Here’s how you test your intracranial pressure in space. First, you collect baseline samples of your blood, saliva, and urine, and take ultrasound images of the vessels in your heart, neck, head, and eyes, lining up the scanning device on black dots tattooed on your body before you left Earth.

Then, you clamber into the Chibis, Russian for “lapwing,” a pair of hard, corrugated-rubber pants whose waist can be sealed. The pants suck: A vacuum imitates how gravity on Earth pulls blood, mucus, the water in cells, and cerebral and lymphatic fluids from our skulls to the bottom half of the body.

In space, fluids won’t drain, and astronauts develop red, puffy faces and complain of congestion or pressure in their ears. There are worse effects, too: 40 percent of the astronauts who lived on the International Space Station suffered some sort of damage to their eyes, including optic disc edema, globe flattening, and folds in the choroid, the blood-filled layer between the retina and the white sclera. NASA posits intracranial pressure is a possible explanation for what it calls “spaceflight-associated neuro-ocular syndrome,” and devised the test to measure fluid shifts to astronauts’ heads and eyes.

Wearing the lapwing is a mildly anxiety-inducing procedure. Once, a Russian cosmonaut lost consciousness when his heart rate dropped. His crewmates thought he was having a heart attack. Another time, the cosmonaut working the controls decreased the pressure too much—ratcheting up the sucking—and the astronaut felt “like I could have my intestines pulled out in the most unpleasant way possible.”

Jason Pontin Ideas Contributor Twitter Jason Pontin ( @jason_pontin ) is an Ideas contributor for WIRED. He is a senior partner at Flagship Pioneering, a firm in Boston that creates, builds, and funds companies that solve problems in health, food, and sustainability. From 2004 to 2017, he was the editor in chief and publisher of MIT Technology Review. Before that he was the editor of Red Herring, a business magazine that was popular during the dot-com boom. Pontin does not write about Flagship’s portfolio companies nor about their competitors.

But if nothing goes wrong, you hang out in the suit for a few hours, taking more ultrasound images. You check your blood pressure. You measure cochlear fluid with an instrument in your ear and record intraocular pressure by tapping a pressure sensor against your anesthetized eyeball. You scan your eyeball with a laser to visualize choroidal folds and optic nerve swelling.

The “Fluids Shifts” experiment was performed by astronaut Scott Kelly when he lived on the ISS from March 27, 2015, to March 1, 2016, the longest spaceflight by an American. At the same time, his twin brother Mark, also an astronaut, tested his intracranial pressure back on Earth.

Over 25 months, the brothers submitted to a parallel routine of cognitive and physical tests—including a spinal tap for Scott—in the lab before, during, and after the mission. In all, 317 samples of stool, urine, and blood from both twins were collected and analyzed for their epigenomic, metabolomic, transcriptomic, proteomic, and microbiome changes. All of this was a first for NASA, which had never conducted a complete multi-omics analysis of an astronaut, let alone of an astronaut and a monozygotic control.

The idea behind the study had a simple logic: Because the twins share the same genome, any changes that occurred to Scott and not to Mark would likely be caused by long-duration spaceflight. The results, whose findings were finally published in Science today, expand our understanding of what happens to the human body after a year in space.

“The NASA Twins Study: A Multidimensional Analysis of a Year-Long Human Spaceflight” is a triumph of cross-disciplinary science. Described as “a Herculean endeavor” by one of the article’s peer reviewers, it integrates the work of 10 different groups at universities around the country and 82 separate authors.

Francine Garrett-Bakelman, one of the article’s lead authors and a molecular biologist at the University of Virginia, said it was the “most comprehensive result possible based on the data available.” But to the essential question “Are humans fit for space?” the study provides only unsettling and incomplete answers. Long-term exposure to spaceflight is dangerous; based on what we know now, a journey to Mars is still too risky to contemplate.

More than 500 people have flown in space, and some of the bodily changes they experienced during missions lasting less than a month or as long as six months are well understood. Fluids shift to astronauts’ heads; the left side of their hearts grow. Unless they exercise vigorously, they lose muscle and bone.