Being weightless makes you hot. That's not to say that having gone beyond the Earth's atmosphere makes people particularly attractive on return, although that's quite likely true as well. Instead, the latest revelation is that astronauts run a persistent fever while in microgravity, a finding with significant implications for long-term space missions conducted without artificial gravity.
Weightlessness has many effects on astronauts' bodies. Most get nauseous initially, and without a punishing exercise routine, bones and muscles start to atrophy to the point where walking can be difficult once back in Earth's gravity. The body operating at higher temperatures is a less anticipated outcome. Although some astronauts have complained of heat stress during spaceflight, the only previous studies have been of short missions. Heat stress is associated with reduced physical and mental capacities, so if we want spacefarers on voyages to other planets in peak condition, we need to start understanding the problem now.
Professor Hanns-Christian Gunga of Charité – Universitätsmedizin Berlin and colleagues attached forehead sensors to 11 astronauts aboard the International Space Station and found that, on average, they developed temperatures 1ºC (1.8ºF) higher in space than on Earth when at rest. Gunga announced in Scientific Reports that during exercise, astronauts' body temperatures were often above 40ºC (104ºF), the point at which body temperatures are considered to become life-threatening. Temperatures also rose more quickly during exercise in space than on Earth.
The higher temperatures were not an immediate response to spaceflight, Gunga found, nor an aftereffect of launch. Instead, temperatures started off normal (37ºC, 98.6ºF) and slowly rose over a period of about 2.5 months before stabilizing. Body temperatures quickly fell back after returning to Earth.
"Under weightless conditions, our bodies find it extremely difficult to eliminate excess heat. The transfer of heat between the body and its environment becomes significantly more challenging in these conditions," Gunga said in a statement. Sweat evaporates slower in space, reducing the effectiveness of one of the body's key cooling mechanisms, and it's certainly not practical for astronauts to take cooling showers.
As the paper notes, long-term space missions are expected to require astronauts to do plenty of exercise to maintain bone density and muscle mass, but this may need a rethink if the consequence is to push temperatures into the danger zone.
Besides the implications for humanity's interplanetary future, Gunga thinks his work may have something to say about our past. Each mammalian species has its own optimum temperature, but little is known about how ours evolved. Perhaps by exploring the effects of an environment we never encountered in our development, we can work out why we settled on 37ºC (98.6ºF).