"Earth is the cradle of humanity, but one cannot live in a cradle forever." The famous quote by the Soviet engineer Konstantin Tsiolkovsky, known as the father of astronautics, encapsulates the unstoppable drive of humans to explore other worlds. A desire that is becoming a reality with small forays outside our planet, to the Moon and, above all, to the International Space Station (ISS), but faces an evident obstacle: our bodies are not designed to live outside Earth or to float in space. In fact, walking upright is considered one of the key moments in human evolution, the result of millions of years of adaptation. A capability utterly useless for life in weightlessness.
Furthermore, long stays in space take a toll on health, as Sunita Williams and Butch Wilmore will attest in the coming days, the NASA astronauts who were 'stranded' on the International Space Station (ISS) for nine months after the malfunctions suffered by the Boeing Starliner spacecraft they tested last June. An odyssey that ended at midnight on Tuesday when they returned to Earth in another spacecraft, a SpaceX Crew Dragon.
Although the mission, which was initially supposed to last about 10 days, turned into a highly publicized space crisis that reached the political sphere, long stays in space like the one just completed are extremely valuable for space agencies. Both NASA and the European Space Agency (ESA) allocate many resources to learn how to survive in space and to develop all kinds of devices and sensors to monitor their health and mitigate the numerous alterations their bodies undergo.
Far from giving up on the endeavor to live in such a hostile environment, they work for humans to adapt to it. To become Homo cosmicus as well. The goal is to send humans to Mars, a much more complex challenge than the brief missions to the Moon carried out between 1969 and 1972 (and which will be repeated this decade with NASA's Artemis program), or the stays on the ISS, the orbital platform located about 400 kilometers above Earth.
Although the nine months -286 days to be exact- that Williams and Wilmore spent on the ISS may seem like a lot, it is common for astronauts to be there for six months, and there have been several who have stayed almost a year in a row, like Christina Koch, Scott Kelly, and Mikhail Kornienko (planned) or unexpectedly, like Frank Rubio, who had to stay 371 days due to a leak in a Russian Soyuz that forced him to return in another spacecraft in February 2023.
When it comes to space endurance records, the Russians are the absolute champions, both in consecutive days in space and accumulated during their missions.
Globally, the record for the longest continuous time in space is held by Valeri Polyakov (80 years old), who spent 437 days on the MIR station between 1994 and 1995. And there is a human who has spent more than three years of his life in zero gravity: the Russian Oleg Kononenko (60 years old), with 1,111 accumulated days in five missions. A record that surpassed in 2024 the 878 days accumulated by his compatriot Gennady Padalka (66 years old). Adding up their three missions, Sunita Williams (59 years old) has accumulated 606 days and Butch Wilmore (62 years old) 464.
Cosmonaut Gennady Padalka conducting a medical check-up on astronaut Mike Fincke on the ISS.NASA
If the space environment is hostile to humans, it is mainly due to two enemies: the absence of gravity and radiation (from the Sun and galactic radiation). Astronauts staying in low Earth orbit, like those on the ISS, have partial protection from Earth's magnetic field. The farther they go, the higher the risk.
Therefore, the impact on organisms is studied on one hand, and on the other hand, technologies and drugs are devised to protect astronauts' bodies from the ravages of space.
"The two main risks associated with space travel are undoubtedly the effects of radiation and microgravity on the eyes and brain. Approximately 70% of long-duration astronauts experience changes in their eyes, which can also affect their vision. In addition, spending months in space can cause alterations in the brain," summarizes Claudia Stern, head of Aerospace Clinical Medicine at the German Aerospace Center's Aerospace Medicine Institute (DLR).
In addition to researching treatments to mitigate the health effects of space, this leading center develops all kinds of sensors to monitor the health of astronauts and patients, including newborns. "These sensors are now incredibly compact, they can even be worn while sleeping and use artificial intelligence for quick and self-sufficient analysis," notes this specialist.
As highlighted by Pedro Juan Moreno Lozano, an aerospace medical examiner at the Spanish Aviation Safety Agency and the US Federal Aviation Administration (FAA), one of the most well-known effects on astronauts is that they "lose bone mass and a lot of muscle mass, so when they return to Earth, they lack the strength to stand, even though they go to space in optimal health and physical condition for which they undergo specific training," says this doctor, a member of the Spanish Society of Aerospace Medicine (SEMAe). "The condition in which some return is comparable to that of a patient who has been immobile in the ICU for a long time," he asserts. And this is despite the mandatory two hours of exercise on the ISS attached to machines every day.
On Earth, fluids tend to accumulate in the lowest area, but in space, "where due to the absence of gravity there is no up or down, the body has to adjust to the volume of fluid that on Earth is in the limbs, so it has to readjust," he explains.
The heart, which is also a muscle, "pumps without gravity in space, that is, without resistance, so it weakens and loses a certain capacity that it then has to regain when returning to Earth," Moreno explains. Although most of the muscle mass can be recovered, he states that the loss of bone mineral density cannot be rebuilt. This explains why astronauts and divers have a higher likelihood of developing osteoporosis: "What they can do upon return is to slow down, with treatments and exercise, the bone loss that also occurs on Earth, to compensate for the rapid loss they experienced in space," he explains.
According to Claudia Stern, "astronauts usually recover within 45 days after returning home, although this period is influenced by the training opportunities on the ISS during their stay. The rehabilitation program involves two hours of individualized training daily. However, it can take up to two years for bone strength and density to fully recover, and in some cases, previous values are never reached after the flight."
It is also common for them to have difficulty sleeping, so many crew members take sleeping pills. In addition to implementing a type of lighting on the ISS that helps regulate circadian rhythms, NASA scientists have designed devices to assess and improve sleep.