In 1972, NASA sealed 3 astronauts inside a 20-foot chamber for 56 days to rehearse Skylab; the ground experiment exposed hardware and medical problems before launch

On September 20, 1972, three NASA astronauts stepped out of a 20-foot altitude chamber in Houston after spending 56 days sealed inside it. They hadn't been anywhere near space. The test, called the Skylab Medical Experiment Altitude Test, or SMEAT, was a rehearsal for the Skylab space station mission scheduled to launch the following year. The agency's official SMEAT technical memorandum, published by the Johnson Space Center, said the ground experiment was designed to catch hardware failures and medical surprises before astronauts ever left Earth.

A test flight that never left the ground

Skylab would be America's first space station, and before committing real astronauts to a real mission, officials wanted to know how the human body would deal with weeks of confinement. Approval for SMEAT came in February 1971. Four months later, the agency picked its crew: Robert Crippen, Dr. William Thornton, and Karol "Bo" Bobko. The trio spent 56 days in a 20-foot chamber in Building 7 at the Manned Spacecraft Center, now known as Johnson Space Center. The air in the chamber was the same as Skylab would use in orbit: 70 percent oxygen, 30 percent nitrogen, at 5 psi, below sea-level pressure. It wasn't a perfect substitute for the real spacecraft, since the chamber didn't have the volume of Skylab's upper deck, but its second level provided the crew with workspace, while its lower deck was a close approximation of where a real crew would sleep, eat, and exercise.

Life inside a sealed room

On July 26, 1972, in front of news cameras and a crowd of well-wishers, the astronauts entered the chamber. Once sealed in, they followed the same rules a real Skylab crew would follow, talking to mission control through a capsule communicator as NASA simulated the radio blackouts astronauts would face by deliberately cutting the link on schedule, since ground stations could only reach Skylab about 20 percent of the time. Over the next weeks, the crew ran 14 medical experiments tracking their hearts, muscles, hormones and nervous systems, along with 15 other tests involving equipment and procedures. Engineers designed a complete miniature life-support system for the test, including a two-gas breathing system, fire detection, a water supply and waste management, all built to behave exactly like the systems planned for the orbiting station, according to a NASA conference paper on the design and operation of the SMEAT facility.

The exercise bike broke down

On just the second day, the bicycle ergometer, the crew’s principal exercise machine and a vital instrument to monitor heart and metabolism, malfunctioned. The crew sent it out through the chamber's manlock for repairs. Engineers fixed the machine and returned it the next day, but they imposed strict limits on how hard the astronauts could use it until the failure was understood. A second ergometer was added four days later, so the crew could exercise at full effort. NASA learned from the episode that a real Skylab crew would not have a backup machine on standby, so redundancy had to be built in from the start.

The astronaut who couldn't stop losing weight

The most notable finding from SMEAT had nothing to do with hardware. Thornton lost 19 pounds during the test, about a pound and a half each week, and continued to lose weight even during the 18-day monitored-diet period that followed. The reason was simple: researchers had planned identical meals for all three astronauts, without taking into account that Thornton weighed more and worked out harder than his crewmates, so he needed more calories than the diet provided. Years later, writing in the journal Aerospace Medicine and Human Performance, aerospace physiologist Chuck Sawin revisited SMEAT as an early data point about how spaceflight-style diets affect the body. Before the real Skylab missions, NASA changed the meal plans, but engineers had also assumed that astronauts would need fewer calories in weightlessness. That assumption also proved wrong, and the first two actual Skylab crews lost weight in orbit before the diet was finally corrected for the third crew.

Chess games and Russian lessons

There was more to chamber life than work. The astronauts got one day off a week and spent their downtime studying Russian on closed-circuit television, building models, reading books and watching movies. Crippen continued two one-move-a-day chess games with the test directors. The team paused to celebrate Crippen’s 35th birthday on September 11, and fellow astronauts joined in through the camera feed.

Why a ground test that never launched still mattered

When the crew emerged from the chamber on September 20, their work wasn’t done. They spent the next 18 days on a monitored diet while writing their mission report. The lessons learned from SMEAT, from the redundancy of the exercise equipment to the diet miscalculations, were applied directly to the way the real Skylab missions were operated a year later. This approach also became a template for future ground simulations, including NASA's ongoing Human Exploration Research Analog program, which still trains astronauts for long missions without ever leaving the ground.