There are certainly a whole host of technological hurdles to overcome before humans successfully travel to Mars, or beyond, but research is also pointing to a growing assortment of fundamental health challenges that astronauts may face from long stretches of time in space. A recent NASA-funded study has found dormant viruses can reactivate in the human body during spaceflight, presenting yet another physiological problem for scientists to solve before we journey out into deep space.

"NASA astronauts endure weeks or even months exposed to microgravity and cosmic radiation – not to mention the extreme G forces of take-off and re-entry," says Satish K. Mehta from the Johnson Space Center, and senior author on the new study. "This physical challenge is compounded by more familiar stressors like social separation, confinement and an altered sleep-wake cycle."

Prior research has revealed prolonged space travel could result in everything from a heightened cancer risk and neurological degeneration to tissue damage in the gastrointestinal tract. It has also been suggested that spaceflight has a detrimental effect on the immune system and it is this process that scientists are hypothesizing allows dormant viruses to reactivate.

"During spaceflight there is a rise in secretion of stress hormones like cortisol and adrenaline, which are known to suppress the immune system," says Mehta. "In keeping with this, we find that astronaut's immune cells – particularly those that normally suppress and eliminate viruses – become less effective during spaceflight and sometimes for up to 60 days after."

The new research looked at bodily fluid samples from astronauts before, during, and after missions to space. Epstein–Barr virus (EBV), varicella-zoster virus (VZV), and herpes-simplex-1 (HSV-1) were all found to have reactivated and shed into saliva and urine samples in more than half of the studied astronauts. In most cases these astronauts did not develop any specific viral symptoms, however these are infectious viruses and they can increase the chances of adverse medical events occurring on longer space voyages.

The level of viral shedding tracked in the astronaut samples seemed to increase in both frequency and quantity during the longer International Space Station missions. This suggests the immune system dysregulation that may be causing the viral reactivation is not just an acute stress reaction from the initial journey into space.

"The magnitude, frequency and duration of viral shedding all increase with length of spaceflight," explains Mehta.

The researchers hypothesize partial-gravity environments may have the potential to curtail this viral reactivation but certainly suggest more research is needed to better understand how the immune system could be modulated by gravity. In the short term the solution to this particular viral reactivation problem may be better and more targeted vaccination protocols for deep space astronauts.

The new study was published in the journal Frontiers in Microbiology.