For decades, the International Space Station has been hovering over Earth, in an orbit somewhere between two hundred and three hundred miles above sea level. Its massive rectilinear structure, resembling an Eisenhower-era TV antenna, contains hundreds of thousands of solar cells and a series of pressurized modules that can support life and equipment, all of it weighing close to a million pounds. Since 2000, people have been living on the station, in an area comparable to a six-bedroom house: humanitys most expensive real estate. The station is also the fastest structure a person can live in. It orbits the planet at more than seventeen thousand miles an hour, many times faster than the Earths rotation. A day on the station, from sunrise to sunrise, lasts just ninety minutes.

In the early hours of July 16, 2015, members of the U.S.Air Force noticed an alarming development involving the I.S.S. Since the Cold War, the military has maintained an extensive space-surveillance network. Every minute, tracking stations across the globe relay a cascade of data to the Cheyenne Mountain Complex, in a bunker carved deep beneath two thousand feet of granite in Colorado. Some of the information is set aside for NORAD and other national-security organizations. Other portions are forwarded to the 18th Space Control Squadron, in California, which works to prevent collisions in the sky.

Sometime before three that morning, the surveillance network glimpsed a hunk of debris hurtling toward the I.S.S. A well-known piece of space trash, it had been labelled Object No. 36912 in an extensive inventory of orbital artifacts known as the NORAD catalogue. It had broken off of a Soviet military weather satellite, which was launched in 1979 from a Cold War facility near the Arctic Circle. The cylindrical satelliteresembling an old-fashioned boilerwas designed to work for less than two years. In the ensuing decades, it had been shedding fragments. That April, another piece of it had threatened the space station.

Object No. 36912 was likely a torn-off piece of thermal shielding; it appeared to be relatively light, and no bigger than a large dinner plate. For years, it had circled safely above the I.S.S. But its mass and shape made it highly sensitive to atmospheric dragits orbit shifting dramatically as the atmosphere expanded and contracted in response to solar activity. Several weeks earlier, the atmosphere had ballooned, causing Object No.36912s orbit to suddenly decay.

As the debris spiralled downward, gathering speed, the Air Force was keeping a close watch, but small things in space can easily evade detection. The object was visible to just two radar stations, in Alaska and in Floridaand then it went entirely dark for more than a week. On July 16th, when it reappeared, Air Force analysts quickly updated their predictions: the object would make a close pass of the space station at 5:29A.M. (Mission Control time, in Houston). It would clear the spacecraft by about fourteen miles, but penetrate a safety zone around the I.S.S. called the box. Then it would loop around the globe, falling farther, and come within striking distancerisking impact, or a conjunction. If the chance that something in the box will collide with the I.S.S. is greater than one in ten thousand, the condition is red. With Object No. 36912, the probability was more than one in a thousand.

At 2:44 A.M., the Space Control Squadron notified Jim Cooney, the I.S.S.s trajectory-operations officer. Cooney, a NASA veteran, was asleep at home, but an app on his phone triggers a high-volume alarm for such alerts. Your brain gets engaged really fast, he told me. He had become accustomed to late-night calls. Only a month earlier, NASA had adjusted the spacecrafts trajectory to dodge a fragment of a Minotaur rocket: a former intercontinental ballistic missile repurposed to ferry cargo into space.

These maneuvers have been performed more than two dozen times, and can be executed without much trouble if Houston has five and a half hours notice. But, when Cooney called the Air Force, he learned that Object No. 36912 would make its closest approach in about four hours. I had them repeat the information to make sure I was doing the math right, he recalled. Never before had the I.S.S. faced such a high probability of collision on such short notice. Moving the station was out of the question.

Instantly, he relayed the news to Houstons flight director, Ed Van Cise, and then rushed to Mission Control, where he joined a tense meeting to discuss the options. There was only one: instruct the crew to lock down the stationclosing hatches between modulesand then shelter in the Soyuz capsule, a Russian vessel that can serve as a lifeboat. There were three people aboard the I.S.S.: one American, Scott Kelly, and two Russians, Gennady Padalka and Mikhail Kornienko. In the Soyuz, they could detach from the failing structure and return to Earth. In the stations history, its crew had sheltered in the Soyuz only three other times.

Van Cise reached out to Kelly, who was exercising on a treadmill mounted on one of the stations walls. Houston on Space to Ground Two, a voice barked, announcing the call. We are privatizing. This meant that the feed from Mission Control, which usually could be accessed freely by the ground crew, would be nonpublic. Kelly later wrote in an expedition log that his first thought was, Oh, fuck. In space, unscheduled private conversations portend bad news: in 2011, on an earlier mission, Houston had privatized the channel to inform him that his sister-in-law, the Arizona congresswoman Gabby Giffords, had been shot.

Hearing that the call was for NASA business, he was at first relieved. Then the enormity of the situation sank in. Fuck, he thought again. The privatized call was a courtesy, so that Kelly would be prepared once the alert was relayed publicly. The space station operates on Greenwich Mean Time; for the crew, the moment that Object No. 36912 either slammed into the structure or zoomed past it would be 12:01. Mission Control instructed Kelly to start closing hatches at 10:30 A.M., then retreat with the Russians to the Soyuz at 11:51, and stay there until notified. Kelly cut short his workout.

