Spaceflight is a harsh experience for humans. First, there's the cosmic radiation that bombards the body, and then the microgravity that can interfere with bodily fluids and pressure. Second, there are the effects of limited mobility in confined spaces and the separation from friends and family that can affect mental health.
Astronauts are among the fittest of the fittest: They train for years to deal with such conditions, but it doesn't stop them having health problems in space. And the recognition of those health problems has led to a whole new branch of medicine: Space health.
Space health research may enable space tourism
Space health research has led to new knowledge about the short and long-term effects spaceflight has on almost every system of the body, from the cardiovascular and metabolic systems to the musculoskeletal and immune systems.
Microgravity makes healthcare in space very complicated: For example, how can someone perform cardiopulmonary resuscitation (CPR) if an individual has a cardiac arrest during spaceflight, without any hard "ground" to press upon?
On Earth, a person needing such first aid would typically lie on the floor. But in space, everything floats, making it almost impossible to apply pressure and get their heart started again. There are work-arounds, using straps and other methods, but how astronauts perform CPR in space is important knowledge, as Jochen Hinkelbein, a space health researcher at University Hospital of Cologne, Germany, has previously told DW.
It's not just professional astronauts who are affected by such concerns. Humanity is on the cusp of space tourism. Increasingly, those who can afford it will be off on regular short trips to space without any extensive training. As a result, societies will soon need an industry of space health experts to help people remain healthy during spaceflight.
Space illness brought from Earth
At the moment, space health continues to focus on professional missions, which can last six to 12 months. The most common conditions astronauts suffer during long-term space missions have been linked to a weakened or "dampened" immune system.
Research has shown that astronauts are susceptible to viral infections, and it's usually old viruses that lurk in an astronaut's body that they've taken with them to space.
"We tend to notice a reactivation of dormant viruses [...] and cysts, as well as skin infections," said Odette Laneuville from the University of Ottawa, Canada.
One example is chicken pox, a disease caused by a type of herpes virus called varicella-zoster.
People are often first infected with chicken pox as a child. The immune system overcomes the infection but the virus stays dormant in bodies, where it lies inactive in the human "virome."
The human virome is a collection of viruses that live in the body. They include good viruses, known as bacteriophages, that protect us by killing harmful bacteria. But the virome also includes bad viruses.
Dormant viruses can reactivate in space during times of stress, partly because the immune system is weakened by stress. And if it's varicella-zoster that gets reactivated, it can cause shingles, which is a serious illness.
But the good news for astronauts, said Laneuville, is that this increased susceptibility to infections is a short-term issue — it resolves itself when astronauts come back to Earth, where the immune system returns to normal function within four to five weeks.
Study tracked astronauts' immune responses
In a study published in June 2023, Laneuville aimed to understand why and how viruses, such as herpes, reactivate during spaceflight.
Laneuville's team investigated how the immune systems of astronauts on the International Space Station (ISS) had changed over a six-month period. They did this by collecting data from the astronauts' immune transcriptomes.
A transcriptome logs all the changes that occur in human genes. Genes, and the DNA they contain, are highly responsive to the environment — they change how they are expressed — and that allows the body to adapt to different situations.
For example, genes that are related to fighting viruses are "switched on" when the immune system need to stop an infection.
As a result, by analyzing the astronauts' immune transcriptomes at different times during the mission, Laneuville's team was able to track how their immune systems responded to spaceflight over the six months.
Blood samples were taken at 10 so-called "timepoints": Three before the mission, four during it, and another three afterwards.
Transcriptome analysis of the blood samples then focused on genes related to the immune system, and, primarily, white blood cells called leukocytes.
The team found 297 genes were affected by six months of spaceflight, and 100 of those were related to immune responses.
They say the most important finding was that the expression of genes related to immune function were changed in space — there's a dampening of the immune response, which the researchers think allows dormant viruses to reactivate and cause illness.
"The opposite happened when they returned to Earth, and the expression of these immune-related genes went up," said Laneuville. That's when their immune systems returned to normal function.
Among the genes affected in astronauts were those involved in "basic housekeeping cell functionality" and "regulation of body fluid," wrote the researchers in the published study.
These changes appear to be ways for the body to cope with changes in blood flow that occur in microgravity.
How does spaceflight dampen the immune system?
Laneuville said researchers don't know how spaceflight weakens the immune system, but they do know there are a combination of factors at play.
"The space environment is harsh for the human body — it has microgravity, cosmic radiation, stress and isolation. Our study demonstrates that the white blood cells of the astronauts are responding to all those stressors, and mainly [through] the pattern of gene expression," Laneuville said.
Of all the factors, Laneuville suspects microgravity may have the most impact on the human immune system.