During his 340-day mission aboard the International Space Station, Scott Kelly breathed carbon dioxide at up to 10 times the levels on Earth. He survived three spacewalks and did a television interview partway through emergency procedures to avoid a defunct satellite hurtling toward him at 28,000 kilometres an hour.
But even Kelly, with four space flights under his belt, was unprepared for the physical challenges he would face after landing back on Earth.
"I felt horrible afterwards," he said in a phone interview. As his body readjusted to gravity, he suffered from stiffness, joint pain and muscle pain. His skin broke out into hives. His legs swelled up like water balloons. He had nausea and flu symptoms. Some of the effects lingered for weeks, others for months. Kelly's previous space mission was a long one, at 159 days, but "I definitely felt worse after 340 days."
The 53-year-old astronaut sheds fresh light on the mental and physical demands of space travel in his new book, Endurance: A Year in Space, A Lifetime of Discovery, published earlier this month. (NASA dubbed it "a year" but technically, it was 340 days.) Part of his mission was to help scientists study the impact of long-term space travel on human physiology, in preparation for future trips to Mars.
Kelly's return to Earth on March 1, 2016, marked the longest continuous stay in space by an American astronaut. (Russian cosmonaut Valeri Polyakov holds the world record of 438 consecutive days, aboard the Mir space station.)
After 340 days in orbit, Kelly discovered he had no more bone loss than after spending less than half that time in space. His exercise habits hadn't changed, "so I think maybe it just levels off after a while."
In his book, Kelly speculates that future generations who spend their whole lives in space may eventually evolve without a skeleton: "They will be able to live as invertebrates."
But in the meantime, bone loss and muscle wasting are inevitable in space, since near-zero gravity makes too few demands on the musculoskeletal system we depend on to walk, lift things or just sit up.
Astronauts learn the true meaning of "use it or lose it." Kelly spent about two hours a day working out on a stationary bicycle and treadmill – equipped with straps to help him stay put – and a weightlifting machine that uses compressed air to provide resistance. (He passed the time watching Game of Thrones.) Getting his heart pumping felt good after all that floating around, he said. When the equipment would break down for a day or two, "you'd crave it."
During his months in space, Kelly had more than 30 times the exposure to radiation of a person on Earth – equivalent to about 10 chest X-rays a day. But while this exposure has increased his risk of fatal cancers, the risk is smaller than a smoker's chance of developing lung cancer, he said, "so it's not something I dwell on."
Space travel has taken a toll on his vision as well. Kelly has gone through multiple eyeglass prescriptions to adjust to vision changes during and after his four NASA missions. More worrisome are the swelling of his optic nerve and permanent folds in the choroid – the blood-filled layer between the retina and the white of the eye. The choroid delivers oxygen and nutrients to the outer layers of the retina. Folds in the choroid can damage the retina, creating blind spots.
Scientists aren't sure what causes vision problems in male but not female astronauts. Increased pressure in the brain's cerebral fluid may be the culprit, or high carbon-dioxide levels, known to dilate blood vessels. If scientists can't pinpoint the cause of these vision problems in male astronauts, Kelly writes, "we just might have to send an all-women crew to Mars."
Of all the discomforts in space, the worst aggravation for Kelly was breathing excess carbon dioxide. As the levels crept up, he'd suffer from headaches and congestion, followed by burning eyes, irritability and trouble thinking straight.
In his book, Kelly is openly critical of NASA for not allowing the crew to run the space station's backup machine to scrub carbon dioxide from the air. "Our CO2 level could come down to a much more tolerable level with the flip of a switch in Houston," he writes, "and yet we can't convince them to do this." Since his return to Earth, NASA has negotiated an international agreement to reduce the acceptable levels of carbon dioxide at the space station by about a third.
Kelly won't be back to breathe the better air, having retired from NASA in April, 2016. But he will continue to serve as a space guinea pig for the rest of his life, along with his identical twin, Mark Kelly, who spent a total of 54 days in space over four space flights before retiring from NASA in 2011.
The astronaut twins gave researchers a unique opportunity to study the effects of a 340-day mission on Scott's cellular and genetic makeup, compared with that of his genetically identical brother, who stayed on Earth.
NASA released preliminary data from the Kelly Twins Study in January. While in space, Scott grew longer telomeres – the protective caps at the ends of chromosomes – compared with Mark's. But this increase in telomere length, associated with healthier aging, reverted to preflight levels shortly after he returned to Earth. Scott showed a decrease in chemical modifications to his DNA, which can affect gene activity. This, too, returned to preflight levels. Researchers also noticed changes in gene-expression signatures between the twins, possibly caused by lifestyle changes such as Scott's space-food diet and sleep habits while floating in near-zero gravity.
Kelly asked NASA for more information from the Twins Study in advance of his book tour, but additional findings were unavailable. For various reasons, he said, NASA may never release the full data.
Even so, Kelly doubts scientists will find any significant long-term differences between the twins caused by space travel. "I would guess that if I don't get some cancer as a result of the radiation, then probably there won't be anything," he said, adding, "I'm an optimist."