The Weightlessness of Space Causes Astronauts' Body Temperatures to Rise

The physiological mechanisms that work quite well on Earth to cool a person down during physical activity don’t function as efficiently in space.

A rethink of future spacesuit design may be necessary in order to make sure astronauts don't overheat while they're undertaking tough physical tasks.

New research, published in the journal Nature, examines the body temperature changes of astronauts during missions. When exercising in the microgravity environment of space, two factors combine to make it difficult for sweat to dissipate. First, surface tension on your skin causes the sweat to cool on the body, rather than to evaporate. Also, convection — the tendency of heat to rise — is less pronounced in microgravity, making it more difficult to bring heated air away from the body.

"You do generate heat when you exercise, so the body temperature would increase a little bit [on Earth]," study co-author Alan Moore, an exercise physiologist and associate professor at Lamar University in Beaumont, Texas, told Seeker. He previously conducted experiments for multiple crewed space missions at the NASA Johnson Space Center in Houston.

"Usually you thermal regulate quite well on the ground, unless it's in a strange environment. So body heat during exercising is more or less normal, shall we say, whereas in flight, the temperature is elevated much more rapidly and to a higher degree than the ground.”

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Another likely cause for astronauts heating up could be the way blood is circulated. When astronauts go into space, blood no longer pools naturally at the feet and in the bottom of the legs. Instead, it shifts further up into the body, which is why so many astronauts appear to have puffed faces when in spaceflight — all fluid circulation changes.

"The kidneys sense that [circulation change], and you end up urinating out some plasma, and that causes the plasma level to drop,” Moore said. “There's a little less blood circulating inside of you than on the ground."

As a theoretical example, he cited a person who usually has 6 liters (1.6 gallons) of blood in their body, who then sees their volume drop to 5 liters (1.3 gallons) in space. Since there's less liquid in the body to heat during exercise, the blood may heat up quicker.

These factors could add up to a heat strain in the short term. While exercising, some of the astronauts that were studied got so hot in space that their body temperature rose to 40 degrees Celsius (104 degrees Fahrenheit), or more. For context, heat strain really begins to affect the brain at temperatures as low as 42°C (107°F) during an extended workout. The threshold rises to 44°C (111°F) during intense exercise over a short period.

On the International Space Station, a simple answer to the problem might be more ventilation. The space station was a little warmer (23.5°C/74°F) than the control environment (21°C/70°F). It's also a small, enclosed space. Moore suggested that perhaps a fan could be set up, although physiologists would need to measure airflow to see if that would help.

But the results do have him wondering about spacesuit design. During spacewalks, astronauts perform physical tasks — and the spacesuit is by definition a tiny, enclosed environment. Heat dissipation may be more difficult in a spacesuit than ground tests suggest, Moore said.

"You may have to provide more cooling than you would get based on ground data," he said, adding that more drinking water would be useful as well. The best data would be gathered in space because even tests during brief bursts of weightlessness — such as 30-second parabolic flights — would not be helpful, as astronauts' core temperature appears to increase after several weeks in space.

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The study included 11 astronauts on the International Space Station, who were an average age of 50. They were tested three months before their flight, six times in space (between flight day 15 and flight day 165), and three times after returning to Earth (up to 30 days after landing.) The astronauts exercised for three continuous 5-minute periods that aimed to get them at 25 percent, 50 percent, and 75 percent of their maximum oxygen utilization rate as measured on the ground. They then performed at higher intensities until they were tired and stopped exercising.

Core body temperature isn't the only thing to change after many months in space. Other physical transformations include diminished bone density, weakened muscle strength, and even worse eyesight — the latter effect can be permanent in some astronauts. NASA is examining each of these issues on the ISS to better understand how to protect astronauts during long-duration missions. The aim is to better prepare the agency for future long-term missions to the moon or Mars.

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