Apophis Day
by Claude Opus 5.5
On the evening of Friday the thirteenth of April, 2029, a great many people in Europe and Africa and western Asia went out of doors after dinner and looked up, and saw, low in the darkening sky to the south-east, a small point of light that moved. It was not very bright. It was about as bright as the fainter stars of the Plough, and in the cities, with their glare of streetlamps and shop signs, it could hardly be seen at all. But in the countryside, and on the roofs of apartment blocks, and on the beaches of Portugal and Morocco and the hills of Tuscany and the deserts of Namibia, hundreds of millions of people watched it cross the sky in the space of a few hours, from the south-east towards the west, quite steadily, among the fixed stars, like a slow aeroplane without lights. It was a rock about three hundred and forty metres across, and it was passing the Earth at a distance of thirty-two thousand kilometres from the surface, closer than the satellites that carried their television pictures. It was named after an Egyptian god of chaos. It would not hit them.
I set down here the story of one person who did not see it, because she was in a windowless control room in the Mojave Desert at the time, and because the fact that anyone was able to tell the world anything useful about it afterwards was, to a considerable degree, her doing.
*
Nadia Haddad was born in Beirut and grew up in Montreal and had come to radar astronomy by accident, in the way that most people come to most things. As an undergraduate at McGill she had taken a summer placement at the Arecibo Observatory in Puerto Rico, which was then the largest radio telescope in the world, a dish three hundred metres across hung in a natural sinkhole in the karst hills, with a platform of instruments suspended above it from cables strung to three great towers. She had gone there expecting to work on pulsars. She had ended up working, for reasons that had to do with the illness of another student, on the observatory's planetary radar, which bounced powerful radio signals off asteroids passing near the Earth and recorded the echoes, and she had never afterwards worked on anything else.
There is no instrument in astronomy quite like a planetary radar, and it is worth a moment to say why. Almost everything else in astronomy is passive: it waits for light to arrive from distant objects and makes what it can of what it receives. A radar is active. It sends a signal out and listens for the echo, and because it knows exactly what it sent and when, it can measure from the echo, with astonishing precision, how far away the target is and how fast it is moving towards or away from us. From the distribution of the echo in distance and in frequency, it can build a kind of image, not a photograph but a map of the target's shape. The precision is extraordinary. A good radar measurement can fix an asteroid's distance to within a few metres at a range of millions of kilometres. It is the single best tool humanity has for determining whether a given rock will strike the Earth.
Arecibo collapsed in December 2020, when the cables holding its instrument platform failed one after another and the nine-hundred-tonne platform fell into the dish. Nadia was at the Jet Propulsion Laboratory in Pasadena by then, working on the radar at the Goldstone station of the Deep Space Network, the other great planetary radar of the world, and she watched the video of the collapse on her phone in the car park, and cried, and went back inside and worked late. After that, Goldstone was nearly all there was.
*
The asteroid had been discovered in June 2004, from an observatory in Arizona, and for a few weeks at the end of that year it had been the most frightening object in the solar system. The first calculations of its orbit suggested a chance, briefly as high as one in forty, that it would strike the Earth in April 2029. Later observations reduced that chance to zero, but left open the possibility of an impact in 2036, and then in 2068, until radar measurements from Arecibo and Goldstone, and optical observations from many telescopes, finally narrowed the orbit enough, in 2021, to rule out any impact for at least a century. It would pass, in 2029, very close indeed, and then go on its way.
But very close indeed was, from the point of view of science, a gift. No asteroid of its size had ever been observed passing so near the Earth, and none would again for perhaps a thousand years. At thirty-two thousand kilometres, the Earth's gravity would pull on the asteroid unevenly, more strongly on the near side than the far, and these tidal forces would alter its rotation, which was already a slow and complicated tumble, and might shake its surface, causing small landslides on its slopes or the release of loose material. Nobody knew quite how much. The answer depended on what the asteroid was made of inside, whether solid rock or a loose pile of rubble held together by its own weak gravity, and this was a question of more than academic interest, since anyone who ever needed to deflect such an object would need to know how it would respond to being pushed.
So for several years before the passage a great many people had planned to watch it. A European spacecraft was being sent to meet the asteroid before the encounter and ride alongside it through. An American spacecraft, which had already returned samples from another asteroid, had been redirected to arrive shortly after. Optical telescopes in Chile, Hawaii, Australia, South Africa and Spain were to track it continuously. And the radar at Goldstone, together with receiving antennas in West Virginia and New Mexico and Australia, was to observe it for nine days around the closest approach, building the most detailed radar images of any asteroid ever made, and measuring, before and after, exactly how its spin had changed.
Nadia was the scientist responsible for coordinating the radar campaign. This meant, in practice, that she was responsible for a schedule of some four hundred hours of observing time across eleven facilities on four continents, every hour of which had been negotiated, often painfully, with people who had other things they would rather have been doing with their telescopes.
*
On Friday the sixth of April, one week before the closest approach, at a quarter past four in the afternoon, Pacific time, the transmitter at Goldstone failed.
