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.