At ten oclock, Mission Control contacted Kelly again, to remind him that he and Kornienko had an interview scheduled with morning news programs in Florida and Kentucky. NASA reasoned that there was time to proceed: the interview would take less than twenty minutes, and lockdown was in half an hour. Seriously? Kelly wrote in his log. We have a satellite coming at us. But he and Kornienko got into their positions without protest. We are ready for the event, Kelly said, dryly, and glanced at his watch. Then he answered questions about the Kentucky Derby, performed zero-gravity stunts, and tried not to show that he was in a life-threatening situation.

As soon as the transmissions ended, Kelly began locking down hatches throughout the American modules. Calmly, he floated through the structurethe lab, the cupola, an air lockwith a flashlight in his mouth, to augment the stations dim lighting. He had asked Houston if the debris hurtling toward the space station would be visible; as he closed the hatches, he got a response. It will be in orbital night, Houston told him. So, no viewing opportunity.

How about relative velocity? Kelly asked.

Fourteen kilometres per second.

Copy, Kelly said, plainly, but the number was terrifying: the debris and the station were closing in on each other at a combined speed of thirty-one thousand miles an hour. In orbit, a one-centimetre bolt can have the explosive force of a hand grenade upon impact. Object No. 36912 was at least ten times larger. When the space stations shielding was being designed, a NASA astrophysicist named Donald Kessler had asked experts to shoot small objects at metal film cannisters at hypervelocities. The ballistics revealed that, even if debris penetrated the I.S.S. cleanly, it could leave a mangled hole upon exit. Object No. 36912 risked triggering a chain of failures that could destroy the entire structure.

Kelly focussed on procedure. Houston had told him to pick up a scientific instrument and a medical kit. He got those, and also some personal items, thinking of an American astronaut,Mike Foale, who had served on the Mir space station, in 1997. Foale had been living in a module called Spektr when a supply ship came in too quicklylike a shark, a cosmonaut onboard recalled, this black body covered in spots sliding pastand then smashed into it. To contain the breach, Spektr was sealed, forever separating Foale from his things, including gold pendants he intended to give his wife and children. You always think about what happened to him when youre closing a hatch with important stuff on the other side, Kelly told me.

After locking down the American modules, Kelly caught up with Kornienko and Padalka in the Russian section. Padalka, the commander of the I.S.S., strove to project confidence; when Moscow Mission Control had asked him about the mood onboard, he responded, Fighting spirit! Kelly noticed that none of the hatches on the five Russian modules were shut. (Padalka and Kornienko say that they remember this differently.) The Russians dont close their hatches like we do, Kelly wrote in his log. They think its a waste of timebasically thinking the two most likely scenarios are the thing misses, or catastrophic destruction. The stuff in-between is way too unlikely to care about.

Kelly was amazed to find the cosmonauts having lunch. We wanted to eat! Kornienko told me. Russians have a proverb, War is war, but lunch runs on time. The Soyuzs food supply was limited to three days, and who knew how long they might be stranded there? There were fourteen minutes to spare, so Kelly joined them for a can of Appetizing Appetizera dish that, he later recalled, resembled cat food, in appearance, consistency, and probably a little bit in taste.

In Houston, the crew at Mission Control waited tensely. A wall-size screen contained a depiction of the space stations orbit and a live feed of its interior. Ed Van Cise fidgeted with a computer mouse. One NASA official stared at a monitor with a hand over his mouth. Another sat with an emergency manual open; he told me, We know what to do, but we dont know what the outcome is going to be. This could be terrible, this could be a loss of life.

At 11:51, the three men in the space station climbed into the Soyuz capsule, a cramped vessel that looked like a pinched cylinder atop the station. It was packed with switches and knobs. Its dark outside, so its darker than normal inside, Kelly wrote in his log. Its cold. He was wearing a black NASA sweatshirt, and he had pulled the hood down nearly over his eyes.

The men were instructed to leave the Soyuz hatch sealed, but unlatchedin case the debris hit the capsule rather than the I.S.S. and they needed to rush back in. Kornienko focussed on the latch, imagining the steps he would take in a crisis. There were no wordssilence, he told me. Kelly, too, was struck by the sudden quiet, as each man retreated into his thoughts. He wrote in his log, I can only hear the sounds of the fans inside the Soyuz, my breathing.

With the tension growing, Padalka said, You know, it will really suck if we get hit.

Da, Kornienko said. Will suck.

A monitor indicated the time, and the men watched the minutes elapse, bracing for 12:01. Kelly noticed Kornienko gazing out a porthole. Finally, I said, Misha, youre not going to see anything, he recalled. That thing is going thirty thousand miles an hour, and its dark outside! Then I noticed that I was looking out the window, and listening, and tensing out, and then at some point you realize, We wouldnt even fucking know if we got hit. We just wouldve been vaporized!

The three men fell into silence again. For a while, Kelly listened to his iPod. As the time approached twelve noon and some odd number of seconds I started to grimace, Kelly wrote. Thirty seconds go by. A minute. At 12:01, nothing happened. Padalka got on the radio. Moscow, he said. Do you read?

Loud and clear. How are things?

Were getting into 12:02, Padalka said. Everything is very quiet up here. After nearly three tense minutes of radio silence, Padalka called in again: Moscow, do we keep waiting?

The radio crackled. Thats all, it finally said. Object No. 36912 had blown past the station. Later, the Air Force put its distance at less than a mile and a halfa gap it could have closed faster than the blink of an eye. Three weeks later, it incinerated in the atmosphere.

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