On March 2, 2016, at around 9 A.M. local time, in Kazakhstan, Scott Kelly plunged through the Earth's atmosphere in a Soyuz spacecraft travelling at seventeen thousand miles an hour. As expected, atmospheric friction warmed up its heat shield so much that molten debris flew off. Rapid deceleration imposed more than six times the force of gravity on Kelly and his crewmates, the cosmonauts Mikhail Kornienko and Sergey Volkov. The Soyuz's descent module, a black sphere measuring about seven feet in diameter, deployed a red-and-white parachute and floated to the surface of the planet, landing in the desert.
A search-and-rescue team, wearing furry ushanka hats and shouting in Russian, rushed to the capsule, twisted open its circular lid, and hoisted Kelly out. He gave a thumbs-up, then grimaced as they lowered him gently into a recliner that sat conspicuously on the barren plain. Someone covered him with a thick blanket and fitted his bald head with a knit hat. Then he lifted a satellite phone to his ear and made his first call back on Earth.
Kelly had spent more time in space than almost any other person -- four missions, each longer than the last, totalling five hundred and twenty days. On this trip, he had taken the longest spaceflight of any American: nearly a year on the International Space Station. He was, in a sense, as accustomed to space as anyone alive. And yet, he told me, "as I flew longer, the symptoms of returning to Earth were worse." After he got back to his home, in Houston, he felt nauseated and dizzy. His joints ached under the force of gravity, and the pressure of simply sitting in a chair felt uncomfortable. A ponderous fatigue set in.
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Some medical effects of space travel are well understood. For decades, scientists have known that near-weightlessness lengthens the spine and causes the wasting of muscle and bone, which is why astronauts have to exercise frequently. Kelly returned to Earth two inches taller than when he left; his body mass declined by seven per cent, in part because his appetite for packaged and freeze-dried fare was lower than NASA planners had anticipated. Some of his other symptoms, however, were strange and unfamiliar. When he stood, his blood seemed to rush downward, causing a painful swelling in his legs. "That was probably most disturbing," he told me. An angry rash spread across his neck, back, and legs.
Kelly has an identical twin brother, the Arizona senator and retired astronaut Mark Kelly. Before the mission, both men had agreed to participate in a comparative study of their bodies -- Mark from Earth, Scott from space and Earth. Because they have the same DNA, the study was a rare opportunity to isolate the physiological effects of long-term missions. And so, before, during, and after Scott's stay on the I.S.S., a team of more than eighty researchers from twelve universities studied him more closely than perhaps any other human in history. "I wish every person was a twin," Christopher Mason, a principal investigator of the NASA Twins Study, has said. Mason and his colleagues were troubled by some of their findings. Cognitive testing, for example, showed declines in Scott's mental speed and accuracy. Markers of inflammation in his blood spiked to levels that laboratory tests had difficulty measuring -- thousands of per cent above normal, which suggested an extreme stress response. "Are these the highest levels ever seen in a human body?" Mason remembers one of his colleagues asking. "How did he survive?"
Fewer than seven hundred people, most of them relatively young and fit men, have gone to space. In the coming decades, this number could grow exponentially as more and more governments and companies -- among them SpaceX, Blue Origin, and Virgin Galactic -- inaugurate what has been called the second space age. Yet many peculiar effects of space travel are only now being identified and investigated. Latent herpes infections often get reactivated; certain medications can become less effective; microgravity, the technical term for near-weightlessness, redistributes blood to veins in the head and neck that aren't used to handling the flow, increasing the risk of clots. Scott Kelly wrote in "Endurance," a memoir, that humans can explore more of the universe only if they strengthen "the weakest links in the chain that makes spaceflight possible: the human body and mind."
Mason is optimistic that we will be able to do so. "At some point, we're going to have thousands of people living or working in space," he told me. "We need to understand how to do that safely." Others have grave concerns. Mathias Basner, a psychiatry professor at the University of Pennsylvania and a member of the Twins Study research consortium, told me that space travel causes profound structural changes in the brain. "Most are probably reversible," he told me. "Some may not be." In microgravity, the brain moves to the top of the skull and compresses an area responsible for absorbing cerebrospinal fluid, leading to a swelling of brain cavities and potentially to an increase in intracranial pressure. Certain biomarkers associated with neurodegenerative disease appear to rise significantly after a long trip. "This could suggest something quite bad is happening in the brain," Basner said. "We need more data."
The record holder for the longest continuous stay in space, a Russian cosmonaut and doctor named Valery Polyakov, spent a little more than fourteen months in low Earth orbit, which is relatively protected from space radiation and communication lags. Only twenty-four people, the Americans who crewed the Apollo missions, have ever exited low Earth orbit -- and that was more than fifty years ago, for less than two weeks at a time. Still, the U.S. and China have already raised the possibility of crewed trips to Mars in the twenty-thirties. Elon Musk, the C.E.O. of SpaceX, who has said that he would like to die there -- "just not on impact" -- has indicated that he wants to send a million settlers. (He noted in 2018 that the job postings would read "like Shackleton's ad for Antarctic explorers: difficult, dangerous, good chance you'll die. Excitement for those who survive.") But Bill Nye, former Science Guy and current C.E.O. of the Planetary Society, has portrayed the dream of Mars colonization as a dangerous delusion. "We can't even take care of this planet where we live, and we're perfectly suited for it," he has said. "Are you guys high?" The truth is that we still know precious little about how humans will fare in deep space. It's also true that we'll never find out until we try.
The Cornell Aerospace Medicine Biobank (CAMbank), where Mason works, is on the fourth floor of a sleek glass building in Manhattan. It contains more than fifteen thousand biological samples from twenty-two astronauts, making it one of the largest repositories of its kind. On a crisp morning in September, I arrived at a nearby hematology clinic to meet Hayley Arceneaux and Sian Proctor, two crew members from Inspiration4, a private SpaceX mission that took off from Florida in 2021. They had come to donate saliva, blood, and urine, as well as microbes from their skin, to science.
Proctor, a community-college geology professor and a futurist with a buzz cut, had visited several times before and seemed to know what to do: she tore a square of gauze out of a plastic wrapper, chewed it as though it were a piece of gum, and placed the wet lump in a box. (Mason developed this method after astronauts complained about the difficulty of spitting into a tube in space.) Next, she removed her socks and swabbed between her toes.
Arceneaux, a health-care worker with wavy auburn hair, pulled up the sleeves of her blue T-shirt, which was decorated with a rocket logo, to show me small scars from skin biopsies. "When you're voluntarily getting sutures for research, you know you're committed," she told me. Then she swabbed her nostril as if she were giving herself a COVID test.
When Inspiration4 was first planned, scientists were eager to study its crew: civilian astronauts greatly expand the pool of potential research subjects, and they also offer more diversity in terms of age, gender, background, training, and fitness levels. (In 2021, William Shatner, of "Star Trek" fame, spent ten minutes in suborbital space at the age of ninety, on a reusable Blue Origin rocket system. He later wrote in the Guardian, "I had to get to space to understand that Earth is, and will remain, our only home. And we have been ravaging it, relentlessly, making it uninhabitable.") Saliva contains inflammatory molecules and hormones, which can indicate an increased risk of heart disease or endocrine problems. Skin swabs and biopsies can reveal alterations in the microbiome and in gene pathways that are linked to cancer. "The Inspiration4 crew are some of the most enthusiastic astronaut research subjects we have," Mason told me.