Scott Kelly, Mark Kelly

Before, during and after a yearlong mission aboard the International Space Station, astronauts Scott Kelly, left, and his twin Mark Kelly went through regular testing on immune systems, gut biomes, biochemistry, genomes, chromosomes, overall physiology and other aspects of health. Scott Kelly was on the space station, while Mark Kelly stayed on Earth.

NASA’s having a heck of a week. Following the photograph of a black hole that broke the internet, the space agency released the final results of its years-long Twins Study. The audacious experiment assessed the effects of space travel versus earth-bound life on the human body, in the test-and-control form of identical twins Scott and Mark Kelly.

The top-line conclusion is that a year in space — about the time needed to fly to Mars and back — drives phenomenal change in the body. And most of it reverts to preflight norms within months of returning to Earth.

“It’s reassuring to know that when you come back, things will largely be back to the same,” said Michael Snyder of the Stanford University School of Medicine, on a Tuesday call with several of the lead authors. “So I think that’s the number one message.”

Decades of exploration have helped space agencies around the world protect and improve their travelers’ safety and living conditions. The NASA Twins Study revolutionizes those efforts at a time of renewed interest and ambition in the public and private sectors. Vice President Mike Pence last month vowed that U.S. astronauts will return to the moon within five years “by any means necessary” as part of a rejuvenated program to eventually bring Americans to Mars. Scott Kelly, an engineer, retired Navy pilot, former Space Shuttle commander and author, spent a year aboard the International Space Station, beginning in March 2015.

Before, during and after that mission, both he and his twin Mark went through regular testing on immune systems, gut biomes, biochemistry, genomes, chromosomes, overall physiology and other aspects of health. Dozens of scientists who’ve never been involved in space research before were brought in to participate.

Snyder called it “the most in-depth study, certainly at the biochemical level, that’s ever been done on people in space.”

Spaceflight delivers all manner of insult to the human body, from terrestrial bothers like intestinal trouble or sleeplessness to more significant challenges such as radiation exposure. Some of the effects are well-known if understudied, such as the “puffy face” and “bird legs” astronauts experience when bodily fluids redistribute in the absence of gravity.

For the first time, medical researchers were able to see just how the body adapts to these particular extreme conditions. Researchers were particularly interested in the possible effects off radiation on the genome and the machines of molecular biology. “As soon as he got into space, there was a really large scale shift we saw in over 1,000 genes,” said Christopher Mason of Weill Cornell Medicine.

The number of genes showing activity increased six-fold in the second half of Kelly’s stay. The genes that lit up included those that help repair DNA and manage the immune system, which made sense given the stresses of Scott Kelly’s new living conditions.Researchers also observed surprising changes in telomeres, strands at the end of chromosomes that prevent the genetic material from unraveling. As people get older, telomeres shorten in length, which makes them a decent indicator of aging and some health risks, like cancer or cardiovascular disease. Genetics plays an influential role in telomere length, which made it particularly useful to have Mark Kelly on the ground as a control subject, in addition to Scott’s preflight data.

Scott’s telomeres lengthened relative to Mark’s during his year in space, then rapidly shrunk upon returning home. “That was very much a surprise to us,” said Susan Bailey of Colorado State University. The finding shouldn’t be taken as a sign that space travel is an cure for aging, she added, “or that people might expect to live longer because they’re in space.”

Mark Kelly was as perfect a control as the scientists could muster, allowing for the variations in diet and activity that come with earthly living. Mark’s been an advocate for responsible gun law since his wife, then-Rep. Gabrielle Giffords, survived an assassination attempt in 2011. In February, he announced his campaign for one of Arizona’s U.S. Senate seats as a Democrat.

NASA has further year-long studies planned. But even though stargazers have looked up for centuries and seen the twin constellation Gemini, there are no more twins above the horizon.

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