There’s never been a study like it before, and there probably never will be again.

Starting in March 2015, astronaut Scott Kelly spent 340 days aboard the International Space Station. A team of scientists tracked his body’s responses to long-term spaceflight in painstaking detail. They simultaneously tracked his identical twin brother, Mark, on Earth.

On Thursday, the scientists began publishing the huge trove of results from the so-called NASA Twins Study — 90 pages, all told, and that’s just the first installment. Here’s a guide to what they’ve found … so far.

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Space travel changes the body.

Scientists tracked a huge range of measurements in the NASA Twins Study, from Mr. Kelly’s performance on cognition tests to levels of gene activity in his cells. Many of the molecular tests had never been carried out on astronauts before.

The exams required Mr. Kelly to draw his own blood and to send fresh samples back to Earth in a Soyuz supply craft. Within 48 hours, researchers were analyzing the samples at NASA labs.