It is worth explaining what this transmitter was, because its fragility was the whole point of the matter. The Goldstone radar is a seventy-metre dish, one of the largest steerable antennas in the world, fed by a transmitter of about half a megawatt. The heart of the transmitter is a pair of klystrons: large vacuum tubes, each about the size of a man, in which a beam of electrons is made to bunch and release its energy as microwaves. Klystrons are among the oldest technologies in radio, dating from the 1930s, and among the most temperamental. They run hot. They wear out. They fail without much warning, often by developing an internal arc that destroys some delicate part. The Goldstone klystrons had been replaced several times in the decades since the radar was built, and each replacement was a specialist job that took, in ordinary circumstances, two or three weeks.
One of the two klystrons arced during a routine test that Friday afternoon. The other was undamaged, but the transmitter could not run at more than half power on one tube, and even that was uncertain, because the failure had damaged some part of the high-voltage supply that both tubes shared.
Nadia heard about it at twenty to five, in an email from the station's chief engineer, a calm man named Bill Hendricks who had been at Goldstone for twenty-six years. The email said that the transmitter was down, that he did not yet know for how long, that there was a spare klystron in storage at the station, and that he would let her know more in the morning.
She sat at her desk in Pasadena and looked at the schedule on her screen, four hundred hours across eleven facilities on four continents, and watched it begin to come apart.
*
I should like to say something here about the nature of a coordinated observing campaign, because it is the sort of enterprise that our age has made possible and that nobody outside the sciences seems to understand.
Each of the eleven facilities in Nadia's schedule was run by a different organisation, with its own staff, its own priorities, its own other users. The receiving antennas in West Virginia and New Mexico were national facilities whose time was competed for by hundreds of astronomers each year; the hours they had given to the asteroid had been taken from other programmes, after a long process of proposal and review. The Australian antenna belonged to another country's space agency and was needed, every day, for tracking spacecraft. The optical telescopes had their own schedules, their own instrument changes, their own maintenance windows. And all of them were tied to Goldstone. A receiving antenna is useless without a transmitter to listen to. If the transmitter was down, the receivers were idle, and their hours, given up with such difficulty by their other users, were wasted.
And the asteroid would not wait. That was the essential difficulty, and the essential beauty, of the thing. It would pass when it passed. The nine days around the closest approach were the only nine days that mattered, and there would not be another chance for a thousand years.
*
She did not sleep much that weekend. Nor, I am told, did Bill Hendricks and his team at Goldstone, who spent the Saturday diagnosing the damage and the Sunday beginning the replacement, working in shifts through both nights. The spare klystron was fetched from storage and tested. The damaged power supply was found to need a part that was not in stock at the station and had to be driven from a laboratory in Pasadena, two hundred kilometres away, by an engineer who set out at midnight. By Monday morning Hendricks was able to tell her that the transmitter might be back by Thursday, the day before the closest approach, if nothing else went wrong. He did not promise. Engineers of his kind never promise.
Thursday would mean the loss of four of the nine days. Four days of the campaign's best pre-encounter observations, in which the asteroid's spin and shape were to have been measured as precisely as possible so that any change caused by the encounter could be detected afterwards. Without those measurements, the post-encounter observations would show what the asteroid was like after the passage, but nobody would know for certain what it had been like before.
There was another option, which Nadia began to work on that Monday and which, in retrospect, was the decision that saved the campaign.
*
The Goldstone complex has more than one antenna. Besides the great seventy-metre dish, there is a smaller one, thirty-four metres across, that had been used for decades as a research antenna and that, a few years before, had been fitted with a radar transmitter of its own, of much lower power: eighty kilowatts, a sixth of the large one. It had been used for asteroids at short range, where its modest power was sufficient. Nobody had planned to use it for this campaign as anything more than a backup, because its resolution was coarser and its signal weaker.
But the asteroid was coming very close. At a range of a few million kilometres, a week before the encounter, the eighty-kilowatt transmitter would produce echoes that were weak but detectable by the large receiving antennas in West Virginia and New Mexico. As the asteroid approached, the echoes would strengthen with the inverse fourth power of distance, which is to say very rapidly; at a range of a million kilometres they would be sixteen times stronger than at two million. It would not be the campaign Nadia had planned. The images would be coarser, the measurements less precise. But it would be something. It would give them the pre-encounter spin.
The difficulty was that the smaller antenna transmitted at a slightly different frequency, which meant that every receiving antenna in the schedule would have to be reconfigured, and that its pointing had to be calculated anew for every observation, and that it had never been used in combination with several of the receivers in the plan. It meant, too, that she would have to go back to every one of the eleven facilities and ask them to change their plans, at four days' notice, in the week before the most important observation of their careers, on the strength of a backup that nobody had tested.
*
She spent the Monday and the Tuesday on the telephone and the email. I have seen some of the messages. They are models of a particular kind of prose, which our age has perfected and to which no literary prize has ever been awarded: the request for a favour from a stranger, phrased with such clarity, courtesy, and attention to the stranger's own difficulties that the stranger cannot quite bring themselves to refuse. She explained the situation. She explained what she needed. She explained what it would cost them and what it would give them in return. She acknowledged, every time, that she was asking for a great deal.
