it cleared, the machines came out and went back to work. A second storm arrived twenty-one days before the landing and did not clear in time.
The pad was seventy-eight per cent complete when the machines sheltered for the last time. Its northern and eastern sectors were finished; its southern and western sectors had a gap of bare graded soil about eight metres wide where the outer ring of tiles had not been laid. The crew's landing target was shifted four metres to the north-east.
14. The landing
The crew landed in the last days of the second storm, through dust that the ship's radar could see through and its cameras could not, at a point three and a half metres from the shifted target, which was well within the accuracy the flight engineers had promised. The ship's southern engine was over tiles. Its western engine was over the last row of tiles before the gap. The plume eroded a shallow channel in the bare soil of the gap, about forty centimetres deep, and threw a spray of sand and pebbles southward and westward, most of which struck the berm and stopped. Some went over. When the crew emerged, two days later, after the storm had cleared, they found a scattering of small pits on the edges of the nearest cargo lander's solar array, about four hundred metres away, and nothing at all on the habitat, the power plant, or the propellant factory.
The propellant factory made the oxygen and methane for their return. The return was, eventually, uneventful.
15. Coda, in South Dakota
Aiyana flew home to Rapid City the following summer and drove out to Pine Ridge to see her uncle, who was eighty-two and had retired from the roads department some years before and spent his days, as far as she could tell, sitting on the porch of his house in Kyle watching the weather. She brought him a printout of a photograph from the crew's cameras, taken on the morning after they landed: the pad, the gap, the channel in the soil, the berm curving away across the plain under a pink sky, with a tall silver ship standing in the middle of it all.
He looked at it for a long time. Then he pointed at the gap with a finger that had been broken, she knew, in a grader accident in 1979, and asked her why she hadn't finished it.
She said that the weather had come in early.
He nodded, as if this were an entirely sufficient explanation, which, in his world and hers, it was. Then he pointed at the berm and said that it was a good berm, and that she had graded it right, with a decent crown on it, so the dust would run off. And he gave her back the printout and went inside to get two cans of beer, and she sat on the porch in the yellow evening light and looked out at the gravel road running straight across the plain to the buttes, and thought that she had never been paid a higher compliment, and never would be.