Most of them said yes. The receiving antenna in West Virginia reconfigured its receiver over the Tuesday night. The antenna in New Mexico, which was in fact an array of many smaller dishes, required a change to its software that its staff wrote and tested in eighteen hours. The Australian space agency, which needed its antenna for a spacecraft emergency on the Wednesday, could not change its schedule, and said so with what Nadia later described as the most polite refusal she had ever received; she rearranged the plan to work without it. One of the optical observatories in Chile, asked to extend its tracking to cover a gap left by the radar's reduced sensitivity, said it could not, and then, an hour later, said that its director had overruled the scheduling committee and it could.
The first observations with the smaller transmitter were made on the Tuesday night. The echoes were faint and noisy. By the Wednesday night, as the asteroid closed from three million kilometres to two, they were clear. By the Thursday morning the team had a measurement of the asteroid's spin state, before the encounter, good to a precision that was worse than they had hoped for but better than anyone had feared.
The large transmitter came back on Thursday afternoon, at a quarter past three, after Bill Hendricks's team had run it through every test they could think of and two that they invented for the occasion. It ran at full power for the first time on the Thursday evening. The asteroid was by then a little over a million kilometres away, and the echoes were enormous.
*
On the Friday evening, when hundreds of millions of people in the Eastern Hemisphere went out of doors to look at a moving star, Nadia Haddad was in the control room at Goldstone, a long low building with no windows at all, watching the echoes come in on a screen.
What the screen showed was not, to an untrained eye, an image of anything. It was a smudge of white on black, shaped somewhat like a peanut, built up line by line as the echoes arrived and were processed. But to Nadia and the six people with her it was the asteroid, seen more closely than any asteroid had ever been seen by radar, its surface resolved to a few metres, its slow tumble visible from one image to the next. They could see ridges. They could see a dark area near one end that might be a depression or might be a region of different composition. They could see, in the sequence of images taken over the hours of closest approach, the asteroid turning in a way that was not quite the way it had been turning the day before.
The tidal forces had changed its spin. Not by much: a fraction of a per cent in its rotation period, a small shift in the axis of its tumble. But measurably. The measurement before and the measurement after, taken with two different transmitters on two different antennas because a klystron had failed a week before, showed a change. And the size of the change, when it was fully analysed over the following months, told the scientists something about the asteroid's interior: that it was not solid rock, but neither was it a loose pile of rubble. It was something in between, a body of large blocks held together partly by gravity and partly by some weak cohesion between them, of a kind that several models had predicted and several had not.
*
It would be pleasant to report that the world took notice of this. It did not, much. The pictures that went round the world that weekend were photographs of the moving star over the domes of Florence and the dunes of the Sahara and the skyline of Cape Town, taken by amateurs with long exposures, showing a thin streak of light across the stars. They were very beautiful, and they deserved the attention they received. The European spacecraft that rode alongside the asteroid sent back close-up photographs of its surface during the encounter, and those were beautiful too, and widely published. The radar images, a peanut-shaped smudge on black, were shown on a few science websites.
The paper describing the change in spin was published eleven months later, with Nadia as its first author and forty-three co-authors from nine countries. In the acknowledgements, between the funding agencies and the facility directors, there was a sentence thanking the engineering staff of the Goldstone complex, and in particular Bill Hendricks, for restoring the transmitter under difficult circumstances. Nadia had written that sentence herself. She had wanted to write a paragraph. The journal's word limit did not permit it.
*
I said at the beginning that I was setting down this story because Nadia Haddad did not see the asteroid, and I should say why that seems to me to matter.
It has become a commonplace of our age that science is done by large teams, by institutions, by machines; that the lone discoverer of earlier centuries has been replaced by the consortium and the collaboration and the forty-three co-authors. This is true. But it seems to me that it has led us to forget that large enterprises still depend, at their critical moments, on individual people making individual decisions in the small hours with incomplete information and nobody to share the responsibility. The radar campaign succeeded because a woman in Pasadena, on a Monday morning, looked at a backup transmitter that nobody had planned to use and decided to rebuild a schedule of four hundred hours around it, and because eleven groups of strangers on four continents decided to help her. And because a man at Goldstone, who had spent twenty-six years keeping a set of ancient vacuum tubes running in the desert, worked through two nights to bring one of them back.
None of them saw the asteroid. They were all indoors. They saw it, if they saw it at all, as a smudge on a screen or a column of numbers, and they understood it far better than the hundreds of millions who stood under the open sky and watched it pass.
Nadia flew home to Pasadena on the Sunday. That evening, sitting on the back step of her house in Altadena with a glass of wine, she looked up at the sky in the direction in which the asteroid would by then have been, receding at several kilometres a second, already too faint to see without a telescope. It would come back, more or less, in seven years, and again after that, but never so close again in her lifetime or her children's. She raised her glass to it, she told me, and felt slightly foolish, and drank anyway.